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Subject

Raisin Molds and Rots

Description

Mold infection on underside of drying fruit on a paper tray.

Molds and rots of raisins are associated with drying the fruit that becomes infected before harvest, during it, and/or as a result of rainfall occurring during drying. Fruit, vineyard, and weather conditions strongly influence the degree of infection. Fruit with tight clusters, mechanical damage during harvest, powdery mildew infections, or rot initiated by omnivorous leafroller or other pests is most susceptible. Rain on drying fruit in north-south oriented rows or on poorly prepared terraces will incur more damage. Although the amount of rainfall received during drying is an important factor, overall weather conditions, such as clouds and high humidity following a storm, influence the amount of damage. For example, the southerly tropical storms received in California during the raisin harvest of 1976, 1978, and 1982 resulted in disastrous industry losses. Rains in excess of 2.5 cm (1 inch) were followed by a week or more of poor drying weather. More typically, northerly storms drop 0.6 cm (0.25 inch) or less and are followed by wind, sunlight, and drier air.

Raisin mold is associated with a wide range of primary and secondary fungal organisms normally found in the vineyard. They include species of Alternaria, Aspergillus, Botrytis, Chaetomium, Cladosporium, Helminthosporium, Hormiscium, Hormodendrum, Penicillium, Rhizopus, and Stemphylium. Yeasts and driedfruit beetle are also important secondary contaminants.

Cultural practices and time of harvest can decrease the risk of raisin molds and rots and rain damage. Powdery mildew infections and bunch rot can be minimized with fungicide treatments, insect control, bloom-thinning gibberillin treatment, and irrigation and canopy management practices. Placing trays on a firm, smooth, steep terrace reduces water retention on trays after a rain. Also, minimizing crushed or broken berries during harvest, keeping clusters with rot off trays, and spreading the clusters evenly in a single layer can also reduce damage.

Fungicide treatments have also successfully reduced mold infections. Studies show that such materials must be applied before a rain to be effective. Post-rain treatments have not been effective, largely because infections have already occurred by application time and much of the mold originates on the inaccessible bottom fruit on the tray. Only use registered materials according to label restrictions.

People

Pete Christensen

Publications

Christensen, L.P. 1992. Raisin Molds and Rots. Page 111-112 in: Grape Pest Management, 2nd edition. University of California Division of Agriculture and Natural Resources Publication 3343, Oakland, CA.

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