Temperature and humidity requirements for germination and infection by ascospores of Pleospora alii, the teleomorph of Stemphylium vesicarium

Pleospora allii is the teleomorph of Stemphylium vesicarium, the causal agent of the brown spot disease on pear. The ability of P. allii ascospores to cause infection has not yet been demonstrated, and no information is available on environmental conditions favouring ascospore germination and infection. These ecological aspects were investigated by environment-controlled experiments. Dynamics of ascospore germination were observed between 0.5 and 48 hours of incubation at different temperatures (T 5 to 35°C), in water or in dry conditions, with relative humidity (RH) between 100 and 67%. Maximum germination occurred after 48 hours of incubation in water at 21-23°C; few ascospores germinated below 15°C and at 30-35°C. At 100% RH germination decreased by about one third and no germination was observed below 80%. Ascospores were inoculated on leaves of three pear varieties showing different susceptibility under orchard conditions (‘Abate Fétel’, ‘Conference’, and ‘William’ in decreasing order of susceptibility). Leaves were incubated at different T (5 to 35°C), 100% RH, and observed daily for the appearance of necrotic spots. Ascospores caused higher infection on ‘Abate Fètel’ than on ‘Conference’, while sporadic symptoms were observed on ‘William’. Highest disease incidence occurred at 25°C.