Influence of temperature and leaf wetness duration on infection of strawberry leaves by Mycosphaerella fragariae

In controlled environment studies, the influence of temperature and wetness duration on infection of strawberry leaves by Mycosphaerella fragariae was quantified by inoculating plants with a conidial suspension and incubating them at various combinations of temperature (5 to 35 degrees C) and leaf w etness duration (0 to 96 h). Infection was expressed as the number of lesio ns per square centimeter of leaf surface and relative infection was used to develop an infection model. Younger leaves were more susceptible to infect ion. Regardless of temperature and duration of leaf wetness, only few lesio ns developed on the oldest (19 to 21 days old) and intermediate leaves (12 to 15 days old), respectively (maximum of 1.7 and 2.3 lesions per cm(2)) as compared to the youngest leaves (5 to 7 days old; maximum of 12.6 lesions per cm(2)). On the youngest leaves, lesions developed at all temperatures e xcept at 35 degrees C, and the number of lesions, for all leaf wetness dura tions, increased gradually from 5 to 25 degrees C and decreased sharply fro m 25 to 30 degrees C. For temperatures of 15 and 20 degrees C, the number o f lesions increased gradually when leaf wetness duration increased from 12 to 96 h. At 25 degrees C, the number of lesions increased with increasing l eaf wetness from 12 to 48 h and then at a higher rate from 48 to 96 h. The optimal temperature for infection was 25 degrees C. For most temperatures, a minimum of 12 h of leaf wetness was necessary for infection (more than 1 lesion per cm(2)). Relative infection was modeled as a function of both tem perature and wetness duration using a modified version of the Weibull equat ion (R-2 = 0.98). The resulting equations provided a precise description of the response of M. fragariae to temperature. The model was sufficiently fl exible to account for most characteristics of the response of M. fragariae to wetness duration. The model was used to construct a risk chart that can be used to estimate the potential risk for infection based on observed or f orecasted temperature and leaf wetness duration.