Experiments on the Sporulability of Cucumber Downy Mildew Fungus in Lesion

Factors affecting the sporulability (capacity for sporulation) of cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., in leaf lesions were investigated. The sporulability was evaluated on the basis of the number of conidia formed on lesion of 1cm2 after being kept for 17hr in a moist chamber under darkness at 20°C.1. Relationship between the age of lesion and sporulability is shown in Fig. 1. The sporulability attained maximun 7 days after inoculation.2. The sporulability was affected by the weather condition of the day when infected leaf specimens were taken from the plant. Conidia were more heavily formed on leaf specimens taken on a fine day than those taken on a cloudy day (Table 1).3. To know the influence of sunlight on the sporulability, a half part of diseased leaf was covered with a sheet of black paper and the other half was left uncovered and exposed to direct sunlight from sunrise to 5p.m., being about 1hr before sunset. The sporulation in the lesion on the uncovered half, which was exposed to sunlight, was always greater than that in the lesion on the covered half, as shown in Table 2. The ratio of the number of conidia formed on the covered half to the uncovered half was 40-64: 100 in the first, 54-60: 100 in the second, and 30-71: 100 in the third experiment.4. The sporulability in lesions either with or without healthy part was compared by using lesions 6 days after inoculation. Conidia were more abundantly formed on lesions with some healthy part attached than on lesions without healthy part.It was considered from these results that the sporulability of cucumber downy mildew fungus closely depends on the photosynthesis of host cucumber leaves.