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Colonization dynamics of red raspberry flowers and fruit by Botrytis cinerea

Olga Kozhar: Washington State University

<div><em>Botrytis cinerea</em>, causal agent of gray mold, is the major pathogen of raspberry in the US Pacific Northwest (PNW) and worldwide, and fungicide applications are the main disease management strategy. Fungicides are applied in PNW on a calendar basis rather than based on infection risk because the disease cycle on raspberry is poorly understood. The objective of this study was to determine the dynamics of flower and fruit colonization, including testing the hypothesis that the primary route of infection of raspberry is through flowers. <em>B. cinerea</em> was isolated from seven developmental stages of raspberry flowers and fruit in northwestern Washington in 2015-2016. Colonization of flowers by <em>B. cinerea</em> was 15%, increased as fruit developed, and peaked on mature fruit (67%). During flowering, the floral part colonized most frequently was carpel (80%). As fruit matured, colonization of other floral parts increased. In 2017, 400 <em>B. cinerea</em> isolates were collected from fertilized flowers, as well as green and ripe fruit from 40 raspberry inflorescences in two fields and genotyped using seven microsatellite markers. Number of colonies and genotypic diversity increased as fruit matured. In addition, different multilocus genotypes colonized flowers, green and ripe fruit in the same inflorescence, suggesting multiple infection events, and that flowers are not the major route of infection. These results will help time fungicide applications more effectively to control gray mold.</div>