How to prevent Apple scab?

A serious disease of apples and ornamental crabapples, apple scab (Venturia inaequalis) attacks both leaves and fruit. The fungal disease forms pale yellow or olive-green spots on the upper surface of leaves. Dark, velvety spots may appear on the lower surface. Severely infected leaves become twisted and puckered and may drop early in the summer.

Symptoms on fruit are similar to those found on leaves. Scabby spots are sunken and tan and may have velvety spores in the center. As these spots mature, they become larger and turn brown and corky. Infected fruit becomes distorted and may crack allowing entry of secondary organisms. Severely affected fruit may drop, especially when young.

Apple scab overwinters primarily in fallen leaves and in the soil. Disease development is favored by wet, cool weather that generally occurs in spring and early summer. Fungal spores are carried by wind, rain or splashing water from the ground to flowers, leaves or fruit. During damp or rainy periods, newly opening apple leaves are extremely susceptible to infection. The longer the leaves remain wet, the more severe the infection will be. Apple scab spreads rapidly between 55-75 degrees F.

Treatment

  1. Choose resistant varieties when possible.
  2. Rake under trees and destroy infected leaves to reduce the number of fungal spores available to start the disease cycle over again next spring.
  3. Water in the evening or early morning hours (avoid overhead irrigation) to give the leaves time to dry out before infection can occur.
  4. Spread a 3- to 6-inch layer of compost under trees, keeping it away from the trunk, to cover soil and prevent splash dispersal of the fungal spores.
  5. For best control, spray liquid copper soap early, two weeks before symptoms normally appear. Alternatively, begin applications when disease first appears, and repeat at 7 to 10 day intervals up to blossom drop.
  6. Bonide® Sulfur Plant Fungicide, a finely ground wettable powder, is used in pre-blossom applications and must go on before rainy or spore discharge periods. Apply from pre-pink through cover (2 Tbsp/ gallon of water), or use in cover sprays up to the day of harvest.
  7. Organocide® Plant Doctor is an earth-friendly systemic fungicide that works its way through the entire plant to combat a large number of diseases on ornamentals, turf, fruit and more. Apply as a soil drench or foliar spray (3-4 tsp/ gallon of water) to prevent and attack fungal problems.
  8. Containing sulfur and pyrethrins, Bonide® Orchard Spray is a safe, one-hit concentrate for insect attacks and fungal problems. For best results, apply as a protective spray (2.5 oz/ gallon) early in the season. If disease, insects or wet weather are present, mix 5 oz in one gallon of water. Thoroughly spray all parts of the plant, especially new shoots.

How to Get Rid of Apple Scab

apple on tree image by Marek Kosmal from Fotolia.com

Apple scab is a fungal disease that affects both edible and ornamental apple tree varieties. The fungus typically develops in late summer and causes the fruit and leaves to develop green spots that eventually turn black and cause fruit loss. The spots also have a distinctive fuzzy appearance. Once apple scab develops on a tree, it is impossible to get rid of it in the same growing season. Treatment measures typically take one full growing season to completely get rid of the apple scab fungus.

Rake up all of the leaves underneath the apple tree to prevent the apple scab fungus from spreading. Although the majority of raking will occur in the fall, you should rake leaves up regularly throughout the year. In addition, collect any diseased fruit that falls from the tree as well.

Place all of the collected leaves and fruit into a trash bag and dispose of it in the trash. Do not place it in a compost area since the fungus will grow.

Wait until the early spring of the next growing season,when trees are just emerging from dormancy. Place a fungicide containing lime sulfur, or sulfur into a garden sprayer and mix it with water as directed by the packaging instructions.

Watch the tree until you notice pink buds appear. Spray the limbs and trunk of the tree with the fungicide until they are saturated.

Wait until the buds open into flower blossoms, which typically occurs within two to three weeks of bud development. Reapply the fungicide as soon as the flowers open.

Reapply the fungicide two more times during the early growing season, once when the petals fall off of the apple tree and again two weeks after that.

IDENTIFY AND TREAT APPLE TREE SCAB

Article by David Marks

Scab is a fungal disease which is spread mainly by spores in the air but also in water droplets. It affects apple and pear trees, hawthorn and mountain ash. The technical name for this fungal infection is Venturia inaequalis. Note the scab also affects pyracantha but it is a slightly different strain and apple trees with scab do not infect pyracantha or the other way around.

This disease is a major problem for commercial growers of tree fruit as well as the amateur gardener. One simple solution which reduces the problem considerably is to choose resistant varieties and avoid those varieties which are susceptible to the disease.

The second key preventative measure is keep the nearby soil and /or grass clear of all fallen leaves and debris from the tree.

IDENTIFYING SCAB

The first signs of scab appear on the leaves of affected plants in spring. Small yellow patches appear on the leaves. After a few weeks the leaves will develop larger brown / dark olive-green marks on them and the leaves will begin to turn yellow and start to prematurely fall off in July and and August.

A leaf infected with Scab

The fruits have greyish, slightly raised or sunken areas on the surface. This affected area will not grow with rest of the apple causing it to split eventually, providing an ideal site for other infections to enter.

An apple fruit affected by scab

LIFECYCLE OF SCAB

SUMMER
The initial infection normally occurs in summer but the effects are not particularly noticeable in the first year.

AUTUMN / WINTER
In first autumn and winter the infected leaves fall to the ground and they overwinter.

SPRING
In Spring, normally around the time that buds appear on the plants, spores are released form the fallen leaves and they spread to new leaves both on the wind and via water splashing. Secondary spores are release in late spring which causes further infection. The higher the humidity and temperature the quicker the spores are released and the more damage they can cause.

AUTUMN / WINTER
The damage becomes apparent in mid summer to winter on leaves and fruit. The cycle repeats itself with more vigour if not treated.

DAMAGE CAUSED BY SCAB

Scab will cause premature leaf drop which not only affects the looks of affected plants but also weakens them and affects their ability to produce fruit. In the short and medium term it is unlikely to kill the plant / tree but it can do so if the disease reappears year after year.

The primary damage is caused to leaves, reducing their ability to convert sunlight to energy, opening up the plant to other infections as well. Severe cases may result in almost all the leaves dropping off causing the plant to die.

The fruits are also affected although scab, of itself, does not make the fruit inedible. Sometimes the scarred area on the fruits split opening it up to other infections which can totally ruin the fruit.

Another apple fruit affected by scab

Sometimes the damage caused to apple fruit by bitter pit is mistaken for scab. However bitter pit scars remain very small whereas the scars from scab increase in size as the fruit grows. If you cut into a fruit affected by bitter pit the damage can often go to the centre of the apple.

PREVENTING AND TREATING APPLE SCAB

The single most effective preventative measure for scab is to plant resistant varieties. This is especially important with apples, crab apples and pears. If you have been affected by scab before then research the varieties you buy in the future to include only those which show good resistance. The list below is for common apple varieties we know which have good resistance to scab:

COMMON SCAB RESISTANT APPLE TREES UK

Adam’s Pearmain

Blenheim Orange

Charles Ross

Discovery

Egremont Russet

Epicure

Lord Derby

Lord Lambourne

Rajka

Red Windsor

Reverend W Wilks

Scrumptious

Winston

The second best way to lessen the effects of scab is to clean up and burn all affected leaves and debris around the the tree / plant. If you religiously pick up all fallen leaves and fruit throughout the year, scab can be significantly reduced – quite possible if you only have a couple of trees but virtually impossible if you have lots of them.

A complication occurs with clearing up leaves however. The spores of scab can travel several hundred metres on the air so you can keep a scrupulously clean garden but still have your trees infected by neighbouring trees.

Couple that with the fact that scab affects pyracantha and hawthorn, both very common in urban, suburban and even country side areas, and you may be faced with a stark choice. Replant with one of the resistant varieties or accept that fruit production will be significantly reduced and those fruits which are produced may well be scarred.

There are no longer any chemical sprays available to the UK gardener for treating scab on apple trees where the fruit will be eaten. Copper based fungicides are also no longer sold in the UK.

Apple Scab: Symptoms and How to Treat

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Scab On Apple Trees: Indentifying And Treating Apple Scab Fungus

Apple trees are an easy-care addition to any home garden. Beyond providing fruit, apples produce beautiful blooms and larger varieties make excellent shade trees if allowed to reach full height. Unfortunately, scab on apple trees is a common and serious problem. Apple tree owners everywhere should read on to learn about controlling apple scab in their trees.

What Does Apple Scab Look Like?

Apple scab fungus infects developing apples early in the season but may not become visible on fruits until they’ve begun to expand. Instead, apple scab first appears on the undersides of the leaves of the blossom clusters. These fuzzy, roughly circular, brown to dark olive green lesions may cause leaves to distort or crinkle. Scabs can be small and few, or so numerous that leaf tissues are completely covered in a velvety mat.

Fruits may be infected at any time from bud set to harvest. Lesions on young fruit initially look much like those on leaves, but soon turn dark brown to black before killing surface tissues, causing a corky or scabby texture. Scabs on infected apples continue to develop even in storage.

Apple Scab Treatment

Apple scab is difficult to control if your tree is already infested, but you can protect future harvests armed with a little apple scab information. Apple scab remains dormant in fallen leaves and on fruit left attached on the tree and lying ground. Sanitation is often enough to control a mild infection; just make sure to burn or double bag all the material to prevent the disease from spreading.

When sprays are necessary, they should be applied between bud break and a month after petal fall. In rainy weather, applications every 10 to 14 days may be necessary to prevent apple scab from taking hold. Use copper soaps or neem oil when apple scab is a risk in the home orchard and keep fallen debris cleaned up at all times. If you can prevent apple scab early in the year, it’s unlikely to cause you problems as fruits develop.

In areas where apple scab is a perennial problem, you may want to consider replacing your tree with a scab-resistant variety. Apples with excellent scab resistance include:

  • Easy-Gro
  • Enterprise
  • Florina
  • Freedom
  • Goldrush
  • Jon Grimes
  • Jonafree
  • Liberty
  • Mac-free
  • Prima
  • Priscilla
  • Pristine
  • Redfree
  • Sir Prize
  • Spigold
  • Williams Pride

Garden diseases – Apple scab

Not only is apple scab unsightly, it also damages the tree itself
Image: Evg Zhul

Apple scab is a common condition which affects apple and pear trees. It attacks both leaves and fruit, and though it doesn’t immediately threaten the tree, an uncontrolled infection weakens it, making it more susceptible to other diseases.

What is apple scab?

Apple scab is a windborn infection that turns leaves blotchy
Image: Tatahnka

Apple scab is the common name for Venturia Inaequalis a primarily windborn fungus that can also spread via contaminated water. It overwinters on fallen leaves and fruit, and because it also affects wild-growing hawthorne, crab apple, and mountain ash trees as-well-as shrubs like pyracantha and cotoneaster, it’s hard to protect your trees against infection from other plants in the local area.

How to recognise apple scab

You’ll recognise an apple scab infection by yellow blotches which form on leaves during the spring. During the summer, the marks darken through olive green to dark brown. By July or August, trees typically shed affected leaves, reducing their overall ability to photosynthesise, lowering their resistance to disease.

Fruits grow greyish raised or pitted scabs which, because they don’t grow at the same rate as the rest of the fruit, cause your apples or pears to become misshapen as they grow, and can cause cracking and splitting.

Venturia is a major problem for commercial growers who, if the spring is particularly cool and damp, sometimes lose up to 70% of their crop. As a home-grower, as long as you’re happy to cut the affected sections from your fruit, apple scab won’t spoil your apples’ texture and flavour.

How do you prevent apple scab?

Plant scab resistant varieties like ‘Bardsey Island’ which is scab and canker resistant
Image: Apple ‘Bardsey Island’ by Thompson & Morgan

Should apple scab strike, make sure you remove and dispose of fallen leaves and fruit. This helps reduce the number of spores in the vicinity, lessening the severity of the infection and minimising the chance of reinfection the following year.

Have a chat with gardeners in neighbouring properties – if you all act promptly to deal with the problem, you maximise your chances of success. Unfortunately, if surrounding gardens and hedgerows contain infected plants, your own trees are much more likely to be reinfected.

It’s no longer possible to buy fungicides for trees whose fruit is intended for human consumption. You may wish to treat the tree, at your own risk, with a fungicide intended for ornamental trees, however you’ll need to treat the entire tree, and even then, reinfection from affected trees in the area is a strong possibility.

The best way to prevent scab is to choose resistant varieties like Bardsey Island or Red Falstaff. Otherwise, accept that if your trees become infected, you’ll just have to chop the bad bits from misshapen apples and pears, and enjoy the otherwise delicious fresh fruit regardless.

CORVALLIS – Wet weather, which is bound to come in the weeks ahead, builds potential for apple scab, a fungal disease of apples.

The apple scab disease fungus overwinters on dead apple leaves and fruit left on the ground, explained Ross Penhallegon, horticulturist for the Oregon State University Extension Service. During spring moisture, scab spores are forcibly discharged and ride air currents to infect developing leaves and fruit of apples. All outer parts of unopened fruit buds are highly susceptible to scab. As the fruit matures it is much less susceptible.

The first visible symptoms of apple scab in the spring are pale, water-soaked spots the size of a pinhead on the new leaves. These spots enlarge, become darker and smoky colored. Later, the spots turn brownish-black color. Spots may be any shape, but tend to be circular, Penhallegon said. Diseased leaves may be curled, distorted and drop off early. Heavy infections can defoliate and weaken your apple trees.

On the fruit, the symptoms of scab include small raised brown or black circular areas (scabs). The skin breaks later in the season and the exposed tissue turns velvety brown or black. As the fruit enlarges, the scab spots become brown and corky. To help control apple scab, Penhallegon recommends:

  • Grow scab-resistant cultivars of apples. Apples with good resistance include Akane, Chehalis, Liberty, Prima and Tydeman Red.
  • Apply nitrogen to leaves that have fallen to the ground in the fall to enhance decomposition of fallen leaves and make them more palatable to earthworms. Hand-apply a liquid fish solution or 16-16-16 fertilizer to help with the decomposition.
  • Shred fallen leaves in the fall with a mower to help speed up decomposition.
  • Prune your apple trees to open up branching and allow more air circulation.
  • When watering your apple trees, avoid getting foliage wet.
  • Apply dolomitic lime in the fall, after leaf drop, to increase pH and to help reduce fungal spores in the spring.
  • Spray fungicide – Bonide Captan, wettable sulfur, summer lime sulfur or Spectracide Immunox – when temperatures are above 60 degrees and the leaves or blooms are wet.

For more information, consult the Extension publication on Apple Scab.

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URBANA, Ill. – Apple scab, a fungal disease affecting apple orchards in Illinois and worldwide, can significantly reduce fruit quality and yield. In fact, the disease recently damaged more than 50 percent of some apple varieties in Illinois orchards. When samples from those orchards were tested, some strains of the fungus were found to be resistant to traditional fungicides.

“I rushed to do something to prevent this disaster. We did an experiment in 2014 and 2015 and were lucky to get very good results,” reports University of Illinois plant pathologist Mohammad Babadoost.

Babadoost and his team tested a new protocol using combinations of systemic and contact fungicides. Dithane M-45 (mancozeb), a contact fungicide, should be applied at the green-tip stage at 3 to 4 pounds per acre, along with the systemic fungicide Inspire Super (difenoconazole + cyprodinil) at 12 fluid ounces per acre. After seven days, the treatment should be followed up with a combination of Dithane M-45 and Fontelis (penthiopyrad) at 20 fluid ounces per acre. Each treatment should be repeated three times, seven days apart, for the most effective control.

“When we tested this combination of chemicals, we could not find even a single scabby apple,” Babadoost says. “Growers that trialed the treatment in 2015 reported no scab.”

Despite the success of the treatment, Babadoost notes that it should not be seen as a silver bullet. “We are in a battle with the pathogen almost all the time,” he says.

Apple scab causes lesions on leaf and fruit tissue that thicken and take on a scabby appearance. In later stages of the infection, the skin of the fruit can crack, allowing in secondary pathogens that can lead to fruit rot or other symptoms. All growing portions of the tree are susceptible to the fungus.

Babadoost warns, “Any green tissue is subject to being attacked. It starts very early in the season. If growers are able to control it effectively as soon as growth starts in the spring, there will be almost no disease by summer. But if they miss the window in spring, summer will be a disaster.”

In addition to the new fungicide treatment protocol, other control options are available to growers. For example, growers can choose apple varieties that are resistant to apple scab; including ‘Honeycrisp’, ‘Jonafree’, and ‘Gold Rush’; avoiding susceptible varieties, such as ‘Fuji’, ‘Gala’, ‘Honeygold’, ‘Winesap’, and others. A more extensive list of resistant and susceptible apple varieties is provided in Babadoost’s recent U of I Extension Fact Sheet.

Again, Babadoost issues a warning: “Even if an apple variety is resistant, it might not be resistant forever. Resistance might break down.”

Small growers, organic growers, and home gardeners can prevent infection by removing or applying a five percent solution of urea to all dead leaves on the ground, as the fungus overwinters in leaf litter. Removing nearby crab apple trees will also be beneficial. Organic growers can apply organic sulphur- or copper-based fungicides, but Babadoost is not confident that organic fungicides will provide good control of resistant strains of apple scab.

“Production of organic apples in Illinois is not an easy task,” he says.

Growers should monitor and treat trees early and often to prevent widespread infection. With the new treatment protocol in place, the 2016 growing season holds a great deal of promise for apples in Illinois. For more information, read the U of I Extension apple scab Fact Sheet.

Often called apple scab, this fungus is responsible for important damage to this fruit tree, but also to plum trees and pear trees.

As it forms brown spots on fruits and leaves of trees, scab can lead to their growth clearly slowing down.

You’ll also observe that fruits are malformed, and even start to rot.

Follow the steps listed here, and your harvest will be protected and effectively guarded against scab.

Conditions that lead to scab appearing

Like all fungal diseases, moisture is the most relevant appearance factor. When in conjunction with heat, the risk of seeing scab colonize your fruit trees is even higher.

It is often at the end of spring that this fungus appears, and it will develop during the entire fruit formation phase.

But sometimes scab appears only in fall when moisture is very present, especially since nights grow longer.

  • Wounded trees are much more vulnerable to fungus attacks.

Effective treatments against scab

Preventive care is the best solution

  • In fall, it is critical to pick up and burn all the leaves.
    Also mow around the tree once leaves have fallen to speed their breakdown.
    Pick fallen fruits up regularly, so that they don’t stay on the ground.
  • Provide compost at the foot of the trees in fall.
  • Spray fungicide based on a solution of Bordeaux mixture in fall and again at the beginning of spring.
    In fall, once leaves have fallen
    In spring, renew the treatment 2 to 3 times depending on rainfall.
    Indeed, treatment must be reapplied after a rainy span of time.

There isn’t really any curative treatment

  • Removing infested leaves, and burning them while renewing the bordeaux mixture-based fungicide treatment is about the only thing to do.

Note that the fungus overwinters on leaves that have fallen from the tree when diseased.

In spring, the ascospores, which are how this fungus reproduces, are ejected in the air during rainy days and reach the trees born by the wind.

That is why it is critical in fall to pick up and burn all the leaves of your apple trees, pear trees and plum trees.

Read also on the following common plant diseases

Treat against: downy mildew Treat against: whiteflies Treat against: black vine weevil
Treat against: rust Treat against: powdery mildew Treat against: aphids (organic)
Treat against: caterpillars (organic) Treat against: fruit flies Treat against: cherry fruit fly

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