Botrytis Bunch Rot: Who, Where, When, And What to Use

In the 2020 field season, nine field sites from eight different vineyards (seven commercial vineyards and one research vineyard) in the Willamette Valley were sampled for Botrytis from June to September. Grape clusters, vineyard floor debris, and nearby wild blackberries were collected, incubated, and then visually accessed for Botrytis. Botrytis incidence on clusters over the field season ranged from about 3% to over 30%. Botrytis isolates generated were screened for fungicide resistance to Benomyl (FRAC 1), Iprodione (FRAC 2), Difenoconazole (FRAC 3), Boscalid (FRAC 7), Cyprodinil (FRAC 9), Trifloxystrobin (FRAC 11), and Fenhexamid (FRAC 17). Some level of tolerance was seen in all fungicide classes among isolates with tolerance to more than one fungicide class observed in 22% of the isolates. Monitoring for sources of inoculum was done by sampling dead grape rachis and cane tissue from vineyard floor and wild blackberries adjacent to the vineyard. Vineyard floor debris decreased as the season progressed but for field sites with wild blackberries (3 vineyard sites), Botrytis on blackberry flower parts and berries was found throughout the season and at one site increased dramatically at the end of the season near harvest after a significant rainfall. These results suggest that vineyard Botrytis resistance levels are of concern and should be continued to be monitored along with the changes of Botrytis inoculum to better time applications of fungicides and other integrative pest management tools.