Mycosphaerella ribis in Blackcurrant: Overwintering Sources of Inoculum and Spore Release Patterns

Disease symptoms, sources of inoculum, and patterns of spore release of Mycosphaerella ribis, the cause of Mycosphaerella leaf spot, were studied over 3 years in an organic blackcurrant planting receiving no fungicide applications. In addition to typical foliar symptoms, fruit lesions were also observed on the cultivars included in the study. Ascospores from leaf litter on the ground were trapped from bud break in April to mid-to-late July, but 99% were released by 1 month before. Conidia formed in old fruit cluster stalks overwintering on the blackcurrant shrubs were present from bud break to early August, but 99% were trapped from late May to mid-July. Conidia were found in leaf litter but were never captured in the spore trap, and ascospores were observed in old fruit cluster stalks. Degree-day models (base = 0°C) were used to estimate the proportion of mature spores. Extended periods of dry conditions slowed spore maturation in the field. Models halting degree-day accumulation after 4 or 7 days with no rain (<0.2 mm) or leaf wetness of <12 h per day gave the best performance for ascospore or conidial release, respectively, if validated by data from controlled conditions in the laboratory. Ascospore release was suppressed during night, and if rain and wetness started during night and continued the following day, very few spores were released before sunrise. The present investigation provides new information that may be used when planning sanitary measures to reduce primary inoculum and predict spore release patterns for Mycosphaerella leaf spot.