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Weather Variables Associated with Infection of Lettuce by Downy Mildew (Bremia lactucae) in Coastal California. H. Scherm, Department of Plant Pathology, University of California, Davis 95616; A. H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 84:860-865. Accepted for publication 12 May 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-860.

Weather conditions and downy mildew (Bremia lactucae) development were monitored in 13 commercial lettuce fields in the central coast production areas of California during 1991 and 1992. Days on which infection likely had occurred were identified based on observations of the appearance of new lesions combined with quantitative information about the length of the latent period of the disease. Univariate (Kolmogorov-Smirnov tests) and multivariate (stepwise discriminant analysis) procedures were applied to statistically differentiate infection days from days on which infection had not occurred and to characterize infection days in terms of weather. With both methods of analysis and during both years, the duration of leaf surface wetness in the morning (AM-LWD) was the most important variable for infection. AM-LWD had a mean of 4.2 h (median 4.0 h) during infection days and a mean of 1.9 h (median 2.0 h) during days on which infection had not occurred. Additional weather variables did not help to differentiate between the two groups of days. Our results suggest that measured or predicted AM-LWD could be used in a decision support system for lettuce downy mildew management. The relevance of our findings is discussed within the framework of known cause-and-effect relationships regarding the infection cycle of B. lactucae.

Additional keywords: disease prediction, Lactuca sativa.