Most of the table grape production in the U.S. is located in the San Joaquin Valley. Rainfall at harvest is uncommon. However, when it does rain, gray mold can reach epidemic proportions if fungicides are not used. Gray mold is caused by a fungus that is activated by rainfall. The fungus produces a short tube with a suction cup and a peg that forces its way through the grape cuticle. Inside the grape, the fungus grows and exudes enzymes that degrade the fruit. Cracks form in the grapes and spores are produced that spread gray mold to other grape clusters. Even a single infected fruit within a table grape package can cause severe losses.
“Timing of fungicide applications to control gray mold is primarily driven in vineyard environments by the occurrence of rainfall. Rainfall at harvest, an uncommon event in the San Joaquin Valley of California, causes abundant production of inoculum and epiphytotics of the disease in vineyards, and fungicide applications are critically needed when this rare event occurs. This area, where most of the table grape production in the United States is located, is typically rainless throughout these periods and B. cinerea seldom causes significant vineyard bunch rot, but it routinely causes substantial postharvest decay if measures to control it are not taken.”
Authors: J.L. Smilanick, et al.
Affiliation: USDA ARS
Title: Control of postharvest gray mold of table grapes in the san Joaquin valley of California by fungicides applied during the growing season
Source: Plant Disease. 94:250-257. 2010.