Effects of interrupted wet periods and different temperatures on the development of ascochyta blight caused by Mycosphaerella pinodes on pea (Pisum sativum) seedlings

The effect of interrupted wet periods on pycnidiospores of Mycosphaerella p inodes was studied by assessing spore viability, infection and disease deve lopment on pea seedlings. Pycnidiospores survived dry periods of up to 21 d ays after inoculation. Rewetting restored the infection capacity of the pyc nidiospore, resulting in high levels of disease. The effects of wet-dry-wet cycles depended on when the dry period occurred during the infection proce ss. No disease symptoms appeared when dry periods occurred during germinati on. A low level of disease occurred after rewetting in high relative humidi ty if the interruption of the wet period was long. However, a wet period re sulting in leaf wetness after a dry period gave similar levels of infection to those achieved with a continuous wet period. Pycnidiospores formed appr essoria but hyphae did not penetrate if a 6-12 h wet period preceded the dr y period, and only a few flecks appeared during the dry period. Coalescent necrosis occurred when the dry period followed penetration. The disease was severe in each case when planes were returned to wet conditions after a pe riod of dryness. Lesion development depended on the duration of the initial wet period, and the characteristics (temperature and duration) of both the dry period and the final wet period.