- Eco-friendly Ways to Control Black Rot on a Grape Plant
- Eradicate black rot in grape arbor
- What Is Black Rot On Grapes: Learn About Black Rot Grape Treatment
- What is Black Rot on Grapes?
- How to Save Grapes with Black Rot
- Grapes: proper pruning and thinning can prevent black rot
- Managing Black Rot
- GRAPES 101
- Cooperative Extension: Insect Pests, Ticks and Plant Diseases
- Black Rot of Grape
Eco-friendly Ways to Control Black Rot on a Grape Plant
Sometimes we get really good questions via Facebook.
Jessica has a problem with black rot on a grape plant, and is looking for a home remedy as she would like to avoid chemicals if possible.
We reached out to Emily Tepe from the University of Minnesota Department of Horticultural Science, and author of The Edible Landscape, to help Jessica solve the problem.
The best thing you can do to control black rot on grapes doesn’t involve chemicals at all.
The first, and most effective practice, is good sanitation. As soon as you see any sign of black rot on any part of the plant, get it out of there. Clean up any berries, leaves, and pruning’s from under the vine and keep it spotless and weed-free all year. The fungus overwinters in this kind of debris and will wreak havoc as soon as spring rains come.
This leads me to tactic number two: good air circulation. Black rot spores love moisture, so you want to reduce the amount of moisture held in the canopy by providing great air circulation. During dormancy, prune heavily keeping only a few healthy, strong canes from last year’s growth. Prune out any diseased parts of the vine too. Keep the vine tied up to a trellis to allow air to flow through.
Another good practice is cultivating the soil under the vine shortly before bud-break in the spring. This can bury spores, thus preventing them from reaching the plant. This has proven quite successful for many growers.
As far as home remedies go, I’ve never tried any on grapes. You could certainly try them, but here’s a thought: Most home remedies involve a water-based solution being sprayed on the plant. This doesn’t sound too good to me, considering how much this fungus likes moisture.
Chemical sprays are effective, but have to be applied numerous times throughout the season at very specific intervals. Every inch of the plant has to be covered to be effective. That’s a lot of work and a lot of exposure to some nasty stuff. I suggest diligently following a good sanitation, pruning, and air circulation routine. That usually does the trick.
Good luck Jessica, and anyone else who has this problem!
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Eradicate black rot in grape arbor
Question: We have had our grape arbor for over 50 years and always had a good crop of Concord grapes to make jelly. The last three years we have not had any grapes due to frost or some kind of blight. The grape bunches would form and have green grapes, but they would all turn black. If this is some kind of blight, what can I do to get rid of it and have a good crop of grapes again?
Answer: It sounds like your grapevines have fallen victim to black rot. This fungal disease is caused by an organism known as Guignardia bidwellii. It’s a common grape pathogen, especially when we have a wet spring. It leads to half-grown grapes that rot and fall off the vine before ripening.
Initially, black rot appears as tiny, yellow spots on the foliage. A few weeks after the infection starts, the spots turn dark and disperse thousands of spores, causing lesions on the stem tips and tendrils. Soon enough the spores move to the developing fruit and cause the rot you describe.
Unfortunately, this disease is a tough one to control once it takes hold, but if you follow a few steps, you can have your concord grape harvest back.
1. Every fall, clean up the vines. This is the most important step in disease control. Pluck off and destroy all grape “mummies” by burning them or tossing them into the garbage. Then, remove diseased leaves by raking them up off the ground underneath the plants and throw them into the trash. “Mummies” that fall to the ground or stay attached to the vine will automatically reintroduce spores to the plants the following season. They must be removed.
2. In the late winter, when they’re still dormant, spray the vines with lime-sulfur to control any overwintering spores. This step is not a substitute for cleaning up the plants and disposing of all “mummies,” but it does help kill any overwinter spores clinging to the bare vines.
3. Provide good air circulation. To help the developing fruits dry off faster after rains, summer pruning is critical. Head out to the grape arbor in late June or early July and cut off all grape leaves, shoots and tendrils that hang over any clusters of developing grapes. Use a pair of clean, sharp pruners to expose the fruit clusters to sunlight and air.
4. Apply a bio-fungicide, such as Serenade, very early in the season. Vines should be sprayed well before any symptoms occur, especially since you’ve dealt with this fungus in previous years. Spray as soon as the initial vine growth reaches about 2 to 3 inches in length. This allows you to control the pathogen before it becomes established on the leaves and spreads to the fruit. Spray both the upper and lower leaf surfaces as well as the vines themselves.
5. Continue spraying at 10-day intervals until four weeks after the plants finish blooming. This is the most critical time for control. Spraying early, before the plants come into flower, is very important for controlling this disease. Once the grapes form and they reach four to five weeks old, they’re naturally resistant to the fungus that causes black rot and sprays are no longer needed.
With proper cultural practices and the described organic spray program, you’ll have wonderful grape harvests in the coming years. Be diligent and it will pay off.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.
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Black rot is one of the most important grape diseases, and it caused by fungus Guignardia bidwellii. It originated from Northern America, but can now also be found in Europe, South America, and Asia. The fungus appears in warm and humid climates and can cause significant economic damage. It can infect all young green parts of the plant, but makes the most damage when it appears on fruits.
Photo (wikipedia): Symptoms of black rot on grape leaves
Symptoms of the black rot infection appear approximately 2 weeks after the primary infection. On leaves, the infections first appear as small brown circular spots, which can enlarge and develop light brown centers with dark margins.
After a few days, numerous tiny black fungal fruiting structures called pycnidia develop. The disease can develop only on young leaves, older ones are resistant to it. Black rot symptoms can appear also on shoots, petioles and tendrils as brown to black irregular lesions. Similar as on the leaves, also here tiny black fungal fruiting structures (pycnidia) appear within lesions. The fungus infects also berries, usually while they are still green. On berries, infection is first seen as light brown spots that eventually become dark brown and cover the entire berry. Also, here pycnidia’s can be seen. Infected berries shrivel and become black mummies.
Photo (wikipedia): Black mummies
The black rot fungus overwinters in black mummies, on the ground, fallen leaves and/or in the infected shoots. In the spring, ascospores released infected fruits with a rain, which are then dispersed by water and the wind. These ascospores initiate the disease cycle and cause primary leaf and blossom infections. Infections depended on the length of leaf wetness period, and air temperature in that period. The secondary infection is caused by spores’ conidia during the whole growing season if only the environmental conditions are favorable.
Photo (Cornell University): Black rot disease cycle
The most efficient way to control black rot is through a combination of good cultural practice and chemical methods. Good cultural practice includes: sanitation – removal of all mummies and infected leaf from the vines, choosing resistant varieties, selecting a site with good air circulation, removal of weeds and tall grass, and wise pruning of the vines. For successful black rot management program also fungicides can be used, where the proper timing of spraying is essential. It’s crucial to control primary infection, which can prevent secondary infections later in the season. In case of infection, protective spraying has to begin before bloom through four weeks after the bloom. Fungicides should be used only when the risk of black rot infection is high, what result in lower numbers of applications during the season and protection of the environment. For determination of high-risk infection periods, available disease prediction models should be used.
Although black rot disease can cause a lot of damage in the vineyard, it’s not that hard to control it. Especially if winegrowers are familiar with the characteristics of infection conditions, and take good care of the vineyard.
So winegrowers, what are your experiences with black rot fungal disease? Let us know in the comment below!
Integrated pest management; University of Illinois Extension
Black rot of grape; N. A. Ward and C. A. Kaiser, Kentucky Cooperative Extension Service
Black rot; Cornell University
Black Rot; University of Minnesota
Many of us skimp on the spotty fruits and vegetables at the grocery store for aesthetic reasons. The spots and mushy patches on produce makes us wonder how fresh they are, and most importantly, how safe they are to consume. Host Elizabeth Brauer of TED-Ed’s latest video, “Are spotty fruits and vegetables safe to eat?”, suggests the stigma against blemished produce should be erased — they’re not harmful after all.
In 2010, 30 billion dollars worth of fruits and vegetables were wasted by American retailers and shoppers because of its funky-looking texture and perceived spoilage. However, Brauer reassures us pathogens that affect food often don’t have the same ill effects on humans. So, what are these blemishes on our produce anyways?
These spots are evidence of a battle between plants and microbes. Similar to humans, plants coexist with billions of fungi and bacteria. Some of these microbes are beneficial to the plant, suppressing disease and helping it extract nutrients. Others are pathogens that continue to live off of produce even as it sits in a store display or in our refrigerator. Most of these pathogens are never bad for us.
These microbes can reach plants in a number of ways, such as getting splashed onto them during watering or fertilization. Under the right conditions, they grow into large enough colonies to attack the waxy outer layers of fruit or leaves. Their target is the delicious sugars and nutrients inside.
They often form spots, draining the nutrients and color from the fruit’s cells, making that yellow halo seen in tomatoes. They next move outward, leaving a black spot of dead cells; each spot possibly contains hundreds of thousands of microbes. This is actually caused by a combination of the microbial attack and the host’s defense mechanisms.
Meanwhile, when it comes to mushy blemishes, microbes usually attack the fruit after it has detached from the plant. If the plant is wounded during transport, necrotic fungi can infiltrate through the wound, kill the cells and absorb their nutrients, leaving our food looking mushy or brown. These spots can taste pretty bad, since we’re eating dead and decomposing tissue.
Usually, we can salvage the rest of the fruit by cutting off the mushy parts and eating the others.
Now, microbes that can make us sick like E.coli and salmonella can be on veggies too. Unlike plant pathogens, they don’t typically cause spots, and instead just hang out invisibly on the surface.
Therefore, it’s important to look past the appearance of a fruit or vegetable, and rinse thoroughly to avoid contamination.
What Is Black Rot On Grapes: Learn About Black Rot Grape Treatment
Growing grapes in the home garden is a labor of love. All the training and pruning and years and years of waiting for the first batch of grapes can be a lot to bear for any grower. When grape black rot ruins your harvest, you may want to throw in the towel. Fear not! There is black rot grape treatment, and with some effort you can defeat this merciless fungal disease.
What is Black Rot on Grapes?
Black rot of grapes is a fungal disease that persists in grapevines for many years without treatment. The earliest signs of disease appear as yellow circular lesions on young leaves. As these lesions spread, they brown and sprout black fungal fruiting bodies that look similar to grains of pepper. With advancing disease, lesions may girdle the petiole of individual leaves, killing them. Eventually, the fungus spreads to the shoots, causing large black elliptical lesions.
Although leaf symptoms are annoying, the real damage from grape black rot comes from fruit symptoms. In many cases, fruits are about halfway grown before they start to show signs of infection — the same small brown lesions on leaves will begin to appear on grapes. These areas soften, sink and rot in just a few days and what remains of the fruit shrivels up into a tiny, hard raisin-like fruit, mummy covered in fungal fruiting bodies.
How to Save Grapes with Black Rot
Grape black rot is difficult to stop once it has taken hold of growing fruit.Many gardeners would consider this year’s crop a lost cause and work toward preventing a recurrence of disease.
The best time to treat black rot of grapes is between bud break until about four weeks after bloom; treating outside of this window is likely to end in frustration. However, if you want to try, captan and myclobutanil are the fungicides of choice.
Prevention is key when dealing with grape black rot. During your fall clean-up, make sure that all mummies have been removed from the vine and all plant material on the ground below is destroyed. Prune out any and all areas with lesions, grapevines can handle a severe pruning — when in doubt, cut it out. If leaves appear the following spring with new lesions, remove these immediately and start a spray treatment program with one of the above-listed fungicides.
Grapes: proper pruning and thinning can prevent black rot
Q. Every year I get a nice initial production of grapes from my backyard vines, but they eventually develop blackish spots, dry up and die. My father says it’s ‘black rot.’ Is there anything I can do to prevent it?
—Angelo in Springfield (Delaware County) PA
Every year in the late spring I get beautiful little green grapes. But by mid-summer they’re all dry and dark looking. I’ve tried all kinds of fungicide sprays, but none helped. I showed the grapes to my local extension office, but they had no idea what it was. I remember Mike mentioning a product called “Surround” in one of his shows. Do you think that it’s applicable here?
—Jing, “doing well in Clinton, NJ”
A. Let’s answer that last question first. “Surround” is the brand name for a micronized clay spray; it forms a thin film on fruits that protects them against many diseases and insect pests. It seems to work great; it’s the only thing I spray on my peaches (to help keep stink bugs off and prevent BROWN rot—the bane of peach growers); and it should work equally well on grapes.
But if I personally had grapes I wanted to spray for disease prevention, I’d first turn to a fungicide whose active ingredient is a very interesting naturally-occurring organism known as Bacillus Subtillis. It’s very effective at disease prevention; it’s specifically EPA registered for use against black rot; and it’s approved for certified organic agriculture. It’s sold under brand names like Plant Guardian and Serenade.
Now, I especially wanted to include Jing’s email here (we had a LOT of similar ones to choose from) because of the follow-up back and forth we had, which is shockingly typical of people with grape vine problems.
Mike: “Do they get good sun and airflow?”
Mike: “Do you prune the vines every winter?”
Jing: ‘Yes, but can you explain exactly how to prune them?’
Mike: “And finally, do you remove grape leaves and fruit clusters during the growing season?”
Jing: ‘No; and I don’t even know what that means.’
Which makes me very disappointed with the response Jing got from that Extension office. As with tree fruits, proper care of grapevines often prevents these kinds of disease problems—but none of that care is intuitive; you have to learn what to do and then do it every year.
…Like removing grape leaves and baby fruit clusters, which is very similar to what I have to do with my peaches (we remove three-quarters of the fruits early in the season, while they’re still tiny). The ‘problem’ with both plants is their natural lushness. Left on their own, they’d quickly get overcrowded and block airflow to the fruits. And both plants also have a strong tendency to produce too many fruits.
So let’s take this step by step. First, select the proper location: grape vines are full sun plants; they’ll never do well in any kind of shade–or in soil that doesn’t drain well. They also can’t be crowded by other plants and should never be fed chemical fertilizers or overfed in any way. A nice mulch of compost underneath is all they need.
And what about those specific rules for pruning?
All grapevines need to be pruned pretty dramatically in late winter. But our fruit growing expert, noted author Lee Reich, Ph.D., explains that the exact style depends somewhat on the specific varieties you’re growing and a great deal on what trellis system you’re using. Grapes are vines that require strong support, and the exact method of winter pruning will have a lot to do with the style of trellising you have in place.
Your local extension office should have Bulletins on proper pruning and trellising, but given the past performance of the office that Ying tried to use, I’d rely on them as a supplement to the excellent advice in Lee Reich’s books; he’s my guide for organic fruit growing, and I highly recommend the advice in his Taunton Press books “Grow Fruit Naturally” (2012) and “The Pruning Book” (2010).
OK. So, let’s say your grapevines are out in the open in full sun, pruned according to their specific variety and your type of trellis, and the growth is wonderful in the Spring. Now what?
Now comes the part where a lot of people fall down. In “Grow Fruit Naturally”, Lee notes that “grape arbors are notorious for becoming tangled messes of low-quality, disease-ridden grapes”. Preventing this, he explains, is “all about light and air”. If you prune properly and have an excellent trellising system, you may not have to do much else to get good grapes; the fruit thinning I’ve been harping on and hinting about might not even be necessary.
But it takes many years to develop true pruning courage—and sometimes the vines still get too lush. So if you can see that’s its crowded, you’ll need to remove both grape leaves and whole clusters of baby fruits to prevent disease and assure a good harvest.
Is there some kind of guide as to when and how much to remove? Yes; and it’s called personal honesty. You need to be able to look at the vines in June and July with the perspective of “I probably need to remove some leaves and fruit clusters to get more air and light in there.” You’ll always get a nice harvest with that attitude. Be in denial and you’ll get black rot—especially in a wet and cloudy year.
Managing Black Rot
Grapes 101 is a series of brief articles highlighting the fundamentals of cool climate grape and wine production.
By Tim Weigle
Figure 1. Various stages of black rot infection on berries. Photo by Tim Weigle.
Black rot (Guignardia bidwellii (Ellis)) is a potentially devastating fungal disease that can infect the leaves, shoots, berries and cluster stems of grapes. Susceptibility to black rot varies greatly by variety, but it can be a concern whether the grape is an American, French Hybrid or vinifera variety. Black rot is considered to be the bane of organic grape growers due to the limited materials that are available for its control as well as the devastating crop losses that can occur due to berry infections. Complete crop loss can occur in warm, humid climates like those of the eastern United States, but black rot is rarely found in arid growing regions.
Impact on clusters. Grape growers often find black rot to be an insidious disease: the grape clusters will appear to be developing normally until suddenly —as late as mid-summer — the fruit will start to turn brown, then black, with numerous round, black spheres called pycnidia on the surface. The grape berry will eventually shrivel up into a hard, raisin-like mass called a mummy. The pycnidia on the mummy’s surface contain inoculum, or spores, that will overwinter and be available to infect the grape crop the following year.
Overwintering inoculum and weather and the disease cycle. The amount of overwintering inoculum and the current season’s weather conditions are the key factors affecting the level of black rot found in a vineyard. Black rot inoculum can overwinter within cane lesions and the mummified fruit. In the spring, when the combination of temperature, precipitation and leaf wetness are favorable, the pycnidia on the mummies explosively propel their spores into the air, where they can land on susceptible grape tissue. No infection by black rot occurs when temperatures are below 45°F. At 50°F it takes 24 hours of leaf wetness to provide the conditions necessary for a black rot spore to germinate and infect the green tissue of grapes.
Figure 2. Black rot Leaf lesion with black spore-containing pycnidia, Photo by Tim Weigle.
Leaf lesions. The earliest, most recognizable indicator of black rot infections are the leaf lesions. These appear as circular, tan lesions that have a darker margin. Within these lesions are small black spheres, or pycnidia, which are containers for spores that can continue to infect the current year’s crop. While there can be many causes of tan lesions on grape leaves, only black rot lesions will have the black pycnidia within the tan field.
Timing and environmental conditions for infections. The ideal conditions for black rot infection are temperatures between 70°F to 80°F, when it takes only six to seven hours of leaf wetness for infection to occur. The availability of primary inoculum peaks around bloom and drops off dramatically post bloom. It is at this time that spores from the present season’s leaf infections become important because they can mature and produce secondary infections. Over the growing season berries become resistant to black rot infections. Concord is one of the first to become resistant at about four to five weeks after bloom. V. vinifera varieties are the latest to obtain resistance, about five to six weeks after bloom.
Sanitation. While there are a number of fungicides available for management of black rot, the importance of sanitation cannot be overemphasized. Sanitation plays a huge part in limiting the amount of black rot inoculum found in a vineyard. Removal of infected canes through pruning during the dormant season will reduce the level of overwintering inoculum. However, the largest reservoir of inoculum is typically found in the mummies. Mummies can be found either on the vineyard floor or retained in the canopy attached to old cluster stems. Removal of mummies from the canopy is critical: research has shown that these mummies provide inoculum much later into the season than those that have fallen to the vineyard floor. If mummies cannot be removed from the vineyard, the next best option is to make sure that all mummies are dropped to the vineyard floor to reduce the length of time spores will be available in the coming year. In a small planting, removal of infected fruit as it is discovered during the growing season is an excellent way to limit the amount of inoculum. Any mummies not removed from the vineyard during the dormant season should be, at a minimum, dropped to the ground where they can be covered with a dirt berm or cultivation, which effectively buries many of the mummies and limits the number of spores available for infection.
Spray timing. Even the best sanitation practices will leave low levels of inoculum in the vineyard, and this is all it takes for black rot to get a foothold. Using fungicides that are effective in managing black rot is critical, as is the correct timing of the applications. The most effective spray programs will target the overwintered inoculum to limit the number of primary infections in the beginning of the year. If primary infections occur, they can produce and release spores about two weeks after the initial infection, resulting in the continued spread of black rot through secondary infections. As spore production peaks just prior to the bloom period, it has been shown that the period just before bloom through two weeks after bloom is the most important time period to protect against black rot. However, the threat will change yearly depending on the level of black rot in the vineyard the previous year and the current season’s weather conditions, as these factors can result in the need to add an earlier season spray or an additional fungicide application after bloom. The Spots chart, a model for determining the weather conditions necessary for black rot infection periods to occur, can be found on the Network for Environment and Weather Applications (NEWA) website at http://newa.cornell.edu by using the grape forecast models in the Pest Forecasts drop down menu.
For more information on black rot, see Integrated Pest Management Disease Identification Sheet #102 : Black Rot , by Cornell’s grape pathologist Wayne Wilcox.
Tim Weigle is senior extension associate with the New York State Integrated Pest Management Program and team leader for the Lake Erie Regional Grape Extension Program, housed at the Cornell Lake Erie Research and Extension Laboratory in Portland, NY.
Cooperative Extension: Insect Pests, Ticks and Plant Diseases
Black Rot of Grape
Pest Management Fact Sheet #5106
Bruce A. Watt, Extension Plant Pathologist
For information about UMaine Extension programs and resources, visit extension.umaine.edu.
Find more of our publications and books at extensionpubs.umext.maine.edu.
The fungus Guignardia bidwellii causes black rot of grape. It is the most common and serious disease of grape in Maine and during years when the weather is favorable losses can range up to 80% of the crop.
The fungus infects newly emerging leaves, fruit, canes, shoots, and tendrils. Infections are usually first noticed on the leaves where they appear as small spots which may enlarge to 1/4 inch in diameter. The spots are commonly light tan in the center and may be circled by a dark tan band (Fig.1). As the infection ages, two types of tiny, dark, spore-producing structures form within the spots (Fig.2). These superficially similar structures (Fig.3) are either spermagonia (Fig.4) or pycnidia (Fig.5) and produce spermatia (Fig.6) and conidia (Fig.7) respectively. Spots formed by infection of the shoots are larger and darker and will also produce spores.
Symptoms of infected fruit begin as small brownish spots that quickly expand to involve the entire fruit in a matter of days. As the infection continues, the grape shrivels from a soft brown rotted fruit to a small, black, hard mummy (Figs. 8, 9). A few, many, or most of the fruit may be infected in each cluster. Once again, spores are produced from these infected fruit.
The spores that initiate infections in the spring are of two types. Ascospores are produced in pseudothecia that develop in infected, over-wintered leaves and fruit. These spores are ejected after bud-break in the spring after rain has fallen. Conidia from pycnidia in the over-wintered fruit and cane lesions (and later from new infections) are also infective. After the spores land on tissue susceptible to infection, they require sufficient time in the presence of free water in order to germinate and infect. Optimally, six hours at 80oF is sufficient for infection whereas at 50oF, 24 hours is necessary (Fig.10).
F 50 55 60 65 70 75 80 85 90 Hours 24 12 9 8 7 7 6 9 12
Figure 10. Hours of leaf wetness required for infection.
Spore dispersal continues through the middle of the summer and then declines until by the end of the summer no new infections occur. Through this period only new growth is susceptible to infection except for the fruit, which can be infected until the onset of color.
- Fall clean up may be the most important component of a control strategy because this can remove most of the inoculum (spores) source from the planting. Rake up all leaves and fallen mummies. Be especially careful to remove mummies that remain attached to the vine and, when pruning, preferentially prune out infected canes and be careful to remove infected tendrils.
- Plant in locations that will provide plenty of sun and air circulation, and orient rows parallel to the prevailing winds (generally west-east). Provide for proper vine spacing when planting, try to maintain an open canopy, and maintain good weed control. These practices will allow for rapid drying of the plants.
- Select resistant varieties when planting
There are two strategies for chemical control of black rot. The first strategy involves the use of protectant fungicides that must be present during infection periods to prevent infections from occurring. The second strategy relies on the ability of certain fungicides to eradicate early infections after they have occurred. All fungicides have protectant action. Protectant only fungicides include Bordeaux mix (may cause phytotoxicity), mancozeb, and ferbam. Fungicides that will move into the plant tissues (systemic) providing kickback activity include the strobilurins and the sterol inhibitors (e.g. Abound, Flint, Pristine/ Elite, Rally, Rubigan) A good general protectant schedule would be to spray at 1/2-1 inch shoot growth, immediately pre-bloom, immediately post bloom, and mid-season depending on the weather until the fruit starts to color. Care should be taken to rotate amongst chemical classes.
Becker, C.M. and Pearson, R.C. 1996. Black rot lesions on overwintered canes of Euvitis supply conidia of Guignardia bidwellii for primary inoculum in spring. Plant Disease 80:24-27.
Hoffman, LE et al. 2004. Integrated control of grape black rot: Influence of host phenology,Inoculum availability,sanitation , and spray timing. Phytopathology 94:641-650
NE Small Fruit Management Guide. 2013-2014.
Spotts, R.A. 1977. Effect of leaf wetness duration and temperature on the infectivity of Guignardia bidwellii on grape leaves. Phytopathology 67:1378-1381
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