Mid and Late-Season Grape Disease Control

By: Bryan Hed, Plant Pathology Research Technologist, Erie County

2018 was a disastrous season for many grape growers in Pennsylvania. Excessive rainfall occurred almost everywhere below Interstate 90 and some growers have told me it was their worst crop, ever.  Now, looking at various NEWA weather station locations across PA, it’s been shaping up to be another wet season in a lot of places, yet again. May rainfall was heavy in all but the Lake Erie region, with 6-9 inches of precipitation recorded across most of the state. Conditions lightened up a bit in June but were still wetter than average in most places (even in the Lake Erie region). But now, conditions in July actually appear to be drying up in a few (but not all) locations, giving some growers a break in terms of fungal disease management. 

Hopefully, most premium wine grape growers have already applied fruit-zone leaf removal to open their fruit to better sunlight and aeration and better pesticide penetration. The benefits of this practice cannot be overemphasized, and in our wet, humid climate, it is one of the most effective cultural treatments we know of for reducing the susceptibility of the crop to disease of all kinds (especially bunch/sour rots), and improving coverage, and therefore efficacy, of fruit protection sprays. If you haven’t yet applied this treatment, it is not too late, though the benefits of leaf removal may be reduced the later it is applied. There is also a greater danger of sunburn on your fruit the later it is applied, and for that reason you may want to confine your leaf removal at this time to the east or north side of the trellis (depending on row orientation), especially in areas where very hot mid/late summer temperatures are expected. 

And with that, let’s talk about diseases and their control for the remainder of the season. Much of this information has already been covered in previous blogs in previous years, and I have borrowed some information from those blogs here (no need to reinvent the wheel). 

Downy mildew

As you know all too well, wet years are ideal for downy mildew. At about this time, the fruit of most grape varieties are resistant to this disease, but cluster stems may remain susceptible for a couple of weeks after fruit are resistant, and leaves will remain susceptible all season. If the weather remains wet or wet weather returns, downy mildew can be a serious threat to grape canopies and ripening, until harvest. Continue scouting for the distinctive white ‘downy’ sporulation on the undersides of leaves. Growers of susceptible varieties need to keep closely monitoring their vineyards for active sporulation and use that information in combination with the DMCast model on NEWA (http://www.newa.cornell.edu/) to determine if and when infection periods occurred.

If you see active, white sporulation on the undersides of leaves, the downy mildew pathogen is capable of spreading quickly under wet conditions. Even humid nights that result in heavy dews by morning, can continue to fuel downy mildew development. Once out of control, it can strip vines of their leaves and effectively end fruit ripening for this year and shoot ripening for next year’s crop. It could also mean your grapevines will go into winter dormancy at less than optimal hardiness and more vulnerable to damage by severe cold, leading to another bout with crown gall and trunk renewal to have to deal with for years to come. All these issues are connected, and this is definitely a disease you want to keep under very tight control, especially on Vitis vinifera. 

If you find yourself trying to control this disease well into the ripening period, be aware that your list of chemical control options will start to dwindle as we get within 30 (Ranman, Reason), then 21 (Ziram), then 14 (Revus, Revus Top, Zampro) days of harvest, until in the end you’ll be left with Captan, copper, and phosphorous acid products (0 day pre-harvest interval), which have their shortcomings, discussed below.

Another reason to keep this disease well under control is that products like Ranman, Reason, Revus/Revus Top, and Zampro, all contain chemistries that are prone to the development of resistance, and should not be used to put down an epidemic, which will speed up the resistance development process. Even phosphorous acid products, which are less prone to resistance development, can be lost to resistance through repeated applications on a heavily diseased vineyard. I know this is probably the last thing on your mind when your vineyard is under an attack of epidemic proportions, but still another good reason to keep downy mildew well in hand. 

Conversely, Captan or copper fungicides would be least risky in terms of the development of resistance and can be an effective means of controlling downy mildew late into the growing season. Just be aware that formulations of Captan have seasonal limits, so plan ahead if you can. There are also some insecticides that should not be applied with Captan. Also, keep in mind the risk for injury by copper applications, and that copper injury will be exacerbated by application under slow drying conditions and application to wet canopies (for example, don’t make applications to dew covered canopies in the early morning). It’s also important to consider that copper is poisonous to yeasts and that excessive copper residues at harvest can interfere with fermentation, and wine stability and quality. Unfortunately, it’s impossible to predict how high residues will be on fruit at harvest; that’s going to depend on the copper formulation (fortunately some of the newer coppers utilize lower copper concentrations), rate of material used, spray coverage, and amount of rainfall from application to harvest. I am not aware of any information that establishes a nice, clean cut-off date or pre-harvest interval for avoiding excessive copper residues at harvest. There is also some evidence that late Captan sprays can cause problems in the winemaking process, in terms of delaying fermentation and negative effects on wine quality but the consequences seem less severe and irreversible.  For more on this, consider this online article by Dr. Annemiek Schilder, former fruit pathologist at Michigan State University.

If you are protecting a non-bearing, young vineyard from downy mildew (you’re not selling/harvesting a crop), you can continue to use mancozeb products to control downy mildew past the 66-day pre-harvest interval. You can also consider using mancozeb after harvest to keep canopies clean of downy mildew and ‘firing on all cylinders’ until that first frost. The longer your vines can continue to produce and store carbohydrates after harvest, the better prepared they’ll be to withstand winter cold. 

Fluffy, white downy mildew sporulation on the underside of a grape leaf

Powdery mildew

Good control of powdery mildew is also very challenging in wet years when humidity levels remain ‘through the roof’ and cloud cover occurs for extended periods of time. Now that we are largely past the fruit protection period, our focus is on keeping leaves clean, especially on V. vinifera, for about 6-8 more weeks. I say this for many of the same reasons expounded in the section about downy mildew (ensure optimal ripening of fruit and shoots/canes, ensure optimal cold hardiness, more effectively and more easily manage fungicide resistance, etc). But there is another very important reason, demonstrated by some excellent research conducted by Wayne Wilcox, Dave Gadoury and graduate students at Cornell University, who showed that controlling powdery mildew up to about Labor Day can also go a long way to reducing overwintering inoculum and disease pressure the following spring. Why Labor Day? When powdery mildew infected leaves die by that first hard frost in fall, the mildew on those leaves stops developing and also dies…unless it has had time to form fully mature, winter resistant structures called chasmothecia. In other words, if the chasmothecia in a powdery mildew colony on a leaf, do not have time to fully mature before the leaf dies, they will not be tough enough to survive the dormant period (winter) and will not contribute to the bank of primary inoculum that infection periods draw upon the following spring. Knowing this, a grower can get a better handle on the ‘size’ of the powdery mildew problems he/she will potentially face next spring and the spring after that, and so on. If, for example, you had heavy powdery mildew development earlier in this season (on clusters and/or leaves), expect to have to deal with powdery mildew early next season and you’ll have to take appropriate action during early shoot growth stages with preventive fungicide sprays. Once again, this is particularly important if you are growing Vitis vinifera and much less important for growers of native varieties like Concord and Niagara. 

Greyish-white colonies of powdery mildew growing across the upper surface of grape leaves

Botrytis bunch rot control

If you’re growing bunch rot susceptible wine grape varieties, you have already applied a Botrytis specific fungicide at full bloom and probably pre-closure (?) This is because Botrytis infections can occur during bloom and early fruit development under wet conditions (which most of us have had). These Botrytis infections of the clusters usually remain dormant, or ‘latent’, and do not result in active rot of the fruit…until after veraison, when injury to berries or high humidity, or some other factor (research has not completely determined all the factors involved) may lead to activation of a percentage of these infections and cause clusters to rot. 

In varieties with very compact clusters, the pre-closure application may be extremely important as it may be your last opportunity to get protective fungicide residues onto the interior surfaces of clusters. Along with the bloom spray, this spray will also help to reduce ‘latent’ Botrytis infections that continue to accumulate throughout the ‘green’ berry development period. The pre-closure spray may also be a good opportunity to clean clusters of bloom trash (dead cap and stamen tissue that got stuck in the clusters after bloom). Bloom trash provides a substrate for Botrytis and serves as a focal point for bunch rots to develop later in the season, from inside clusters. The compactness of clusters plays an important role in not only the retention of bloom trash (the tighter the cluster, the more bloom trash retained) but also the effect of retained bloom trash on cluster rot; as compactness increases, the enhancement of bunch rot by retained bloom trash increases. 

Another bunch rot control measure is leaf removal around clusters, which we’ve already discussed above. It is an expensive operation to add to your production costs and is most cost-effectively applied by machine (machinery costs aside). We have found that it can be mechanized most effectively if vines are trained to a vertical shoot positioned (VSP) or some other two-dimensional trellis system with a relatively focused and narrow cluster zone. One additional benefit of leaf removal that I haven’t mentioned yet is the fact that it can also reduce bloom trash retained in clusters: when comparing clusters of vines treated with and without leaf removal, we noted a significant reduction in bloom trash where leaves were removed, regardless of timing or method (by hand or machine). 

Our next fungicide application for Botrytis is made just before or at veraison. As fruit begin to soften and skins become thinner and more easily penetrated by fungal pathogens like Botrytis, an application at this time, to rot prone varieties, is a good way to stave off bunch rot development. After veraison, fruit also becomes more susceptible and more likely to become injured by birds, insects, excess moisture/humidity, and overcrowding of berries in tight clusters. Botrytis fungicides can protect intact fruit surfaces and may help to reduce the spread of Botrytis rot on fruit, even after they have become injured. 

Lastly, an application about 2-3 weeks after veraison, especially under wet weather conditions, can reduce further rot development during the last stretch of ripening. Keep in mind that Botrytis fungicides control Botrytis and will not provide protection against sour rot organisms that often destroy the fruit of overly compact clusters, despite the application of a full Botrytis fungicide program.

And speaking of sour rot…

In case you haven’t already heard, there is some relatively new information on sour rot control that I would like to impart. It’s been included in previous blogs as well and that information was presented earlier this year at the Mid Atlantic Fruit and Vegetable Convention in Hershey PA. However, it bears repeating it here. It originated from work conducted by Dr. Megan Hall, a former graduate student of Wayne Wilcox at Cornell University, and it demonstrates how additional pesticide applications during the latter stages of ripening (beginning around 15 brix) can significantly reduce the development of sour rot, which for many premium wine grape growers in PA, is public enemy no. 1 at harvest. Her incredibly thorough work has shown a close connection between fruit flies and sour rot development. It turns out that the presence of the flies is important to the accumulation/generation of acetic acid in rotting fruit. Treatments composed of weekly, tank-mix applications of an insecticide (to control the flies) and an antimicrobial (to kill bacteria) have been found to reduce sour rots by 50-80% over unsprayed vines. So far, the best results appear to occur when weekly sprays are initiated before sour rot symptoms are observed (preventive sprays before about 15 brix). This exciting work should provide yet another effective option for sour rot control in the wet, humid parts of the eastern U.S. and we are looking forward to hearing more about this rot control option in the near future.

Lastly, don’t forget how important good canopy and fertility management is to the efficacy of your expensive Botrytis fungicide and sour rot pesticide applications. It’s always a good idea to make sure your shoots are well tucked and spaced within the catch wires, and summer pruning has removed shoots ends that may block sprays from thoroughly penetrating the fruit zone, just before you make each Botrytis fungicide application. We like to wait as long as possible to trim shoot tips because of the effect on lateral growth stimulation, but make sure excessively long shoots have not flopped over to block spray penetration into the fruit zone. Limiting shoot growth after veraison with good canopy and fertility management will also limit the supply of new green tissue that is hyper susceptible to powdery and downy mildew and will contribute to more effective late-season management of these diseases as well.   

For further reading on this and many other disease management topics, refer to the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes. If you don’t have a copy, you can get one through Cornell University Press. Every commercial grape production operation should have one! 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: