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

In the 2021 field season, eleven field sites from nine different vineyards (eight commercial vineyards and one research vineyard) in the Willamette Valley were sampled for Botrytis from May to September. Grape inflorescence, clusters, vineyard floor debris (grapevine rachis), and nearby wild blackberries were collected, incubated, and then visually accessed for Botrytis. Botrytis incidence on clusters over the field season ranged from <1% to 11%. Botrytis isolates generated were screened for fungicide resistance to Benomyl (FRAC 1), Iprodione (FRAC 2), Myclobutanil, Tebuconazole, Difenoconazole (FRAC 3), Fluopyram, Boscalid (FRAC 7), Cyprodinil (FRAC 9), Trifloxystrobin, Azoxystrobin (FRAC 11), Fenhexamid (FRAC 17) and Polyoxin-D (FRAC 19). In 2020 isolate collection, some level of tolerance was seen to all fungicide classes examined, with tolerance to more than one fungicide class observed in 35% of the 144 isolates examined to date. In 2021 isolate collection, all but FRAC 9 fungicide classes tested had some level of tolerance, with tolerance to more than one chemistry seen in 18% of tested 48 isolates examined to date. 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. Incidence of Botrytis on vineyard floor debris in all but one site was over 75% in late April and all sites sampled decreased over time to under 25% by September. For field sites with wild blackberries (3 vineyard sites), Botrytis on Blackberry flower parts and berries was found at low levels throughout the season. The results from 2020 and 2021 collections have been published and are available online. These results indicate a fungicide class for Botrytis management should not be used more than once in season and the Botrytis inoculum is potentially available throughout the growing season but from vineyard debris or nearby blackberries. They also indicate the benomyl resistance is not as stable as previously thought.