WEATHER VARIABLES ASSOCIATED WITH INFECTION OF LETTUCE BY DOWNY MILDEW (BREMIA-LACTUCAE) IN COASTAL CALIFORNIA

Weather conditions and downy mildew (Bremia lactucae) development were monitored in 13 commercial lettuce fields in the central coast produc tion areas of California during 1991 and 1992. Days on which infection likely had occurred were identified based on observations of the appe arance of new lesions combined with quantitative information about the length of the latent period of the disease. Univariate (Kolmogorov-Sm irnov tests) and multivariate (stepwise discriminant analysis) procedu res were applied to statistically differentiate infection days from da ys on which infection had not occurred and to characterize infection d ays in terms of weather. With both methods of analysis and during both years, the duration of leaf surface wetness in the morning (AM-LWD) w as 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 weath er variables did not help to differentiate between the two groups of d ays. Our results suggest that measured or predicted AM-LWD could be us ed in a decision support system for lettuce downy mildew management. T he relevance of our findings is discussed within the framework of know n cause-and-effect relationships regarding the infection cycle of B. l actucae.