Effect of pea cultivar, pathogen isolate, inoculum concentration and leaf wetness duration on Ascochyta blight caused by <i>Mycosphaerella pinodes</i>

Abstract

The effect of host leaf wetness duration, Mycosphaerella pinodes inoculum concentration and pathogen isolate on the latent period and the incubation period of the pathogen or disease severity were quantifi ed on pea (Pisum sativum L.). Seedlings of two widely grown pea cultivars, Onward and Merveille de Kelvedon, respectively susceptible and moderately resistant to M. pinodes were subjected to six leaf wetness durations of 6, 12, 24, 48 and 72 h, and inoculated with fi ve inoculum concentrations, 2.5×103, 4×104, 3.5×105, 4x106, and 5.2×107, in order to determine whether the cultivars reacted differently to M. pinodes isolates inoculated under identical conditions. Increasing the duration of leaf wetness and inoculum concentration caused signifi cant (P<0.001) increases in disease severity within each cultivar. Both the incubation period and the latent period decreased with increasing conidial concentration and leaf wetness duration. Generally, the cv. Onward had a signifi cantly shorter incubation period, and latent period and higher disease severity than cv. Merveille de Kelvedon. Isolates differed in aggressiveness at higher levels of leaf wetness (48 h) duration and of inoculum concentration (4×106), but there was no signifi cant interaction between isolates and leaf wetness duration, or between isolates and inoculum concentration. The optimum levels for obtaining a consistent infection and for readily separating the susceptible and the partially resistant cultivars were a leaf wetness of 48 h and an inoculum concentration of 4×106. The study also showed that continuous leaf wetness for 48 h was a threshold for application of fungicides to control the fungus in the susceptible cultivar.