What is potato blight?


Brown leaf spot in potatoes: Plan for this early blight look-alike in 2013

Early blight, caused by Alternaria solani, is a very common fungal disease present in most regions that produce potatoes. Brown leaf spot (Alternaria alternata) of potatoes has gained attention recently for its similarity to early blight. Just as common, yet underestimated, brown leaf spot presents symptoms that are often incorrectly attributed to early blight. Successful control of both pathogens depends on accurate identification and a tailored approach.

Brown leaf spot

The fungus that causes brown leaf spot, Alternaria alternata, is closely related to early blight (Alternaria solani). Like early blight, brown leaf spot overwinters as spores and mycelia on the infected tissue of various Solanaceous hosts. Warmer temperatures in spring trigger fruiting and the release of spores from both fungi, which are moved by wind and water onto potato plants. When moisture is available, the spores germinate and penetrate potato tissue, often through existing wounds. Initial infections of the two pathogens create similar symptoms: small, dark spots on lower leaves.

(A) Symptoms of early blight on potato leaves caused by Alternaria solani and
(B) symptoms of brown leaf spot on potato leaves caused by Alternaria alternata.
Photo credit: Willie Kirk and Phillip Wharton, MSU

However, the foliar lesions caused by brown leaf spot never develop the dark concentric rings characteristic of early blight. They coalesce across large veins until whole leaves turn brown and hang from the plant. The tuber symptoms of brown leaf spot are commonly referred to as black pit, appearing as small black holes in the tuber surface. Brown leaf spot can also result in skin patches similar to those caused by early blight, most often observed after washing.

Brown leaf spot can be managed using cultural controls and fungicides. Crop rotation including non-host crops and cultural practices such as the planting of certified seed, timely irrigation, and post-harvest refuse management should be the foundation of any disease management program. Fungicides can be applied after flowering to improve control. Strobilurins used for early blight will not provide adequate control of brown leaf spot. Yet, protectant fungicides also often prescribed for early blight (e.g., maneb, mancozeb, chlorothalonil and triphenyltin hydroxide) are moderately effective against brown leaf spot when applied at approximately seven- to10-day intervals. This highlights the point that strobilurin fungicides should always be paired with a protectant fungicide to manage pathogen diversity and pesticide resistance.

More information on brown leaf spot of potatoes can be found in a new bulletin from Michigan State University Extension and the Michigan Potato Industry Commission titled Michigan Potato Disease: Brown Leaf Spot (E3182). This bulletin and others are available free through the Michigan State University Extension Bookstore.

  • Michigan Potato Disease: Brown Leaf Spot, MSU Extension Bulletin E3182
  • Michigan Potato Disease: Early Blight, MSU Extension Bulletin E2991

Dr. Kirk’s work is funded in part by MSU’s AgBioResearch.

Potato plants have brown dead leaves and stems are drying up

Potatoes are susceptible to a bacterial wilt called verticillium wilt. Here is a to a publication that explains the symptoms. I would also look for environmental causes. The heat spell we just had could be a cause. I lost some melon vines in exactly the same way. They withered and turned brown with the immature melons still attached. Potatoes are a cool weather crop, so it’s possible that they couldn’t handle the recent high temperatures. Here is a quote from a publication on potato temperature requirements
“Potatoes are cool season crops which grow best in cooler climates or as a winter crop in areas with warm summers. They are sensitive to heat but can tolerate a light frost. Potatoes require a deep, fertile, loose, well-draining soil with a pH between 5.8 and 6.5 and will grow optimally at daytime temperatures between 18 and 27°C (65–80°F) and night time temperatures between 12 and 18°C (55–65°F). “
It’s also possible they didn’t get enough water. Although we had plenty of thunderstorms, we didn’t have many soaking rains. Here is a quote about moisture requirements.
“Potatoes are sensitive to soil moisture and grow best when soil moisture is consistent. The plants generally require about 1 inch of water a week from rainfall or irrigation. Water-saturated soil should be avoided as it can poorly formed tubers and rot. In addition, potatoes are heavy feeders and the addition of a balanced fertilizer every two weeks can help increase tuber yields.”
Once the vines are dead, there is nothing left to feed the potatoes. I recommend you dig up any potatoes that are attached to dead vines. Perhaps some of the crop will be suitable as seed for the next growing season. Also, you can take a sample of the vine into your local Penn State Extension and have it analysed for possible disease. The plant clinic will send you a report and recommend any remedy they think is necessary.
With the tricky weather conditions we have had in the last couple of years, it is a good idea to invest in a rain gauge so you can monitor the amount of water your crop is getting. I also put in a low tunnel made of PVC flexible conduit and used clothespins to attach sheer curtains from the local box store. This gave my heat sensitive plants some shade during the hot spell. There are lots of directions on the internet for cheap ways to make a low tunnel.

Potato Pests and Diseases

Increasing potato production while protecting producers, consumers, and the environment requires an integrated approach encompassing a range of strategies: encouraging natural pest predators, breeding varieties with pest/disease resistance, planting clean seed, rotating with other crops, and organic composting to improve soil quality.


  • Colorado potato beetle (Leptinotarsa decemlineata) is a serious pest with strong resistance to insecticides.
  • Potato tuber moth, most commonly Phthorimaea operculella, is the most damaging pest of planted and stored potatoes in warm, dry areas.
  • Leafminer fly (Liriomyza huidobrensis) is a South American native common in areas where insecticides are used intensively.
  • Cyst nematodes (Globodera pallida and G. rostochiensis) are serious soil pests in temperate regions, the Andes, and other highland areas.


Viruses are disseminated in tubers and can cut yields by 50 percent.

  • Late blight, the most serious potato disease worldwide, is caused by a water mould, Phytophthora infestans, that destroys leaves, stems, and tubers.
  • Bacterial wilt, caused by the bacterial pathogen, leads to severe losses in tropical, subtropical, and temperate regions.
  • Potato blackleg, a bacterial infection, causes tubers to rot in the ground and in storage.

Some of Potato’s Enemies

Potato Blight Cause, Identification. Prevention, Treatment Potato Blight

Cause of, Identification of, Prevention of and Treatment of Potato Blight – Phytophthora infestans

Potato blight is the worst problem that the potato grower faces. Once it arrives it can devastate a crop in a day or two and when the infection moves down from the foliage to the potato tubers, cause them to rot as well.

Potatoes infected with late blight are shrunken on the outside, corky and rotted inside They also stink and once smelt, never forgotten.

Most famously the potato blight was, if not the only cause, certainly the major contributor to the Irish Famine of the 1840’s.

The blight had started in Europe and eventually reached Ireland where the potato was a staple food. Worse still, the Irish all grew the same very susceptible variety (Irish Lumper). Not only did they lose crops in the ground but crops in store were lost as well.

There’s a lesson for growers today in the Irish Famine. Lack of genetic diversity and mono-culture increases the risk of devastation. Sadly farmers ignore it at our peril.

Cause of Potato Blight

Potato Blight is caused by the Phytophthora infestans fungus This fungus can also infect other members of the potato family, Solanaceae such as tomatoes. It spreads via airborne spores on the wind until it lands on a susceptible plant and the weather conditions are right for it to develop, warm and humid.

Blighted King Edward Potatoes in July.
One day the haulm is fine, next like this – browning and obviously dying.

Being near to other potato growers increases your risk as there are more spores around but even growers in isolated spots with no nearby potatoes are vulnerable.

If you’ve been growing for some time and you think potato blight is getting worse, you’re right. This is for two reasons.

First the weather. Over recent years winters have been warmer and summers wetter in Britain. The conditions are more favourable to blight than they used to be. Whether this is a long term effect of climate change or just statistical chance doesn’t matter – the fact is the weather favours blight.

Secondly we have more strains of blight than we used to. Until the 1990s we basically had one strain of blight that would change over the years overcoming blight resistant potato varieties. Then another strain of blight was imported in a shipment of potatoes from Mexico.

The two strains mingled and reproduced so producing more strains and currently there are 4 or 5 variants on potato blight in the UK. Very similar to the way flu develops. One year we have Hong Kong Flu and the next another type as the virus mutates. Hopefully we won’t have a blight version of the 1919 Spanish flu which killed more people than the first world war did.

Smith Periods & Hutton Periods

The warm and humid conditions ideal for blight to develop have been formally defined and are known as Smith Periods. A ‘Smith period’ is a 48 hour period in which the minimum temperature is 10°C or more and the relative humidity exceeds 90% for at least 11 hours during the first 24 hours and for at least 11 hours again during the final 24 hours.

Recently a new, more simple, measure of blight risk known as a Hutton Period has come into use. A Full Hutton Period occurs when the following criteria are met on 2 consecutive days:

  • Minimum air temperatures are at least 10°C
  • Relative Humidity is 90% or above for at least 6 hours

By watching for Smith or Hutton periods it is possible to predict when blight is most likely to appear and use that warning for growers to take preventative action. You can get automated warnings of Hutton Periods sent direct to your inbox by signing on to the blightwatch web site.

Symptoms of Potato Blight

Initial symptom of Potato Blight is small brown patches on the leaves like this.

Initially a spore or more likely a number of spores will land on the potato haulm. In dry weather they lie dormant but when the humidity is right, the fungus starts to grow. You may notice brown freckles on the leaves or sections of leaves with brown patches and a sort of yellowish border spreading from the brown patch.

In a severe attack you may discover that all the foliage is down and brown, starting to rot. It’s frightening how quickly potato blight can spread in the right circumstances.

The blight spores will fall or be washed down by rain onto the soil and any exposed tubers. Once in a tuber it will spread from tuber to tuber and infect the entire crop.

Potato blight affected tubers (the actual potato) by can be told by dark patches on the skin. Cutting the potato in half will reveal brownish rot spreading down from the skin. Eventually the potatoes almost liquefy, becoming jelly like and give off a distinctive stench. The smell of blight is unmistakeable.

If you are storing potatoes that smell can actually help identify the infected potatoes when there is still little to see. On a good sunny day, spread the potatoes out on a mat outside. You’ll notice the flies make a beeline for the infected potatoes.

Prevention and Treatment Of Potato Blight

The Sarpo Mira being blight resistant are unaffected.
Photo taken at the same time as the King Edward potatoes above

The blight fungus is generally killed by cold weather although with the new crossbred strains the signs are some are better at coping and over-wintering. The main disease reservoir is infected tubers in the ground or in storage sacks. Wherever it comes from, the spores can travel miles on the wind and there is little you can do if the weather is right for blight.

The farmers have a range of chemicals available to spray their vast acreages of mono-culture potatoes but the chemicals available to non-licensed home growers are more limited. Even with repeated weekly spraying in the horrendous wet summer of 2012 many farmers lost the battle to the blight anyway.

The traditional spray for blight is Bordeaux mixture. I’m not sure this is such a good idea as it contains copper and that is hardly good for your diet although it was organically approved. It is worth checking in horticultural suppliers what anti-fungals are currently available and approved for use on potatoes as the list shrinks every year.

If you’re relying on spraying, then it should be as a preventative rather than a cure. When a Smith or Hutton period is reported you spray and at the first sign of blight striking the haulm, remove the affected foliage and spray the rest. You just might get away with it, so long as only a few leaves are affected, there is a good chance of repelling the blight.

If however you don’t win or don’t like using chemical sprays, cut off the haulm and dispose of it. You can hot-compost the haulm but the safest answer is to bin or burn it. The potato tubers won’t develop any further but leave them be for 3 weeks nonetheless. This is to hopefully stop the blight spores getting to the tubers as you harvest.

After harvesting and storing, check potatoes regularly for signs of blight and remove any suspect tubers at once from your store.

Good practice helps prevent potato blight

  • Be sure to get all the potatoes out from the ground when you harvest, even the tiny ones if you can so you don’t leave a reservoir of infection on your plot.
  • Ensure potatoes are well earthed up to protect tubers so preventing it spreading into them even if you get blight in the foliage
  • Water from the base rather than spraying potatoes. If the weather is right but there is no rain, your artificial rain will give the blight a hold. Leaky hoses are a good way to water potatoes, unless you have hosepipe restrictions

Avoiding Potato Blight

Since potato blight tends to hit later in the season when the weather conditions favour it, growing first and second earlies can dodge the fungus. Not every year, obviously, but most years they’re out of the ground when blight arrives.

Blight Resistant Varieties of Potato

Some potatoes resist the blight better than others, although a lot depends on which variety they get now. Some varieties resist better in the foliage and some in the tubers so crops are measured for both.

The relatively recent development and release to market of Sarpo Hungarian varieties of potato that are extremely blight resistant has been a boon to home growers. I’ve had the Sarpo Mira standing healthily when every other potato on the allotment site had gone down to blight.

Best Potatoes for Foliage Blight Resistance

First Early – Orla, Premiere

Second Early – Cosmos, Nadine

Best Potatoes for Tuber Blight Resistance

First Early – Orla, Colleen

Second Early – Cosmos, Nicola

Main Crop – Cara, Sante, Record, Kondor, Sarpo Axona, Sarpo Mira, Sarpo Gwyn, Sarpo Shona, Sarpo Una, Lady Balfour, Valor, Picasso

Note: Sarpo Mira has the highest rating for tuber blight.

Potato Growing Articles

  • Growing Potatoes Overview – How to Grow Potatoes Guide
  • Growing Potatoes – Standard Traditional Method
  • Growing Potatoes Under Straw Mulch
  • Growing Potatoes Under Black Plastic (Polythene) Sheet
  • Potato Growing in Raised Beds & Ridge Planting Potatoes
  • Growing Potatoes in a Barrel – Patio Growing Potatoes
  • Growing Potatoes in Bags | Greenhouse Potatoes
  • Second Crop Autumn Planted Christmas New Potatoes
  • Can you chit supermarket potatoes?
  • Potato Varieties for Flavour -Boiled Baked Roasted Mashed
  • Potato Fertiliser Program Program & (NPK) Requirements
  • Potato Blight Cause, Identification. Prevention, Treatment Potato Blight
  • Wireworm in Potatoes Cause Identification Prevention Control Potato Wireworm
  • Eelworm Potato Cyst Nematode – Control Potato Eelworm
  • Dry Rot in Potatoes Cause Identification Prevention Control of Potato Dry Rot
  • Potato Scab – Common Scab in Potatoes
  • Potato Scab – Powdery Scab in Potatoes
  • Hollow Heart, Splitting & Spraing Potatoes
  • White Spots on Potatoes Lenticels & Potato Stem Rot

See Also:

  • Growing Potatoes for Show, Introduction & Best Varieties
  • Growing Potatoes for Show, Cultivation of Show Potatoes
  • Growing Potatoes for Show Harvest & Showing Potatoes
  • Harvesting Potatoes Guide
  • Storing Potatoes Guide
  • Potatoes from the Allotment Shop

Blight Control In Potatoes: How To Treat Early And Late Potato Blight

Potato blight diseases are the bane of gardeners everywhere. These fungal diseases wreak havoc in vegetable gardens throughout the growing season, causing significant above-ground damage to potato plants and rendering tubers useless. The most common potato blights are named for the part of the season when they’re common — early blight and late blight. Blight control in potatoes is difficult, but armed with some knowledge you can break the disease cycle.

How to Identify Potato Blight

Both types of blight are common in American gardens and pose some risk to other closely related plants like tomatoes and eggplants. Symptoms of potato blight are distinct when the timing of their appearance is taken into account, making blight easy to diagnose.

Potato Early Blight

Potato early blight is caused by the fungus Alternaria solani and attacks older leaves first. Fungal spores overwinter in plant debris and tubers that were left behind after harvest, but waits to activate until the humidity is high and daytime temperatures first reach 75 F. (24 C.). Alternaria solani penetrates the leaf tissues quickly under these conditions, causing visible infection in two or three days.

Lesions start as small, dark, dry flecks that soon spread into dark circular or oval areas. Early blight lesions may have a bull’s eye appearance, with alternating rings of raise and depressed tissues. Sometimes, these ring groupings are surrounded by a green-yellow ring. As these lesions spread, leaves may die but remain attached to the plant. Tubers are covered in spots similar to leaves, but the flesh below the spots is usually brown, dry, leathery or corky when potatoes are cut open.

Potato Late Blight

Potato late blight is one of the most serious diseases of potatoes, caused by the fungus Phytophthora infestans, and the disease that single-handedly caused the Irish Potato Famine of the 1840s. Late blight spores germinate at humidity levels above 90 percent and temperatures between 50 and 78 F. (10-26 C,), but grows explosively at the cooler end of the range. This disease is often seen in early fall, toward the end of the growing season.

Lesions start out small, but soon expand into large brown to purple-black areas of dead or dying leaf tissue. When humidity is high, a distinctive white cottony sporulation appears on the undersides of leaves and along stems and petioles. Late blight-infested plants may put off an unpleasant odor that smells like decay. Tubers frequently become infected, filling with rot and allowing access to secondary pathogens. Brown to purple skin may be the only visible sign on a tuber of internal disease.

Blight Control in Potatoes

When blight is present in your garden, it can be difficult or impossible to kill entirely. However, if you increase the circulation around your plants and carefully water only when needed and only at the base of your plants, you may be able to slow the infection significantly. Pick off any diseased leaves carefully, and provide additional nitrogen and low levels of phosphorus to help potato plants recover.

Fungicides can be used if the disease is severe, but azoxystrobin, chlorothalonil, mancozeb and pyraclostrobin may require multiple applications to destroy the fungus completely. Most of these chemicals must be discontinued two weeks before harvest, but pyraclostrobin can safely be used up to three days before harvest begins.

Prevent future outbreaks of blight by practicing a two to four year crop rotation, removing volunteer plants that may carry disease and avoiding overhead watering. When you’re ready to dig your tubers, take great care not to injure them in the process. Wounds can allow post-harvest infections to take hold, ruining your stored crop.

Management of Late Blight of Potato

5.1. Chemical management

Chemical management is very popular strategy for the management of late blight. Since the discovery of Bordeaux mixture in 1885 and it was first important landmark in the history of chemical disease control. Bordeaux mixture belongs to first generation of fungicides along with other inorganic chemicals. After more than 130 years, the introduction of Bordeaux mixture (Copper sulfate, hydrated lime and water), large numbers of fungicides (first generation Bordeaux mixture to fourth generation Mandipropamid & Azoxystrobin) were evaluated at worldwide against late blight of potato/tomato. In practice, the traditional management of late blight depends highly on preventative fungicides, application on a regular calendar basis (e.g. weekly) during the growing season . The population diversity and disease incidence of P. infestans has been increased through the development of systemic fungicide resistance (insensitivity) and the transcontinental shipment of the late blight infected potato tubers and tomato plantlets . Metalaxyl fungicide which comes under Phenylamide group with FARC 4, was introduced against oomycetes, very effective for late blight management and highly adopted worldwide. However, after introduction within three years metalaxyl resistant isolates were detected on field grown potatoes in Ireland, The Netherlands and Switzerland . The site-specific systemic fungicide, mefenoxam (the active isomer in metalaxyl), inhibits sporulation and mycelial growth inside host tissues by specifically inhibiting RNA polymerase-1, a mutation that changes the affinity of target sites could easily lead to fungicide resistance . In Indian scenario, metalaxyl based fungicides were introduced on experimental basis for management of late blight during late 1980’s however, their commercial use started only during 1994–1995 . In India, 200–400 ppm tolerance level was observed with metalaxyl. After 12 years, its introduction during 2006, the metalaxyl based fungicides failed to protect the potato crop from the late blight in temperate highlands leading to 40–70% crop losses. Systemic fungicide metalaxyl is cause of concern for management of late blight disease due to quickly developed resistance. Pathogen had developed.

Tolerance up to 400 ppm and genetic studies crosses indicated that a semi dominant major locus determines resistance to metalaxyl, since insensitive and sensitive parents usually yielded progeny with those phenotypes at a 1:1 ratio . The heterothallic single mating type isolates of P. infestans was exposed to 9 of the 11 commercial fungicide formulations for assess their effect on formation of oospores. The highest numbers of oospores were observed on media amended with Ridomil 2E (metalaxyl) and Ridomil Gold EC (mefenoxam) at 0.1 to 10 μg a.i./ml, when averaging it was found that 471 and 450 oospores/petri dish, respectively. The remaining fungicides viz., Maneb, Manzate (Mancozeb), Curzate (cymoxanil +mancozeb), and Acrobat MZ (dimethomorph + mancozeb) also induced oospore formation, which ranged from 0 to 200 oospores/petri at fungicide concentrations from 0.1 to 10 μg a.i./ml. No oospores were formed on media amended with Bravo (chlorothalonil) or Tattoo C (chlorothalonil + propamocarb HCl), moreover both the compounds completely suppressed growth of the isolates at 0.1 and 1 μg a.i./ml. The metalaxyl resistant isolates formed oospores in response to the fungicides more often than the metalaxyl sensitive isolates . Metalaxyl + mancozeb (Ridomil MZ) and ofurace (Orafce 50WP) were reported to provide highly effective control of late blight . The fenamidone is a novel fungicide, which acts on cytochrome bc1 in mitochondrial complex III of P. infestans at a number of points in its life cycle . Cymoxanil based fungicides possess a novel mode of action by preventing electron transfer between cytochrome b and c1 in mitochondrial complex III and provide good scope for the control of late blight of potato and tomato . Efficacy of seven fungicides was tested under in vitro conditions and the fungicides, which showed promising results, were further evaluated under field conditions and fenamidone based fungicide was found most effective in controlling late blight followed by cymoxanil based while mancozeb was found least effective; similarly the systemic fungicides viz., fenamidone and dimethomorph were reported most effective in vitro for management of late blight . Various studies showed that a reduced use of fungicides lowers the selection pressure for mefenaxam-resistant strains and mixture with a contact fungicide improves efficacy and may slow the development of resistance to mefenoxam . The systemic fungicides have better persistence on the host surface and are being used as mixture with contact fungicides against late blight so as to avoid development of resistance in pathogen . The fungicide mixtures, containing two or more fungicides with different modes of action, have been developed with the twin objectives of broadening the activity spectrum against diverse plant diseases and to check the development of resistance in the target pathogens . In commercial production of potato is not viable without fungicides for management of late blight. Fungicide mixtures and targeted application based on late blight forecasting model are very important for managing late blight. However, due to delisting of many fungicides products under the EU Pesticide Directive and environmental concerns, provides impetus for potato breeding and more effective fungicide application . It has been reported from European country that the same fungicide should not applied more than two sequential applications . The severe late blight can be effectively managed with prophylactic spray of mancozeb at 0.25% followed by cymoxanil+mancozeb or dimethomorph+mancozeb at 0.3% at the onset of disease and one more spray of mancozeb at 0.25% seven days after application of systemic fungicides in West Bengal . Similarly, one spray of mancozeb followed by three spray of cymoxanil + mancozeb was effective on cv. Kufri Bahar under western UP . Due to development of resistance to fungicides, a new fungicide, Victory 72 WP was first used in controlling late blight of potato and tomato in West Shoa of Ethiopia . The late blight specific spray scheduling method and a method of scheduling sprays for both diseases (early and late blight) suppressed early and late blight as well as did weekly sprays (conventional methods) and with the same average number of applications as with weekly sprays . The customarily, spray schedules were one prophylactic spray using contact fungicides followed by systemic fungicides and one more spray of either same contact or same systemic fungicides. A unique combination of treatments was developed keeping in view the sensitivity of P. infestans to develop fungicide resistance. The post spray (curative spray) of same mode of action fungicide was not taken. Prophylactic sprays of chlorothalonil/mancozeb followed by systemic/trans laminar fungicides were found effective than post symptom sprays. This will be useful to minimize the yield losses due to late blight and assist in reducing development of resistance against fungicides in pathogen . The spray schedule of mancozeb 75% WP (0.2%- before appearance) followed by two more spray with mancozeb 75% WP (0.2%) + dimethomorph 50% WP (0.2%) at 7–10 days intervals showed less terminal disease severity (24.55%) with highest disease controlled (74.45%), which was at statistically par with treatment mancozeb 75% WP (0.2%, before appearance) followed by cymoxanil 8% + mancozeb 64% WP (0.3%) with two more spray at 7–10 days intervals, with 27.56% terminal disease severity along with disease controlled 71.29%. One spray of mancozeb (contact fungicides: before appearance) and latter two more sprays of translaminar/systemic + contact fungicides at 7–10 days interval give better results for managing late blight of potato . The highest marginal benefit was achieved by applying first Ridomil then Dithane M-45 at 14–21 days interval. The lowest marginal benefit was with alone application of Ridomil at 21 day spray interval. At 7 days sprays was more economical to apply Dithane M-45 than Ridomil first followed by Dithane M-45 subsequently . Twelve fungicides were evaluated on isolates of three identified clonal lineages (US-22, US-23, and US-24) of P. infestans using a detached tomato leaf assay in preventative and post-infection methods. The results revealed that these fungicides were suitable in conventional and organic systems, which can effectively control late blight caused by new clonal lineages of P. infestans when applied preventatively and late blight caused by the US-24 clonal lineage may require less fungicide than US-22 or US-23 to manage the disease . The efficacy of Ametoctradin 27% + dimethomorph 20.27% (w/w) as a new molecule for management of late blight of potato was reported in India . Initium (ametoctradin) is a new fungicide for management of Phytophthora infestans. It affects mitochondrial respiration inhibitor interfering with the complex III (complex bc1) in the electron transport chain of the pathogen, thus ATP synthesis in the fungal cells is inhibited. It is a non-systemic fungicide that remains primarily on the leaf surface where it is adsorbed with high affinity to the epicuticular wax layer of the epidermis . Many oomycete-specific fungicides such as QoI compounds, dimethomorph, propamocarb, etc. were commercialized, but currently, we are unaware of any fungicide that could effectively halt epidemics caused by metalaxyl-resistant strains under conditions favorable to P. infestans growth and development . Isolates of Phytophthora infestans showed 10-fold or more variation in baseline sensitivity to many fungicides including cymoxanil, dithiocarbamates, mandipropamid, and strobilurins . Various substances other than fungicides also were tested for management of late blight of potato. Ammonium molybdate, cupric sulfate and potassium metabisulfate at 1 mM partially inhibited the growth and spore germination of P. infestans, whereas ferric chloride, ferrous ammonium sulfate and ZnSO4 at 10 mM completely inhibited growth and spore germination . The foliar spray of ZnSO4 and CuSO4 (0.2%) micronutrients, 12 days delayed the onset of late blight when used with host resistance, subsequently reduced disease severity with higher yield . Sub-phytotoixc dose of boron with reduced rate of propineb + iprovalidicarb has been found more effective than treated with fungicides alone . β-aminobutyric acid (BABA) has been known as an inducer of disease-resistance. However, only the R but not the S enantiomer of BABA primes for resistance. Unfortunately, BABA can also impose growth stress in some treated plants therefore BABA analogs with reduced stress effects are highly desirable for agricultural field . Plant activator viz., BABA and phosphoric acid was evaluated against late blight by various researchers with combination of fungicides or alone . A 20–25% reduction of the fungicide dose in combination with BABA gave on average the same result on late blight development as full dose Shirlan alone in field condition, while reduced dose of Shirlan alone sometimes resulted in less effective protection. However, in vitro results indicated that the efficacy was lasted for only 4–5 days after BABA treatment and subsequently efficacy was lowered. The partially resistant cultivars Ovatio and Superb reacted to lower concentrations of BABA where no effect was found in susceptible cv. Bintje . Two SAR activators (BABA and phosphorous acid) were found effective against late blight of potato with significantly reduced disease severity (40–60%). The expression of the defense related genes and P. infestans effecter proteins β-1,3 glucanase, PR-1 protein, phytophthora inhibitor, protease inhibitor, xyloglucanase, thaumatin protein, steroid binding proteins, proline, endochitinase and cyclophilin genes were up regulated with the SAR activator treatment compared to unsprayed . Since last one and half decades, various fungicides have been developed for management of late blight. Isolates of P. infestans might develop resistant over the period. Fungicides resistance with currently used fungicides, including dimethomorph, has been reported . There are three key phases in the development of fungicide resistance (i) emergence, (ii) selection, and (iii) adjustment. In emergence, the resistant strain has to arise through mutation and invasion whereas in selection, the resistant strain is present in the pathogen population and a small portion of the pathogen population carrying the resistance increases due to the selective pressure imposed by the fungicides. In case of adjustment phase, the resistant fraction of the pathogen population has become large, crop managers have to adjust fungicide programs, by changing the dose or active substance(s) used, in order to maintain control .

Scientists have long known that it was a strain of Phytophthora infestans (or P. infestans) that caused the widespread devastation of potato crops in Ireland and northern Europe beginning in 1845, leading to the Irish Potato Famine.

P. infestans infects the plant through its leaves, leaving behind shriveled, inedible tubers. The most likely culprit, they believed, was a strain known as US-1, which even today is responsible for billions of dollars of crop damage each year. To solve the mystery, molecular biologists from the United Kingdom, Germany and the United States examined DNA extracted from nearly a dozen botanical specimens dating back as far as 1845 and held in museum collections in the UK and Germany, which were then sent to the Sainsbury Laboratory in Norwich, England. After sequencing the genome of the 19th century samples and comparing them with modern blights, including US-1, they were able to trace the genetic evolution of P. infestans around the world and across centuries.

The researchers concluded that it wasn’t in fact US-1 that caused the blight, but a previously unknown strain, HERB-1, which had originated in the Americas (most likely in Mexico’s Toluca Valley) sometime in the early 19th century before spreading to Europe in the 1840s. HERB-1, they believe, was responsible for the Great Famine and hundreds of other potato crop failures around the world. It wasn’t until the early 20th century that improvements in crop breeding yielded potato varieties that proved resistant to HERB-1 that the deadly infection was stopped in its tracks. Scientists believe that the HERB-1 strain is now extinct.

First domesticated in southern Peru and Bolivia more than 7,000 years ago, the potato began its long trek out of South America in the late 16th century following the Spanish conquest of the Inca. Though some Europeans were skeptical of the newly arrived tuber, they were quickly won over by the plant’s benefits. Potatoes were slow to spoil, had three times the caloric value of grain and were cheap and easy to grow on both large farms and small, backyard lots. When a series of non-potato crop failures struck northern Europe in the late 18th century, millions of farmers switched to the more durable spud as their staple crop.

Nowhere was dependency on the potato more widespread than in Ireland, where it eventually became the sole subsistence food for one-third of the country. Impoverished tenant farmers, struggling to grow enough food to feed their families on plots of land as small as one acre, turned to the potato en masse, thanks to its ability to grow in even the worst soil. Requiring calorie-heavy diets to carry out their punishing workloads, they were soon consuming between 40 and 60 potatoes every day. And the potato wasn’t just used for human consumption: Ireland’s primary export to its British overlords was cattle, and more than a third of all potatoes harvested were used to feed livestock.

By the early 19th century, however, the potato had begun to show a tendency toward crop failure, with Ireland and much of northern Europe experience smaller blights in the decades leading up to the Great Famine. While the effects of these failures were largely ameliorated in many countries thanks to their cultivation of a wide variety of different potatoes, Ireland was left vulnerable to these blights due to its dependence on just one type, the Irish Lumper. When HERB-1, which had already wreaked havoc on crops in Mexico and the United States, made its way across the Atlantic sometime in 1844, its effect was immediate—and devastating. Within a year, potato crops across France, Belgium and Holland had been affected and by late 1845 between one-third and one-half of Ireland’s fields had been wiped out. The destruction continued the following year, when three-quarters of that year’s harvest was destroyed and the first starvation deaths were reported.

As the crisis grew, British relief efforts only made things worse: The emergency importation of grain failed to prevent further deaths due to Ireland’s lack of working mills to process the food; absentee British landlords evicted thousands of starving peasants when they were unable to pay rent; and a series of workhouses and charity homes established to care for the most vulnerable were poorly managed, becoming squalid centers of disease and death. By 1851 1 million Irish—nearly one-eight of the population—were dead from starvation or disease. Emigration from the country, which had steadily increased in the years leading up to the famine, ballooned, and by 1855 2 million people had fled, swelling the immigrant Irish populations of Canada, the United States, Australia and elsewhere. Even today, more than 150 years later, Ireland’s population has still not recovered its pre-famine level. Those that stayed behind, haunted by their country’s suffering, would form the basis of an Irish independence movement that continued into the 20th century.

America: Promised Land The 2-part special premieres Memorial Day at 9/8c on HISTORY.

Everyone who has grown potatoes will have heard of potato blight, it’s the most serious disease affecting potatoes and one which is almost impossible not to get. The disease is thought to have originated in South America (where the potatoes themselves originally hailed from) and brought to Europe in infected ‘guano’ or fertilizer. Whatever way it arrived here it’s here to stay and unfortunately for growers has developed more virulent and destructive strains.

There are ways of avoiding blight but for domestic growers there is no way of treating the disease once it has made an appearance in your crop. Growing potatoes and dealing with blight is all about trying to work around the fact that sooner or later you will get it so it’s all about avoidance and minimising the effects once the disease hits. If you haven’t grown potatoes before don’t let this put you off, gardeners everywhere grow potatoes year after year and enjoy great crops of healthy and tasty potatoes. This article is intended to make sure you enter the battlefield fully prepared not only to recognise the enemy but to know how to tackle it when it appears.

What is potato blight?
Potato blight or late blight disease is caused by a fungus-like organism Phytophthora Infestans. It spreads rapidly in warm humid weather in the foliage of potatoes and outdoor tomatoes causing collapse and decay of the foliage and infection of the potatoes or tomatoes. Late blight disease can also infect potato tubers causing rapid tuber rot making to unfit for storage or consumption.

How does potato blight spread?
Blight spores are produced on infected leaves and are dispersed by the wind to neighbouring plants and gardens. Blight spores can travel long distances and while isolated gardens may be less at risk than more concentrated city gardens it is inevitable you will get blight at some stage.

How does potato blight infect my potato crop?
When a blight spore lands on a wet leaf it sends out zoospores which swim over the surface of the leaf where they settle and grow into the tissues of the potato leaf. If conditions are warm and humid a dark lesion will form after 2-3 days with a white fuzzy mould under the leaf. The white fuzz consists of tiny stalks containing new spores which detach and spread to other plants to start a new infection.

During wet weather light spores are washed off leaves and down stems into the soil where they can infect the developing tubers.

How do I recognise potato blight?
The first sign of infection is likely to be brown freckles on the surface of the leaf which will spread to form larger dark brown patches. The dark brown spots are usually on the margins of the leaves and can be surrounded by a light green halo. These dark spots or ‘lesions’ will quickly spread in humid conditions to cover the whole leaf which will soon die off.

Dark brown lesions may also develop on the stems of the potato plants, this often starts where the leaf joins the stem and spread downwards. The stems will show the same white mould over time with the stem eventually becoming soft and collapse.

Infected potato tubers have a brown discolouration on the skin and a brown marbled appearance to the flesh when cut. The tubers may remain firm but are highly likely to develop soft rot in storage.

Blight seems to be more of a problem recently, is this true?
Unfortunately yes. There are varieties which previously had good resistance to blight which are now coming under attack. This is because of new strains of the disease most notably Blue 13 and Pink 6 which are particularly damaging to potato crops. New varieties are being developed with resistance to these strains like the Sarpo varieties mentioned later.

How do I avoid potato blight?
There are a number of things you can do to reduce the instance of potato blight from the type and variety of potato you grow to good garden hygiene and finally damage limitation when it does hit. We’ll take them in order so here we go:

Growing early varieties
Blight is most common in July and August so growing early varieties which are harvested before blight arrives allow you to avoid the disease. You will need to grow early potatoes for immediate use rather than maincrop potatoes for storage.

Growing blight resistant varieties
Breeders are constantly working on new blight resistant varieties to try to delay the instance of potato blight. There is really no such thing as a blight resistant variety, even the best performers will get blight eventually if conditions are right but they will remain healthy for longer and won’t succumb as quickly as less resistant varieties once they do become infected.

The appearance of stronger strains of blight like the aforementioned Blue 13 makes this process an ever shifting battlefield. The current leaders in the UK are the Sarpo Varieties (Sarpo Mira, Axona, Una and Shona) with Irish varieties like Setanta, Red Cara and Avondale also showing good resistance.

Garden Hygeine
Make sure all potatoes are removed from the soil at the end of the season, this can be difficult as some very small tubers can be difficult to find. Avoid dumping infected potatoes near your garden. Potatoes from the previous season growing on a compost heap are a common source of infection in many gardens so try to keep a potato free heap. Blighted potatoes can be buried in a hole at least 12 inched deep, crush the potatoes completely before burying.

Blighted foliage should not be left around the garden, it can be composted in a good active heap. Foliage on an open compost heap should be buried inside the heap as the spores will remain active until the green material dies. Obviously a closed plastic bin is fine as the material is kept enclosed anyway.

If you don’t have a working compost pile they can be buried in a trench of soil, again at least 12 inches deep..

How do I control potato blight?

Spraying with Bordeaux Mixture
Bordeaux Mixture which consists of a copper formulation has been traditionally used to control potato blight. It works by creating a temporary film on the leaves which prevents the blight from taking hold, it is preventative rather than curative. The plants need to be sprayed repeatedly, especially after rain meaning many treatments in damp climates like ours. The issue here is copper will build up in your soil over time and is harmful to one of our best friends in the garden – the earthworm.

Removing infected foliage
Keep a close eye on your potato patch for any signs of blight and remove any infected leaves as your see them. Obviously the more diligent you are in this respect the longer you can prevent the spread of the disease. Removing infected leaves is particularly successful if the weather turns dry after doing it.

If over 10% of the foliage is affected cut off all growth at soil level, this will stop any growth but should prevent the tubers in the ground from infection. Wait for 2 or 3 weeks before digging the crop which allows a thicker skin to build up and any spores left in the soil will have died preventing infection when your crop is lifted. Harvesting in dry weather is also recommended as it lowers the risk of infection.

Storing Potatoes
Don’t wash potatoes destined for storage but discard any tubers showing signs of blight. Dry off tubers before storing in dry, dark conditions. Inspect your stored potatoes regularly and discard and diseased specimens.

Crop Rotation
This should be standard practice anyway but growing your potatoes in a different part of the garden the following year will help reduce the chance of disease spreading from the previous crop.

The potato blight that killed about a million people in Ireland in the 1840s originated in South America, a new genetic analysis finds.

Until now, the origin of the fungus-like blight that devastated potato crops in Ireland and throughout Europe had not been pinned down. Now researchers at North Carolina State University and the Norwegian University of Science and Technology Museum say the blight was caused by a pathogen with a particular genetic lineage, dubbed FAM-1.

The researchers conducted a new analysis of 183 samples of this pathogen, the oldest dating back as far as 1941. They found that the genetic strain that caused the European blight likely made its way from South America to the United States and then to Europe via potato shipments and the seed trade, they reported Dec. 28 in the journal PLOS ONE.

The Great Famine

Beyond killing hundreds of thousands of people, Ireland’s Great Famine of 1846 to 1851 triggered an exodus of emigrants from the island to North America. The potato blight that caused all of this death and destruction had actually been detected on U.S. shores in 1843, two years before it showed up in Europe, wrote study leader Jean Ristaino, a plant pathologist at NC State. (Ireland was particularly hard-hit by the widespread blight because the potato was a staple in that country and because of pre-existing poverty exacerbated by British policy, scientists noted.)

The origin of the blight had been a source of scientific and historical debate, however. Some studies suggested that the strain of the blight-causing pathogen, Phytophthora infestans, arose in Mexico. Others pointed to the Andes. Ristaino and her colleagues cast a wider net than any previous research, sequencing genomes from both modern P. infestans samples and historical samples from Mexico, Central America, South America, Europe and the United States. The U.S. samples spanned the time period between 1855 and 1958 and included the oldest known sample of the pathogen still in existence today from North America. Likewise, the European samples dated from between 1846 and 1970 and included the oldest surviving European sample. The oldest samples from South America dated to 1913, and the oldest from Central America were from 1941. The oldest Mexican samples were from 1948.

The researchers sequenced portions of the nuclear genomes and one segment of the mitochondrial genome — the separate genetic sequence that resides in the cell’s mitochondria, a structure that converts energy into a useable form. The researchers also sequenced 12 microsatellites, or simple sequence repeat (SSR) segments, of DNA. These repeating chunks of DNA have a high mutation rate, which allows researchers to detect mutations and changes over time.

Global spread

The researchers found that both New World and Old World blight outbreaks were caused by pathogens with an SSR lineage that they dubbed FAM-1. (Previous research by another group had suggested the culprit was a different genetic variant called HERB-1, but that variant was not exclusive to the P. infestans pathogen, Ristaino wrote.)

After analyzing the patterns of mutations in these samples, the researchers used computer models to determine the probability of scenarios that could have led to those patterns. They determined that the most likely scenario was that the pathogen strain originated from a South American ancestor and then split into U.S. and Mexican strains.

The FAM-1 variant hung around long after it caused Ireland’s famine. The same sequence was found in samples in the United States 100 years after 1843, when it first showed up around ports in New York City and Philadelphia, the researchers reported. It was also present in samples from 1913 in Colombia and in samples from 1942 in Costa Rica.

“FAM-1 was widespread and dominant in the United States in the mid-to-late 19th century and the early 20th century,” Ristaino said in a statement. It was later displaced by a genetic strain of the pathogen called US-1, she said, which in turn has been replaced by a strain of Mexican origin that is still active.

Original article on Live Science.

How to stop potato blight

Keep your potato stock healthy and blight-free this season.

To prevent blight, plant your potatoes in a breezy spot with plenty of space between plants, and treat with fungicide before blight appears. It’s also important to rotate crops regularly to prevent build up of the disease in the soil, and to remove and destroy infected plants and tubers as soon as blight develops.

What is potato blight?

Late blight can destroy potato crops in a fortnight.

The plant disease that led to the Irish potato famine, late blight, is a widespread disease of the Solanaceae family. Often called potato blight or tomato blight because it particularly affects these crops, it can destroy your entire haul of potatoes in as little as ten days.

Blight is a fungal disease caused by spores of Phytophthora infestans which are spread on the wind and which can also contaminate potato tubers in the soil. Here’s what you need to know about blight and what you can do to stop it.

When does blight strike?

Blight can strike as early as June in the UK.
Image: Wikimedia Commons

Warm, humid weather, especially during the late summer is a perfect breeding ground for blight. In southern counties of the UK, potato blight can strike as early as June, and though it’s most damaging to outdoor crops, it can affect greenhouse produce too.

What does potato blight look like?

Blight turns the leaves brown and fungal spores develop.

Dark brown blotches appear around leaf tips and edges, spreading towards the middle, shrivelling and rotting the leaf. At the same time, white fungal spores develop on the undersides of the leaves, around the lesions, and further brown lesions develop on the stems. The leaves and stems rapidly blacken and rot, and the plant collapses.

Spores are released on the wind and quickly spread to infect neighbouring plants. They’re also washed into the soil where they can infect potato tubers causing a red-brown rot directly beneath the skin which slowly spreads towards the centre of the tuber.

How does late blight spread?

Late blight spreads really easily.

Heavy rain washes the fungal spores of late blight into the soil, where it overwinters. The disease also persists in infected potato tubers left in the ground or on the compost heap. Sometimes these tubers grow the following year to produce infected shoots which release fungal spores onto the wind to infect new crops.

Blight resistant potatoes

Late blight spreads really easily.
Featured: ‘Valor’ Potatoes from Thompson Morgan

The best way to reduce the possibility of blight ruining your potato crop is to choose blight resistant potato varieties like the Hungarian Sarpo range, and others renowned for their superb resistance to late blight and viruses:

  • • Potato ‘Sarpo Mira’ (maincrop)
  • • Potato ‘Orla’ (first early)
  • • Potato ‘Carolus’ (early maincrop)
  • • Potato ‘Valor’ (late maincrop)

How stop potato blight – top tips

Careful harvesting and crop rotation reduces your chances of getting potato blight.

There’s little you can do to save an infected crop, so stopping blight is all about taking precautions to reduce the chances of the disease attacking your crop:

  • • Plant healthy, disease-free seed potatoes from a reputable supplier. Early crops, harvested before the worst of the blight season, have less chance of being exposed.
  • • Always choose an open planting site with good airflow and leave sufficient space between plants. Better airflow helps the foliage to dry quickly after rain, slowing the spread of blight between plants.
  • • In dry weather, water plants in the morning so that any moisture on the leaves can evaporate during the day. Water at the base of the plant only.
  • • Crop rotation helps prevent a build up of disease spores in the ground, and avoids infected plants growing from potato tubers that were missed during last year’s harvest.
  • • When they’re ready for harvest, make sure you dig up every last potato so blight has nowhere to hide during the winter.
  • • Spray potato crops with a protective fungicide before signs of blight appear. Start from June, especially if the weather’s wet. Spray again after a few weeks to protect new growth.

What to do if blight strikes

Quickly remove and destroy any plants that show signs of blight.

Should your potatoes show signs of blight, remove and destroy all affected plants. If your potatoes have already developed tubers, you might be able to save them by cutting away the foliage and stems. Leave the soil undisturbed for 2–3 weeks to kill off any lingering spores so that they don’t infect the crop when it is lifted.

Given that old potato tubers can harbour blight spores over winter, it’s important to destroy any unwanted or diseased tubers. Don’t put them on the compost heap. Also remove any plants that spring up the following season from old tubers that were left in the ground over winter.


The spores of the Potato Blight fungus are spread in the air and they can travel several miles. A less likely source of infection is through water. Potato Blight thrives in warm and damp conditions. When these conditions occur together for a couple of days or more then infection by potato blight is a distinct possibility. There are two standards which are used to determine if the conditions are correct for a blight infection, the Beaumont Period and the Smith Period. Both have relatively similar criteria, essentially warm and damp weather over a period of two days or more.

Those standards might be useful for professionals but a more practical predictor can come in the form of an email! Simply sign up to the free Potato Council blight predictor service, , and they will email you when those conditions are occurring in your area.

Top view of a leaf with potato blight
Click picture to enlarge


The first sign of potato blight will be small, dark brown marks on the leaves. These will quickly increase in size and number. In many cases the stems of the plants will also have some brown marks on them. If you turn an infected leaf over you will see a light white layer of mould, these are the spores and these will be spread by wind.

Bottom view of a leaf with potato blight
Click to enlarge picture

If the potato tubers below ground are affected as well they will have dark marks on them. Take a potato from the ground and cut it in half. If it is infected with potato blight the flesh will be stained brown and it will have started to rot.


First is cleanliness and hygiene, this will prevent the potato blight starting from your garden or allotment. Remove all traces of the plant and potatoes at the end of each season. If anything looks even remotely infected then burn it.

Store only potatoes in perfect condition and regularly inspect stored potatoes for signs of deterioration. Practice good crop rotation and buy seed potatoes only from certified suppliers as explained earlier.

Choose potato varieties which are naturally resistant to potato blight. See our page on pest and disease resistance potato varieties, . The top blight resistant potatoes are Setanta, Sarpo Axona and Sarpo Mira. Although they will resist most attacks of blight the not so good news is that they are not the most versatile potatoes. A mention must be made of Orla which also has good tuber blight resistance, not quite as high as the others but it can be grown as a first early.

If you grow potatoes in a greenhouse then they are much less likely to suffer from potato blight because the spores can’t reach them so easily. The problem with this though is that you will need to replace the soil every year to avoid the build up of other pests and diseases. One alternative is to grow potatoes in containers in the greenhouse (or temporarily move them there when blight is likely. See our page on growing potatoes in containers. You will be surprised at how many potatoes can be harvested from a couple of container fulls of potatoes.

If you use the Potato Council Blight Predictor service, , then you can take action before blight has significantly affected your potato crop. The recommended course of action is to spray your potato crop with either a Dithane 945 replacement (Dithane 945 is no longer available) or with Bordeaux Mixture. But before you do use these chemicals, remember that they are only partially effective and definitely only to be used before the Potato Blight infection is established.

The copper fungicide Bordeaux Mixture has previously been recommended as a spray to prevent canker in fruit trees. However it has now been withdrawn from sale in the UK. Currently there are no alternatives which have been scientifically proven to be anywhere near as effective as Bordeaux Mixture

Various alternatives are suggested, for example aspirin solution or milk, but none have been proven to have any effect.


There is no cure for potato blight when your plants are infected. The first action to take is to cut off all growth above soil level and burn it as soon as possible. This will minimise the infection on your soil and also reduce the risk of you passing potato blight on to neighbours and that includes neighbouring farms.

If the above ground growth has been removed early enough it is likely that the potatoes below ground will not have been affected by the potato blight, especially if you have previously earthed up your potato plants. Test the potato tubers by removing a couple from the ground.

If they are infected (see Potato Blight symptoms above) then your crop is beyond help and all potatoes should be dug up and burnt. Be thorough in doing this so that all trace of infected potato growth above and below the ground is removed.

If however your potatoes below ground have not been infected then they can be harvested as normal. Without the top growth they will no longer continue to grow but they will be fine to eat.

For those who are adventurous and willing to try something new, the link here is from the Independent and it claims that a couple of aspirin in 4.5 litres of water gives some protection against blight as far as potatoes and tomatoes are concerned. It may well be worth a try, after all, aspirin are very cheap.

Need help with what to do in your garden?

Q What is potato blight?

A Potato blight (Phytophthora infestans) or ‘late blight’ is a devastating fungus disease that spreads rapidly in wet weather. Like a mildew disease, to which it is related, it attacks the foliage, but can get washed down to damage the tubers. Blight also attacks tomatoes, and has been recorded on aubergines. Related plants, such as peppers, seem immune.

Caption: Cut off the foliage at ground level if blight strikes and wait two weeks before harvest

Q How do I recognise potato blight?

A The first signs are small, dark spots, often on the edges of leaves. If the weather is wet, a white mould surrounds these spots, usually on the underside of the leaf. This shows that spores are being produced. The spots grow, covering the whole leaf. They also spread onto stems.

Infected tubers have brown and purple skin blotches that go into the flesh of the tuber. The flesh is brown/red and granular. Infected tubers may shrivel and dry, but are often infected by rotting organisms and liquefy in storage, smelling strongly and contaminating other tubers.

Caption: Potatoes infected by blight often rot and liquefy in storage

Q Could I mistake potato blight for anything else?

A Early blight also blackens leaves, but with concentric rings of darkened leaf. It usually attacks older leaves and does little damage. Viral diseases can also speckle leaves with black spots and stunt growth.

Tubers can be infected with scab, spots, rots and gangrene. The spots and rots can be difficult to tell apart from blight as the end result, a liquefied potato, is the same. Fortunately, the same counter-measures apply.

Q When should I expect a potato-blight attack?

A Following recent research by the James Hutton Institute, blight alerts for potato and tomato growers are now being issued under the Hutton Criteria, which has replaced the Smith Period.
The minimum temperature needed by the disease, 10°C over two consecutive days, is unchanged, but it’s now known that relative humidity only needs to be above 90% for six hours on both consecutive days for the
disease to spread, rather than 11 hours, as previously thought. In these conditions, the disease spreads rapidly. In hot, dry weather it temporarily ‘dries up’, but breaks out again if conditions become favourable. This typically happens during early summer in the west and late summer in the east.

Q How can I avoid potato blight?

A Dig out as many tubers as you can when you gather the potatoes. At the end of the season, remove all potato tubers, even tiny ones, and destroy them. Digging over the plot so frost can kill any remaining tubers will help. Throw away, burn or bury deeply any leftover or rotting tubers from storage, so they can’t grow the following year. Protect new crops by earthing up well. A layer of soil will protect tubers from spores falling from infected foliage.

Q Can I compost plants that are affected by potato blight?

A Potato blight persists on living material so you can safely compost the stems and leaves as they are dead. However you should not compost the tubers as they are alive and will carry the blight spores.

Q Can I reuse compost that potatoes were grow in?

A Yes, you can but you must be careful to remove all of the tubers, including tiny ones. Also carefully check for the c-shaped grubs of vine weevils. As with any compost that you reuse, add controlled-release fertiliser before you replant to replace the depleted nutrients.

Q Can potato blight be sprayed?

A No, there are no longer any chemical controls available for potato blight.

Q Can I rescue plants infected by potato blight?

A Cutting off infected foliage can prevent spores from reaching the tubers. Leave at least two weeks between removing the foliage and lifting the tubers, so viable spores lurking on the soil surface don’t contaminate the tubers as you lift them. Store the tubers in dry, cool conditions to reduce the disease’s activity and subsequent rotting. Check potatoes every month; get rid of rotting ones.

Q Can potato blight be prevented?

A Using certified disease-free seed potatoes and good garden hygiene will help to prevent blight, but the spores of the disease are carried on the breeze, so few gardens will escape. You could try covering the crop with an open-ended polythene tent to keep the foliage dry. Avoid high-nitrogen fertiliser, as this stimulates growth of soft, blight-susceptible foliage.

To avoid the worst of blight outbreaks you could also grow early or second-early varieties and lift before August.

Q Are there any varieties that are resistant to potato blight available?

A ‘Sarpo Axona’ and ‘Sarpo Mira’ were both extremely resistant in our trial.

Caption: ‘Sarpo Mira’ potatoes are resistant to potato blight

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