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 sulfur dioxide, but this can make the fruit more difficult to export or market domestically to some buyers. We applied a variety of treatments to replace sulfur dioxide that avoided its residues, and some were effective enough to merit consideration for commercial use by growers. This research provided insights into a new approach to increase the storage life of fresh table grapes use that could be adopted as a practical management tool for grape growers.
Technical Abstract: Gray mold, caused by B. cinerea, causes severe losses since it spreads easily among berries during cold storage. Currently, it is controlled by fumigation with SO2 or SO2 emitting sheets within boxes. Alternative methods, such as storage in ozone atmospheres, are needed because SO2 is banned in organic agriculture and consumer preferences to avoid SO2 residues. The effectiveness of three alternative treatments to control postharvest gray mold alone or in combination with ozone atmosphere storage were evaluated. About 1.5 kg of freshly harvested, organic ‘Princess Seedless’ grapes were dipped for 60 seconds in 1% vol/vol hydrogen peroxide (30% a.i. hydrogen peroxide, Brenntag Pacific, Inc., Fresno, CA) containing 6mM copper sulfate (Sigma-Aldrich Chemical Co., St, Louis, MO), 1% wt/vol chitosan (99% a.i. chitosan, Chito Plant, ChiPro GmbH, Bremen, Germany), or 0.5 % wt/vol potassium sorbate (99% a.i. potassium sorbate, Fruit Growers Supply, Exeter, CA) and placed into clamshell boxes. All were stored at 2C, either in air or ozone at 150 parts per billion (ppb). Ozone concentration in the storage room was maintained automatically with a system that employed an air drier, oxygen concentrator and corona-discharge ozone generator (Purfresh, Inc. Fremont, CA). For comparison purposes, grapes were either not treated or an SO2 sheet (3 g anhydrous sodium bisulfate, Uvas Quality Grape Guard, Imal Ltda, Santiago, Chile) was placed in boxes. Five or six clamshell boxes were prepared for each treatment. Two single berries were infected by the injection of 20 µl of a suspension containing 106 conidia of B. cinerea (isolate 1440) per ml and placed in each side of each clamshell after treatment at the beginning of storage. After 3 weeks at 2oC, natural incidences of gray mold and other rots were recorded. Spread of gray mold from the inoculated berries was recorded as the number of infected berries near it. Aerial mycelial growth on inoculated berries, shatter (naturally detached berries) and rachis appearance were evaluated. Chitosan, potassium sorbate, or hydrogen peroxide + copper sulfate, followed by storage in air or ozone, reduced the incidence of natural gray mold and other rots. Ozone consistently reduced the spread of gray mold from the inoculated berries, but was not very effective alone. The combination of ozone storage with treatments of chitosan, potassium sorbate or hydrogen peroxide + copper sulfate generally had both a lower incidence of gray mold and other rots and a reduced rate of gray mold spread from inoculated berries. The number of shattered berries was significantly reduced by all the treatments. The rachis appearance was variable and possibly harmed by potassium sorbate. Although none matched the effectiveness of the SO2 emitting sheets, these alternative treatments combined with constant low-level ozone storage were promising, although the use of processes that wet the grapes after harvest are not used by most grape growers.