Location: Commodity Protection and Quality
Title: Control of postharvest gray mold of table grapes in the San Joaquin Valley of California by fungicides applied during the growing season Authors
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 13, 2009
Publication Date: January 4, 2010
Citation: Smilanick, J.L., Mansour, M., Mlikota Gabler, F., Margosan, D.A., Hashim-Buckey, J. 2010. Control of postharvest gray mold of table grapes in the San Joaquin Valley of California by fungicides applied during the growing season. Plant Disease. 94(2):250-257. Interpretive Summary: Table grapes rot after harvest unless actions are taken to retard this process. Sulfur dioxide gas is used to stop the rot, but it harms flavor and appearance of the grapes and leaves residues harmful to some people, so we evaluated pre-harvest applications of USEPA ‘reduced-risk’ fungicides to see if they could stop postharvest rot sufficiently so as to make postharvest sulfur dioxide fumigation unneeded. Unfortunately, although postharvest rot could be reduced significantly by vineyard applications of these fungicides, their effectiveness was insufficient to make postharvest sulfur dioxide fumigation unnecessary, even when several fungicide were applied repeatedly and directly to clusters in a vineyard with the canopy pruned to optimize thorough coverage.
Technical Abstract: Fungicides applied in vineyards before harvest were evaluated to control postharvest gray mold of table grapes, caused by Botrytis cinerea. Under the arid growing conditions of the San Joaquin Valley of California, it causes vineyard bunch rot rarely, but it often causes substantial postharvest decay. We determined the effectiveness of fungicides in tests with inoculated, detached berries and in practical vineyard tests, fungicide residues, and EC50 values to inhibit growth of B. cinerea isolates from vineyards in central California. Thiophanate methyl (THM), iprodione (IPR), cyprodinil (CYP), pyraclostrobin+boscalid (PS/BO), pyrimethanil (PYR), or fenhexamid (FEN) reduced colony size of 4 fungicide-sensitive isolates by 50% at 12.4, 2.5, 0.61, 0.29/0.57, 0.26, or 0.17 mg L-1, respectively. In a laboratory experiment, THM, IPR, CYP, PS/BO, PYR, or FEN were applied to run-off to detached ‘Thompson Seedless’ (TS) berries at the equivalent of maximum approved rates of 600, 500, 270, 59/116, 370, or 290 mg L-1, respectively, 24 or 48 h before or after inoculation with B. cinerea. Postharvest gray mold after 2 weeks at 15C was lowest after FEN, followed by PYR, CYP, IPR, PS/PO, or THM. In commercial vineyards, one application of FEN, PYR, CYP, or PS/PO two weeks before harvest reduced subsequent postharvest gray mold by approximately 50%. When fungicides were applied repeatedly in mixtures or alternated with fungicides in different mode of action classes beginning after berry set, postharvest gray mold was reduced about 50% when applied by a commercial air-blast sprayer and to between 70 and 87% when applied with a hand held, powered sprayer where excellent coverage was obtained. Fungicide applications near harvest provided most of the control of gray mold, because the benefit by the addition of earlier sprays was small. Fungicide sensitivity of 85 isolates collected from many vineyards was determined. Isolates with EC50 values of THM, IPR, CYP, PS/BO, and PYR, that exceeded the EC50 values of sensitive isolates by two times or more were common, while only a single isolate with reduced sensitivity to FEN was found. However, the effectiveness was insufficient to make postharvest sulfur dioxide fumigation unnecessary, of even the most aggressive fungicide regimes that were applied directly to clusters in a vineyard with the canopy pruned to optimize thorough coverage.