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Grape Cluster Architecture and the Susceptibility of Berries to Botrytis cinerea. M. E. Vail, Former graduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Present address: Viticulture Production Specialist, Crop Care Associates, Inc., P.O. Box 288, St. Helena, CA 94574.; J. J. Marois, associate professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 81:188-191. Accepted for publication 17 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-188.

Detached mature grape berries from seven cultivars were inoculated with conidia of Botrytis cinerea over a range of 102106 conidia per milliliter. The percentage of diseased berries was determined after 7 days at 25 C and 95% relative humidity. The slope of the inoculum density-disease incidence response, used as an index of susceptibility, ranged from 0.07 for Emperor to 2.13 for Muscat of Alexandria in 1988. Cabernet Sauvignon and Muscat of Alexandria berries were more susceptible than expected, considering the low level of disease usually observed in the field. Cluster tightness was also quantified for the seven cultivars. Cabernet Sauvignon and Muscat of Alexandria were less tight than the other cultivars and were below the methodology threshold of 0.0005 N. The cultivars most severely affected by bunch rot in the field had the tightest clusters, while cultivars with loose clusters were least affected in the field. Components of cluster architecture were measured and their respective contribution to cluster tightness determined by path analysis. Cluster weight and the ratio of interior to exterior berries contributed most to cluster tightness, with path coefficients of 0.35 and 0.25, respectively. The number of berries per centimeter of rachis, the component most commonly used by viticulturists to estimate cluster tightness, was least important in its contribution to cluster tightness (path coefficient of 0.09). The rate of surface water loss per hour was also determined for each of the clusters. Cabernet Sauvignon, which had a loose cluster, dried at a higher rate than did Barbera, Colombard, or Semillon, which had tighter clusters.

Additional keywords: Botrytis bunch rot, Vitis vinifera.