Evelyn Alvarez-Mendoza, Samantha Anderson, Stephen Chang, Evan Tamayo, and Shunping Ding*
*California Polytechnic State University, San Luis Obispo, 1 Grand Avenue, San Luis Obispo, CA, 93407 (sding01@calpoly.edu)

Botrytis bunch rot, caused by Botrytis cinerea, is a major concern to grapegrowers and wine producers in California due to significant crop losses and reduction of fruit and wine quality. The increasing occurrence of fungicide resistance in B. cinerea pop­ulations poses a challenge for effective management of this disease in vineyards. In 2020, 35 B. cinerea isolates were screened against the fungicides Scala, Flint, Endura, Elevate, Rovral, and Scholar to determine their sensitivities to these different active ingredients. Grape cluster samples exhibiting Botrytis growth were collected random­ly from six conventional and two organic vineyards in five geographically different locations in the Central Coast of California. Pure culture isolates of B. cinerea were tested against discriminatory doses of each fungicide. Radial mycelial growth was measured following a five to seven day incubation. Measurements were converted to percent relative growth values for four single-colony replicates per isolate. Isolates were separated into four sensitivity categories: highly sensitive, moderately sensitive, highly resistant, and moderately resistant. Of the 35 isolates screened, 88.6, 97.1, and 74.3% were highly resistant to Scala, Flint, and Endura, respectively. Only 5.7 and 2.9% of isolates showed moderate resistance to Scala and Endura, respectively. 62.9 and 50% of the isolates were moderately sensitive to Rovral and Elevate, respective­ly. Isolates exhibited high sensitivity to Scholar (82.9%), Elevate (41.2%), and Rovral (37.1%). A mixed model statistical analysis was then used to identify the effect of type of fungicide and fungicide dosage (mg/mL) on the growth (cm) of resistant and sensitive B. cinerea isolates. Pathogen isolates were obtained again in 2021 to determine if similar resistance frequency patterns were repeated. These results can be used by grapegrowers to implement effective fungicide spray programs for control of Botrytis bunch rot in the Central Coast of California.

Funding Support: CSU Agricultural Research Institute (ARI)