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Effects of Fluctuating Temperatures on the Latent Period of Lettuce Downy Mildew (Bremia lactucae). 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 84:853-859. Accepted for publication 12 May 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-853.

The effects of fluctuating temperatures on the length of the latent period of downy mildew (Bremia lactucae) were studied on six lettuce cultivars. Potted plants were inoculated with B. lactucae, incubated at 13 C for 12 or 24 h to initiate infection, and subjected to different temperature treatments by moving them to growth chambers operating at diurnally alternating temperatures, moving them outdoors to naturally varying temperatures, or holding them at constant conditions. Subsets of plants incubated at fluctuating temperatures were returned to constant conditions at 24-h intervals to simulate a wide range in thermal exposure during the latent period. Cumulative numbers of sporulating lesions were recorded for each plant daily from 5 to 14 days after inoculation. Minimum and median latent periods (LP0 and LP50, respectively) were determined for each cultivar and temperature treatment at the end of the experiments. LP0 and LP50 were expressed as days, in degree-hours (accumulated hourly temperatures above a threshold of 0 C), and in units of accumulated hourly developmental rates derived from a nonlinear function fit to values of LP0 from a previous constant-temperature study. We found that the variability in LP0 and LP50 was greater when latent periods were measured in degree-hours (coefficient of variation [CV] between 6.2 and 30.9%) or developmental rates (6.4% ≤ CV ≤ 48.6%) than when they were measured in days (4.0% ≤ CV ≤ 17.5%). Thus, there seems to be little advantage in using degree-hours or developmental rates to model latent periods of lettuce downy mildew. Values of LP0 were shorter at low and longer at high temperatures with fluctuating temperatures compared to constant temperatures with the same mean. Possible reasons for the difference between constant- and fluctuating-temperature data are discussed.

Additional keywords: Lactuca sativa.