Abstract
Effect of light and darkness on oospore formation of downy mildew fungus, Peronospora manshurica, of soybean, Glycine max, was investigated. Oospores were more abundantly formed in lesions floated on water at a light intensity of 20, 000lux than those in darkness. The ratio of total number of oogonia and oospores under light to that in darkness was 2.9-4.0. The total number of oogonia and oospores increased from the 3rd day of incubation and reached a maximum the 4th day of incubation in darkness. Under light, the total number of oogonia and oospores rapidly increased from the 2nd to 7th day of incubation. The oogonium was differentiated on the 3rd day of incubation and the oospore was formed on the 5th day of incubation in darkness. In contrast, under light, the oogonium was differentiated on the 2nd day of incubation, one day earlier than in darkness, and the oospore was formed on the 3rd day of incubation, 2 days earlier than in darkness. The number of oospores increased with the increase of incubation days in the light and photoperiod. Oospore formation was reduced by the treatment of lesions with 3-(3, 4-Dichlorophenyl)-1, 1-dimethylurea (DCMU) at concentrations of 10-6-10-4M under light. DCMU, however, did not affect the oospore formation at concentrations of ranging between 10-7-10-4M in darkness. This phenomenon supports the idea that oospore formation in lesions was decreased by means of the inhibition of photosynthesis of downy mildew lesions. It was concluded from these results that the oospore formation of soybean downy mildew fungus closely depends on the photosynthesis of host soybean leaves.
