A diagnostic kit developed by researchers at the University of California, Davis, and DuPont is now being used to quickly and accurately determine the level of bunch rot in harvested wine grapes. For more than 40 years, the level of rot in harvested wine grapes has been determined by hand-separating the diseased grapes from the healthy grapes, weighing them and converting the results to a percentage of mold. Although reliable, the process is slow, and it can't be applied to machine-harvested grapes, because the diseased grapes are crushed during harvest. "This technique is accurate, reproducible and can be performed in less than 15 minutes using a juice sample from grapes crushed for sugar and acid analysis," said James J. Marois, a UC Davis professor of plant pathology. Marois is part of a research team asked in 1986 by the California Department of Food and Agriculture to develop a simple, repeatable test that could quantify mold levels ranging from 0.5 percent to 3 percent. Working on the same principle as an at-home pregnancy test, the biological test, called an immunoassay, is targeted at detecting Botrytis cinerea, one of the primary molds associated with bunch rot in grapes. The mold attacks unripened grapes on the vine during high humidity and grows on and through the grape's skin, leaving it vulnerable to attacks by other molds, yeasts and bacteria. Although it presents no human health threat, bunch rot decreases yield and makes the grapes less suitable for winemaking. Truckloads of harvested grapes are monitored by CDFA inspectors before they are delivered to the wineries. A high level of rot may be justification for rejecting a load of grapes or for reducing the price paid by the winery to the grower. The new immunoassay kits are based on the "rapid membrane technique." A 1-inch square cassette holds a filter-type membrane to which the grape juice and the substances necessary for the chemical reaction are added. A color change occurs on the membrane correlating to the level of mold. The higher the level of mold in the grape juice sample, the darker the color change resulting from the chemical reaction. The color is quantified using an instrument with a light meter and a printer that directly prints out the results. That instrument also can be linked to a computer for direct transfer of the information into a data base. During 1,000 tests administered at wine grape inspection stands during the 1991 and 1992 harvests, the new immunoassay kits provided quick, accurate and reproducible results, Marois said. "Each immunoassay costs about $15 and will increase the speed and reliability of the analysis," he said. The immunoassay kits also could be used to monitor rot in other crops, such as strawberries and tomatoes, which are also very susceptible to Botrytis cinerea, he added. Funding for the UC Davis research was provided by DuPont, the CDFA Winegrape Inspection Committee, the American Vineyard Foundation and UC Davis. DuPont is manufacturing the kit as part of its expanding line of diagnostic kits.