2016 Post Bloom Disease Management Review
By: Bryan Hed
Once again, we’ve arrived at that part of the season just beyond the immediate pre bloom and first post bloom spray. For many years now, research has shown that those two sprays are absolutely essential to a fruit disease management program, at least for control of the four major grape diseases (powdery and downy mildew, black rot, and Phomopsis). We have always emphasized the use of ‘best’ materials, shortened intervals, best coverage, etc., for those two sprays, EVERY YEAR!…a no brainer. No matter what varieties you grow, those two sprays are most often the most important for protection of your crop.
Fortunately for some of these diseases, fruit susceptibility is short lived and most control of fruit diseases like powdery mildew is achieved by management right around/shortly after bloom. Indeed, work conducted by Wayne Wilcox and his grad students over the years has shown that fruit are generally susceptible to powdery and downy mildew for only about 2-3 weeks after capfall. Varieties of Vitis vinifera tend to be susceptible a little longer than native varieties like ‘Concord’, but for all varieties, the period of time during the first 2-3 weeks of fruit development is the most critical for fruit protection. Whenever I am approached with questions from growers as to why they ended up with a boatload of fruit disease in a given year, the answer almost invariably lies within the spray program during early fruit development.
Beyond that, things tend to get a little more complicated in terms of ‘what do I spray now?’ It depends on a number of things like the weather (past, present, and future), the variety grown/susceptibility of the host, your overwintering inoculum load (how much disease you had last year and the amount of old wood and debris in your trellis system this year) and your current disease levels. If you’re growing Concord grapes in the Lake Erie region in 2016, where rainfall during June has been scarce and sunshine and low humidity have dominated, diseases have been relatively easy to control so far. For example, there have been but four brief downy mildew (Figure 2) infection periods to date. As a result, this disease simply cannot be found in most maintained vineyards in the Lake Erie region, despite an abundance of downy mildew overwintering inoculum from the widespread occurrence of this disease last year. And, there have been just two mild black rot (Figure 1) infection periods since bud break. The immediate pre bloom and first post bloom spray probably provided all that was needed for control of powdery and downy fruit infections. The threat of black rot fruit infection remains (Concord is susceptible to this disease for about 4-6 weeks after capfall; V. vinifera about a week or two longer), though in vineyards that did not have black rot problems last year and where current disease is almost non-existent it is unlikely that black rot will spiral out of control at this point unless the current weather pattern suddenly turns very wet. Regular scouting of your vineyard will reveal whether or not this disease has gotten started in your vineyard (at this point in the season, it takes about 14 days for symptoms of black rot fruit infections to manifest themselves after an infection period).
The threat of Phomopsis infections depends to a large degree on how much overwintering inoculum is available. Since current season Phomopsis infections (Figure 3) generally do not produce spores until the following seasons (unlike the other fungal diseases we deal with each year), the development of this disease is dependent on overwintering inoculum sources that are normally ‘milked out’ by seasonal rainfall from May through mid-July. That means that in an average rainfall year, there are few spores left to cause infections by mid-July, even though fruit of many varieties do not appear to lose their susceptibility to Phomopsis (research by Mike Ellis and his students at Ohio State University). If spore sources are not being depleted in regions that have experienced a dry spring this year (like the Lake Erie region), enough inoculum may still be available in overwintering sources to cause fruit infections (Figure 3) past the mid-July period, should conditions turn wet. As one would expect, this is more of a concern for vineyards with a previous history of this disease. In addition, vineyards trained to trellis systems that retain lots of older and/or dead wood (cordons as opposed to canes, machine pruning as opposed to hand pruning) and/or vineyards that have not been receiving early shoot (3-6” shoots) sprays in previous years, will be more at risk of retaining significant amounts of overwintering inoculum of Phomopsis past the mid-July period during years with dry springs.
For Lake Erie region juice grape growers, powdery mildew remains in spite of the dry weather. Recall that powdery mildew primary infection periods require rainfall of at least 0.1” (and temperatures above 50 F). However, once primary infections have occurred, the disease can proceed to build without rainfall from spores produced by those primary infections. I suspect the dry, sunny weather will keep disease development moving at a slower than average pace (direct sunlight kills powdery mildew), but the disease will continue to build as it always does. Juice grape vineyards with low to average size crops may require little beyond the first or second post bloom spray for mildew. But keep in mind that we’ve a long way to go and if cloudy, humid conditions become entrenched, it can speed epidemic development. Also, poor ripening conditions after veraison can greatly reduce a mildewed canopy’s ability to ripen a crop, especially a large crop.
For wine grape growers, wherever you are in PA, it’s a ‘given’ that protection against all the major diseases should continue well past the first post bloom spray, for fruit and for leaves. As mentioned earlier, fruit are still susceptible to black rot and Phomopsis, and if you’re in an area experiencing at least some rainfall this year, downy mildew is definitely a continuing threat. As detailed above, the threat of powdery mildew goes without saying and every day is a powdery mildew infection period. So, for the second post bloom spray on wine grapes, include active ingredients for control of all diseases.
For continuing summer sprays, pay close attention to chemical classes for resistance management and always rotate modes of action. The loss of a mode of action (like the QOIs (strobies)) is a big deal to wine grape growers who have to apply many sprays within a given season for control of diseases like powdery and downy mildew. Rotation and resistance management should be an important component of your summer spray program. You’ve applied your best materials around bloom, now it’s time to rotate to other modes of action. Fortunately, we have lots of effective options for powdery and downy mildew control; use as many of them as you can, never applying consecutive sprays of anything (except the old standards like copper, sulfur, mancozeb products, ziram, captan). In Pennsylvania, we have many effective modes of action for powdery mildew like those found in Vivando, Torino, Quintec, the difenoconazole products (the newest, most powerful sterol inhibitor), and Luna Experience, Endura, and now Aprovia (the succinate dehydrogenase inhibitors). And of course, there’s always sulfur, but beware its use on red hybrids. I should mention that Aprovia is also labeled for black rot control. However, our recent tests have indicated that Aprovia’s black rot efficacy may be limited and that further testing is needed to better define this activity before it can be recommended for control of this disease.
For downy mildew we have products like Revus, Presidio, Ranman, Zampro, the old standards (copper, mancozeb products, ziram, captan), and the phosphorous acid products. Unfortunately in parts of PA, the powdery and downy mildew pathogens have developed resistance to the strobilurins, and they may not be reliable choices any longer. Also, the active ingredient in the product known as Reason, has the same mode of action against downy mildew as the strobilurins, and for resistance management purposes, Reason should be considered the ‘same’ as the strobilurins. One last thing: if you use the phosphorous acid products for downy mildew control, keep in mind that although they are extremely rain-fast, do not expect them to provide more than 10 days of protection against this disease, especially under high disease pressure.
For wine grape growers in more southerly regions of PA that have been receiving regular or heavy rainfall, Protection against all diseases obviously needs to continue. Once past the fruit protection period (which may be up to 6-7 weeks past capfall for black rot on V. vinifera), leaves of V. vinifera and some of the more sensitive hybrids will need continued protection from powdery mildew up to veraison or longer. As long as conditions remain wet, downy mildew will also remain a threat deep into the season. A clean canopy is essential for maximum ripeness and fruit/wine quality, maximum winter hardiness (recalling the cruel winters of 2014 and 2015), and minimal overwintering inoculum. For late season powdery mildew control, alternative materials may gradually be used to replace the synthetics and sulfur (particularly for reds where late sulfur applications can create wine quality issues). Avoid oils around/after veraison for powdery mildew control to avoid reducing photosynthesis. I have heard good things about potassium salt use (potassium bicarbonates and nutrol) from colleagues in Ontario, to maintain clean canopies late into the season. There are other alternatives currently available for late powdery mildew control, but for many their efficacy, especially on V. vinifera, is modest at best.
Late Summer sprays are for leaf protection, especially for varieties of V. vinifera. Sprays at this time primarily target powdery mildew, but may also include downy mildew if disease has gotten a foothold in the vineyard and conditions remain wet into fall. Regular scouting and strict attention to weather conditions at this time are very beneficial to making prudent late season spray decisions. For downy mildew, rainfall and leaf wetness is critical for epidemic development and dry late summer periods can sometimes offer relief from this disease. However, beware of heavy over-night dews which can continue to fuel downy mildew infections and sporulation without rainfall and keep the ‘fire’ alive on leaves at a slow burn. Early defoliation by downy mildew will effectively terminate the fruit and cane ripening process and leave vines weakened going into winter.
For bunch rot control, wine grape growers of bunch rot susceptible varieties may have already applied a Botrytis specific fungicide at full bloom. This is because Botrytis infections of the inflorescences can occur during bloom under wet conditions. These infections usually remain dormant and do not result in active rot…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.
The next Botrytis fungicide application is commonly applied at just before closure of the clusters (soon). In varieties with very compact clusters, this application may be extremely important as it represents your last opportunity to get fungicides into the interior surfaces of clusters. This spray may also help to reduce latent infections that research has shown can continue to accumulate throughout the berry development period. It may also be an opportunity to ‘blow out’ bloom trash (dead cap and stamen tissue that got stuck in the clusters after bloom) from the insides of clusters. Bloom trash can provide substrate for fungi like Botrytis and serve 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. Most often applied shortly after fruit set, fruit zone leaf removal exposes fruit to better air, sunlight and pesticide penetration which can improve control of ALL fungal diseases. This practice is most commonly applied to varieties of Vitis vinifera that produce tight clusters, but it is an expensive operation to add to your production costs and is most cost effectively applied by machine (machinery costs aside). It can be mechanized most effectively if vines are trained to a VSP or some other two dimensional trellis system with a relatively focused and narrow cluster zone (Figure 4A and B).
Research generally indicates that the earlier this practice is applied, the larger the effects for bunch rot control. For example, when applied at trace bloom (first flowers opening), it tends to reduce fruit set in addition to exposing clusters. This can be beneficial for varieties that naturally produce compact clusters (Pinot Noir, Pinot Gris, Chardonnay, Vignoles, Riesling) that are very susceptible to rot during ripening (Figure 5). Clusters that are looser (as a result of reduced fruit set) are easier to penetrate with pesticides, and are less apt to become damaged by overcrowding of berries before harvest. However, the potential for yield reduction may make the trace bloom timing unnecessary or undesirable on varieties that do not suffer from compactness/high susceptibility to late season rots. One note of caution: in more southerly climates, some growers remove leaves only on the east (north-south running rows) or north (on east-west rows) side of the trellis to avoid sun damage to fruit in late summer.
Fruit zone leaf removal 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). This can be particularly effective if utilizing air-pulse technology to remove leaves. This type of leaf removal mechanization applies high speed pulses of air to shatter leaves in the cluster zone, while blowing bloom trash from clusters.
The next fungicide application for Botrytis is made just before or at veraison. As fruit begin to soften and skins become thinner and more ‘breachable’ by fungal pathogens like Botrytis, an application at this time, to rot prone varieties, is a good way to stave off bunch rot development as fruit become 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 fruit of overly compact clusters, despite the application of a full Botrytis fungicide program.
Don’t forget that there is abundant information available in the 2016 New York and Pennsylvania Pest Management Guidelines for Grapes. This is one of the very best guides for grape growers in NY and PA (and the Northeastern U.S. in general). It represents the compilation of many years of excellent grape research and includes the most recent updates on pesticide use and disease and insect pest control. If you don’t have a copy, get one through Cornell University press. Every commercial grape production operation should have one! At about half the cost of a single pesticide spray, it is well worth it.