The Biology and Control of Botrytis Bunch Rot
Botrytis bunch rot observed at harvest is often the result of infections initiated at bloom. Typically, the vast majority of such early infections remain latent (dormant) through harvest, and never cause disease. However, a fraction of these latent infections can become active as the crop matures, causing affected berries to rot and subsequently serve as sources for disease spread during the preharvest period. The factor or factors that promote activation of latent infections are poorly understood. In the research conducted last year, we found that by increasing the nitrogen content of berries after veraison, we increased the likelihood that latent infections initiated at bloom will transition to rotten berries at harvest. Higher N content also appeared to increase the susceptibility of berries to disease resulting from post-veraison infections via berry-to-berry contact within the cluster or air-blown spores from external sources. Confirming anecdotal observations, we furthermore determined that activation of latent infections was promoted significantly by periods of high humidity during the 10-day period before harvest, and that the effect was directly proportional to the duration of the humid period. In contrast, similar humid periods at veraison had no effect in this experiment. In field trials, we found that both Elevate and Vangard provided limited but significant postinfection control of latent infections when these fungicides were applied during the first few days after flowers were inoculated with spores of the Botrytis fungus. We also found that sprays designed to provide protective activity when applied before infection were equally effective when sprayed before or after cap fall. If substantiated, this finding suggests that the pedicel (berry stem), rather than stamens (flower parts), may be an important court for the establishment of latent infections. On a practical level, it also would be significant in by providing an element of flexibility in the timing of such sprays.