EFFECTS OF FLUCTUATING TEMPERATURES ON THE LATENT PERIOD OF LETTUCE DOWNY MILDEW (BREMIA-LACTUCAE)

The effects of fluctuating temperatures on the length of the latent pe riod of downy mildew (Bremia lactucae) were studied on six lettuce cul tivars. Potted plants were inoculated with B. lactucae, incubated at 1 3 C for 12 or 24 h to initiate infection, and subjected to different t emperature treatments by moving them to growth chambers operating at d iurnally alternating temperatures, moving them outdoors to naturally v arying temperatures, or holding them at constant conditions. Subsets o f plants incubated at fluctuating temperatures were returned to consta nt conditions at 24-h intervals to simulate a wide range in thermal ex posure during the latent period. Cumulative numbers of sporulating les ions were recorded for each plant daily from 5 to 14 days after inocul ation. Minimum and median latent periods (LP(0) and LP(50), respective ly) were determined for each cultivar and temperature treatment at the end of the experiments. LP(0) and LP(50) were expressed as days, in d egree-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 LP(0) from a previous constant-t emperature study. We found that the variability in LP(0) and LP(50) wa s greater when latent periods were measured in degree-hours (coefficie nt of variation [CV] between 6.2 and 30.9%) or developmental rates (6. 4% less than or equal to CV less than or equal to 48.6%) than when the y were measured in days (4.0% less than or equal to CV less than or eq ual to 17.5%). Thus, there seems to be little advantage in using degre e-hours or developmental rates to model latent periods of lettuce down y mildew. Values of LP(0) were shorter at low and longer at high tempe ratures with fluctuating temperatures compared to constant temperature s with the same mean. Possible reasons for the difference between cons tant- and fluctuating-temperature data are discussed.