Submitted to: Plant Disease Management Reports
Publication Type: Research notes
Publication Acceptance Date: 3/1/2012
Publication Date: N/A
Citation: Interpretive Summary: Table grapes will rot after harvest and most common solution to control rot under commercial conditions is to apply fungicides before harvest and use sulfur dioxide after harvest, but residues of these can make the fruit more difficult to export or market domestically to some buyers. The purpose of this work was to establish the influence of these vineyard fungicide applications on postharvest decay to identify those fungicides effective in this role so the ineffective ones need not be applied so their residues would be avoided. This research provided insights into better management decisions for grape growers to increase the storage life of fresh table grapes so they can avoid the use of ineffective and needless residues in consumers' diets.
Technical Abstract: The purpose for this experiment was to quantify the effect of residues of vineyard fungicides on control of postharvest decay of table grapes. Mature (22% soluble solids content), freshly harvested ‘Princess Seedless’ grape clusters were arranged on metal racks. Fungicides were applied with an air brush sprayer to run-off at rates that approximated those emitted from a vineyard sprayer when the label maximum rates of each fungicide was applied in a water volume of 1,900 Lha-1. Actual active ingredient rates applied were: i) Pristine WG, pyraclostrobin and boscalid, 59 µgL-1 and 116 µgL-1, respectively; ii) Vangard, cyprodinil, 270 µgL-1; iii) Scala SC, pyrimethanil, 370 µgL-1; and iv) Elevate 50WDG, fenhexamid, 290 µgL-1. A single berry infected just before placement by the injection of 20 µl of a suspension containing 106 conidia of B. cinerea (isolate 1440) per ml was placed in the center of each cluster. Two boxes that contained 10 clusters each were prepared for each fungicide treatment; one was examined after storage at 1oC for 4 weeks and the other after 6 weeks. Observations included: i) the spread of gray mold, caused by B. cinerea, from the single infected berry placed within the cluster after fungicide treatment; ii) the natural incidence of gray mold infected berries; iii) the incidence of berries infected by other fungi; and iv) the residues of the applied fungicide after 6 weeks. A sample of 50 healthy berries was collected from each treatment for residue analysis by gas chromatography. Elevate 50 WDG, Scala SC, and Vangard significantly controlled the spread and natural incidence of gray mold among berries during cold storage. Elevate 50 WDG was particularly effective. Decay by other fungi, mostly Alternaria spp., was significantly controlled only by Vangard and Pristine WG. The residues of all of these fungicides were within US EPA tolerances. Gray mold is relatively rare in San Joaquin Valley vineyards before harvest under late summer conditions, but it can cause devastating losses during cold storage. Therefore, to obtain a significant level of control of postharvest gray mold and minimize the development of resistance among B. cinerea populations in vineyards, the use of Elevate before harvest, and not at other times, would be a practice supported by the results of this experiment. Vineyard applications should be directed into the clusters as much as possible to improve coverage of the berries and avoid spraying the canopy where the fungicide is not needed.