The life cycle of Stemphylium vesicarium, the causal agent of Welsh onion leaf blight

Symptoms of Welsh onion leaf blight, caused by Stemphylium vesicarium, are divided into two types, i.e., brown oval lesions and yellow mottle lesions. Yellow mottle lesions exert considerable economic damage on Welsh onion in northern Japan. In this study, we investigated the life cycle of the pathogen in terms of seasonal fluctuation of spore dispersal and its relationship with development of disease, formation period of pseudothecia and overwintering of the pathogen based on field surveys, spore trapping and fungal isolation. Conidia were trapped throughout the cropping season except before mid June, when no ascospores were trapped. Brown oval lesions, which contained a large number of conidia, usually occurred in July followed by yellow mottle lesions with an increasing number of conidia trapped. These observations suggest that conidia released from brown oval lesions play an important role as a secondary inoculum source of the disease, leading to the development of yellow mottle lesions. Pseudothecia on leaves were first observed at the end of the cropping season or immediately after harvest (late October). The pathogen overwintered in the form of pseudothecia produced on leaves with or without symptoms. Ascospores failed to be trap in the field during the interval between before and beginning of the cropping season in April–May. However, pot experiments demonstrated that ascospores were released from leaf debris in November and rapidly increased in number after snow melt. From this circumstantial evidence, we hypothesize that ascospores are the primary inoculum source of Welsh onion leaf blight.