2015 Post Bloom Disease Management Review
By: Bryan Hed
Botrytis bunch rot fungicide applications
Botrytis infections of the inflorescences can occur during bloom, especially under wet conditions. These infections usually remain dormant or latent 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. So a Botrytis specific fungicide application may be advisable at mid to full bloom to limit these infections and reduce the number of sites for active bunch rot to develop during ripening, especially on rot prone varieties like Riesling, Vignoles, Pinot noir, Pinot gris, and Chardonnay.
The next Botrytis fungicide application is commonly applied at just before closure of the clusters. In varieties with very compact clusters, like Vignoles or Pinot gris, this may occur just a couple of weeks after fruit set to around pea size berries. This application represents your last opportunity to get fungicides into the interior surfaces of clusters before the long march to harvest and may help to reduce latent infections that can 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, especially if there haven’t been periods of heavy rain/wind since bloom. This material 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.
The next 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.
If you’re considering cluster zone leaf removal:
Now is the time to plan out your leaf removal strategy if you intend to apply this cultural method. Leaf removal around clusters, opens the cluster zone to better air, sunlight and pesticide penetration which can improve disease control. This practice is most commonly applied to varieties of Vitis vinifera and can be an expensive operation to add to your production costs. It is most cost effectively applied by machine, and can be mechanized most effectively if vines are trained to a VSP or some other 2 dimensional trellis system that creates a relatively focused and narrow cluster zone. Research generally indicates that the earlier this practice is applied, the larger the effects. 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 susceptible to rot during ripening. 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. If you haven’t been applying this early timing of leaf removal, but wish to test it in your vineyard, I would suggest just applying it to a small number of vines first (by hand) and compare it to non-defoliated vines (control vines) and/or vines that you normally pull leaves from later (like after fruit set).
The more commonly applied leaf removal timing is at, or just after, fruit set. Removing leaves at this time will have minimal effect on fruit set and yield and will still provide many benefits in fruit health. The later leaf removal is done, generally the greater is the risk of fruit becoming sunburned. We noticed this in a trial where we compared leaf removal at i) trace bloom, ii) post-fruit set, and iii) veraison in Chardonnay (a tight clustered, rot prone variety). Trace bloom leaf removal reduced rots by about 70% when compared to ‘no leaf removal’, whereas leaf removal at post set and veraison reduced rots by about 50 and 14%, respectively. We also noticed that in one particularly hot season, more sunburn developed on berries subjected to veraison leaf removal than berries subjected to removal at post set or trace bloom. A additional note of caution here: in more southerly climates of the mid-Atlantic, some growers have found it advisable to pull 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. In our ‘neck of the woods’ in Erie county, we have found this precaution to be unnecessary in most seasons, but not all.
First post bloom fungicide application.
I ended my blog back in April by saying that the first post bloom spray is a critical one for every vineyard, every year, for control of every disease!!!…I just said it again. When the caps come off during bloom, they expose an extremely susceptible developing berry that, regardless of variety, is vulnerable to infection by fungal grape pathogens. Young fruit are most susceptible to all the major diseases (Phomopsis fruit rot, black rot, downy and powdery mildew) during the period from bloom to about 2-3 weeks after bloom, and severe damage to fruit from disease can usually be tied to disease control failures that occurred during this period of time. The fruit of some varieties develop resistance to some diseases rather quickly after capfall. For example, Concord grape is very resistant to powdery mildew by the time fruit reach about a quarter inch in diameter, which may occur within 3 weeks of capfall. On the other hand, the fruit of more susceptible wine varieties (Vitis vinifera and some of the more sensitive hybrids) may be susceptible to powdery mildew until bunch closure or about 3-4 weeks after bloom. Generally the susceptibility period to black rot is much longer for most varieties. Concord fruit may remain susceptible to black rot for 4-5 weeks after bloom, and fruit of V. vinifera for 6-7 weeks after bloom. Regardless of susceptibility periods, at this point in time all varieties of grapes should receive the best fungicide protection you can provide, for all the major diseases.
Apply your most effective materials at this time, and again, remember to include a material for every major disease. The strobies (if no powdery/downy mildew resistance issues; may provide activity against all diseases but remember that Sovran is mediocre against downy, Flint is weak against downy, and all are good to only modestly effective on Phomopsis), Quintec (powdery only), Manzates/Ziram/Captan (for Phomopsis, black rot, downy mildew)) may be good choices. This is also the perfect time to consider some of the newer products like Vivando (for powdery mildew only), Revus Top (for powdery and downy mildew and black rot), Inspire Super (for powdery mildew and Botrytis), Luna Experience (wine grapes only, for powdery mildew, Botrytis, and black rot) and/or one of the newer downy mildew materials listed here:
- Revus; contains mandipropamid, registered in 08. Very effective on downy mildew in PA and NY trials.
- Presidio; fluopicolide, registered in 08. Very effective on downy mildew in PA and NY trials. Label requires that Presidio be applied as tank mix with another downy mildew fungicide.
- Reason 500 SC; fenamidone, which is a quinone outside inhibitor; same mode of action as strobies, but not technically a strobie. However, treat it as a strobie with respect to resistance management. Provided excellent control of downy mildew in Cornell trials.
- Quadris Top; azoxystrobin + difenoconozole; for downy and powdery mildew, black rot, and Phomopsis. New combination of current chemistries. Its use on grapes in the Lake Erie region will be greatly restricted: with azoxystrobin in the mix, this can’t be used in Erie county PA, and with difenoconazole in the mix, this can’t be used on Concord.
- Ranman; cyazofamid, a new chemistry for downy mildew. PA and NY trials show good to excellent efficacy against downy when applied alone and mixed with phosphorous acid.
- Zampro; ametoctradin + demethomorph. The newest of the new downy mildew materials; a combination material that is very effective on downy mildew.
A quick note on phosphorous acid products (aka phosphites, phosphonates); these have become favorites for many growers as a means of controlling downy mildew. They are effective and ‘friendly’ to work with. However, if you use these materials at this time as the only material for downy mildew, be mindful that, although they are extremely rain-fast (they are absorbed into the plant and translocated), they still provide only limited protection against new infections. Do not expect phosphorous acid sprays to provide more than 10 days of protection, especially under high disease pressure.
Plan to apply for best coverage, every row, full rates, and shortest intervals (NEVER extend the interval between the last pre-bloom and first post-bloom sprays beyond 14 days). The bloom and early post bloom periods are the most critical for protecting your crop ($) against all diseases; it is never cost effective to cut corners during those stages of crop development.
Second post bloom fungicide application.
This spray is still a ‘no brainer’ for V. vinifera and hybrid wine varieties, because even if fruit have developed some resistance to some diseases, fruit are still very much at risk from black rot and leaves will still require protection from things like powdery and downy mildew. As mentioned earlier, berries of the vast majority of grape varieties are still susceptible to black rot at this point and, unless almost no black rot has developed on the crop, may still require protection from this disease with fungicides. Berries of natives like Concord can remain susceptible to black rot for 4-5 weeks after bloom, whereas the more susceptible V. vinifera varieties can remain susceptible for 6 weeks or more after bloom. Generally, fruit resistance to this disease develops gradually, with the period of highest susceptibility occurring during the first 2-3 weeks after bloom (that critical period of fruit susceptibility to all diseases).
Natives like Concord may be almost completely resistant to powdery mildew at this point, whereas V. vinifera and sensitive hybrids will still require protection of fruit and most certainly will require protection of leaves. The threat of Phomopsis infections at this time may depend on how much overwintering inoculum is left (current season shoot infections will not produce spores until next year). If spring rainfall has been above average, Phomopsis spore sources (in wood) will have been somewhat drained and risk of further infection will be minimized. If weather has been dry, there may be sufficient inoculum still available to prolong the risk of Phomopsis infections during rainfall periods, especially of fruit and rachises, which remain susceptible.
The risk of fruit loss to downy mildew may remain as well, especially for V. vinifera and hybrid wine varieties that are susceptible. Fruit are less susceptible or resistant at this point, but berry stem tissue can still become infected and cause berries to stop developing and shell later. So, for susceptible varieties it is advisable to continue cluster protection against downy mildew through the second post bloom spray, especially if your scouting reveals the presence of the pathogen in the vineyard and conditions remain wet. Leaves continue to be very susceptible to this disease as well.
Summer sprays are generally for leaf protection and leaves of wine varieties, especially those of V. vinifera, may still need protection from powdery and downy mildew through veraison or beyond. This determination is best made through regular scouting and strict attention to weather conditions. If black rot has not been controlled on varieties with lengthy fruit susceptibility periods (mentioned above), an additional spray for this disease may be needed. However, if scouting reveals little or no black rot in the vineyard, sprays for this disease are generally no longer necessary. Berries of V. vinifera are resistant to powdery mildew about 3-4 weeks after bloom and so rarely require continued protection at this point. However, canopies of V. vinifera and sensitive hybrids will require strict control of powdery and downy mildew through harvest for maximum fruit development and vine cold hardiness (which, given the past two winters, is all the more important to Northeastern U.S. growers!). The best way to maintain a handle on leaf disease is to scout regularly. Remember that once powdery mildew gets past the primary infection stage (generated by overwintering inoculum), which usually has run its course by around bloom, secondary powdery mildew cycles can continue to build an epidemic without rainfall, and virtually every summer day is an infection period for this disease. As for downy mildew, rainfall and leaf wetness is critical for epidemic development and dry summer periods can offer a reprieve from this disease. However, heavy over-night dews (which can become common in mid and late summer) can continue to fuel downy mildew infections and sporulation without rainfall and keep the ‘fire’ alive at a slow burn, ready to pounce when conditions turn wet again. Ultimately, scouting is your best bet for keeping yourself informed of what’s going on in your vineyard, and when you should spray a fungicide to protect your leaves during the summer months.
I’ve only touched the surface of post bloom disease management in this discussion, but hopefully you’ll find this information useful. Pay close attention to labels and reentry and ‘days to harvest’ intervals. Lastly, don’t forget the abundant information available in the 2015 New York and Pennsylvania Pest Management Guidelines for Grapes. This is a painstakingly comprehensive guide for grape growers in NY and PA, representing many years of excellent grape research, and you can get a copy through Cornell University press. The price of the guidelines is less than half the cost of a single pesticide spray.