I am attaching reports from wine grape pathology field trials conducted in 2020. Shane Breeden and Brooke Warres, both graduate students in the UGA Plant Pathology Department, were responsible for generating this data. Three of the trials were conducted at the Blairsville research station, so we need to also thank Ray Covington and his staff for their help in executing these trials. This information may be of value to you when developing your spray programs for 2021. Below, you will find my summary of the take-home points from each trial.

Brannen_Grape1: This trial reviewed three different spray programs for managing disease on the new Pierce’s disease resistant grape varieties ‘Camminare noir’ and ‘UC Davis selection 07370-84’. ‘Camminore noir’ may be less susceptible to diseases than 07370-84, but practically, a full spray program will be required for both. Though the low input program provided good downy mildew control in ‘Camminore noir’, it required a full program for rot management. For 07370-84, a high input regimen worked best for downy mildew and rots. Both of these varieties are 94% Vitis vinifera in their parentage. Based on the first year of testing, the limited hybridization with native grape species, though conferring resistance to Pierce’s disease, does not allow for use of more economical spray programs.

Brannen_Grape2: This trial reviewed fungicides and their activity in the presence of a DMI- and QoI-resistant population of Erysiphe necator, fungal causal agent of powdery mildew. Efficacy of various products are clearly rated in this report. In particular, you will note the failure of QoI-only products such as Abound, as well as the reduced efficacy of DMI products such as Rally. However, also note that Inspire Super, which contains difenoconazole (another DMI) does work well; this means that cross-resistance is not complete for the DMI class, and difenoconazole is still active — even against a DMI-resistant population. Note that reduced activity of the contact materials stylet oil and sulfur could have in part been an artifact of the application method, a CO2 backpack research sprayer as opposed to an airblast. We really can’t use the stylet oil due to our need to spray captan and other fungicides that may cause damage when mixed with oils. However, sulfur is a primary part of our spray programs and does not develop resistance. Bottom line — incorporate sulfur in your programs when you can do so without causing damage, and mix sulfur with DMIs to prevent resistance development.

Brannen_Grape3: This test was designed to determine whether low rates of sulfur and various DMIs would provide a synergistic interaction that would provide much better control than expected in the presence of a resistant Erysiphe necator population. Note when the sprays were conducted, as compared to when the ratings were conducted. The severity data is probably of more value than the incidence data for comparing treatments. Though the addition of sulfur with DMIs is a good idea, and though it does add to the total efficacy of a DMI in the presence of a resistant Erysiphe necator population, the selection of the DMI to mix with the sulfur is more important. Again, the cross-resistance in this class is not complete, and Cevya, a new DMI fungicide, clearly provided better control than either Rally or Elite in this trial. In addition to providing a resistance management tool, tank-mixing a low rate of sulfur did generally reduce incidence and severity of powdery mildew on leaves, but this response was less clear on fruit.

Brannen_Grape4: This was one of the more interesting trials, as it clearly shows the difference in efficacy of several DMI fungicides when DMI resistance is observed. Though Cevya was once more shown to be more active than Elite or Rally, the difenoconazole component of Revus Top provided exceptional control of powdery mildew on leaves and fruit. Though difenoconazole may by inherently more active even where resistance does not occur, it is clearly a more active material in the presence of a resistant powdery mildew population. In fact, Revus Top was equivalent to Aprovia, an SDHI fungicide and among the most efficacious materials for powdery mildew control at this time. The data from two trials at Blairsville would imply that use of difenoconazole-containing products would still be warranted — even where resistance to DMIs has generally been confirmed.

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