We are now hitting peak bloom on many of our grape cultivars. Bloom is a critical time for disease management in grapes, and with the rainfall that has occurred this week, we are setting ourselves up for another active downy mildew and rot season. In addition, other diseases, such as powdery mildew, Phomopsis and Botrytis, infect during the bloom phase. In fact, powdery mildew is particularly active during bloom. The information below is oriented mainly for vinifera grapes. For hybrids, the rots and downy mildew are important, but you are less likely to be majorly concerned with powdery mildew management. Note that sulfur will damage grapes such as Norton and Chambourcin, so make sure you know about potential phytotoxicity with chemicals before spraying them; many natives and hybrids are less likely to have powdery mildew, and sulfur would be wasted and cause damage.
One of my graduate students (Cheng-Fang Hong) is running the UCSC (Italian) disease-prediction models to review diseases such as downy mildew. The model predicted the first visible downy mildew symptoms on 4/25 with conducive conditions lasting until 5/6. It also predicted another infection period which started on 5/16, with first visible symptoms predicted for 5/20; the infection period is still continuing. I am not aware of any reports of downy mildew to date, but the infections are likely starting to occur on unprotected leaves; in addition, the young fruit can be infected, so the wet conditions we are experiencing would lead to diseased fruit as well. In terms of Botrytis, powdery mildew, and black rot, the Italian model is now predicting low, medium and high risks, respectively. During bloom, we need to make sure that fungicides are applied that will cover all of these diseases.
Strobilurin fungicides (Abound, Sovran, and Pristine) are locally systemic, and some have had good to excellent activity against downy mildew in the past. Another graduate student (Sarah Campbell) has been surveying vineyards for resistance of downy mildew to these strobilurin fungicides, and she has confirmed that resistance is widespread to this fungicide category – as expected. Grape growers in Georgia should NOT use strobilurin fungicides alone for downy mildew control. If these products are used to control other diseases and downy mildew control is also required, tank-mix strobilurins with another fungicide with activity against downy mildew (mancozeb or captan at a minimum). Also of interest, Sarah has not found resistance to the Revus fungicide, though this has been reported in Virginia and North Carolina. Revus would still be an excellent choice for downy mildew management in our spray programs – good news.
A few powdery mildew management pointers may also be of value as we enter this phase. Luna Experience (fluopyram + tebuconazole) is an excellent, highly efficacious material for powdery mildew control. Therefore, one does not want to use it too often due to the possibility of resistance development. However, targeting a couple of back-to-back applications at the inflorescence, bloom, berry set timeframe can provide excellent management of powdery mildew. If one application is specifically targeted to fall on berry set, this is the best time for a “bang for your buck” timing for powdery mildew management — particularly effective.
In some locations, the powdery mildew fungus has developed resistance to the sterol-inhibiting fungicides (Rally, Rubigan, Elite, and Procure) and the strobilurin fungicides (Abound, Sovran, and Flint [and Pristine]). All of these materials were highly effective for control of powdery mildew when they were first introduced. In the vineyards where these materials have been used for several years, reduced sensitivity or resistance may be present. In such cases, it is recommended that these materials not be used alone when powdery mildew needs to be controlled. In order to provide adequate control of powdery mildew, they should be mixed with sulfur, Quintec, Endura, or potassium salts. Pristine is a combination of a strobilurin fungicide plus Endura; therefore it can be used alone. Sulfur is an inexpensive and very effective fungicide for powdery mildew control. On sulfur-tolerant varieties, the use of sulfur should be considered. Mizuho Nita (Virginia Tech) recently brought some newer powdery-active fungicides to my attention as well. Though I am not familiar with these, he indicates that Vivando (FRAC 50) and Torino (FRAC U6) have good powdery mildew activity. He says that Vivando seems to work better than Torino, but Torino has a very short PHI — if you are concerned about late-season powdery mildew. In addition, he mentions that there are two newer SHDI materials, Aprovia (Syngenta) and Kenja (ISK) that have good powdery mildew activity as well, but the mode of action is the same as the fluopyram in Luna or the boscalid found in Pristine — not a new class or necessarily better activity.
I think everyone likely knows this, but tighten the spray schedules as we are in this wet period. Use 7-10 day intervals, as opposed to 10-14 day intervals. I hope we have a great season, despite the current rains and challenges they bring. Always follow label directions, and please contact your local county agent if you have questions or need additional information.