The relationship between conidium and oospore production in spinach leaves infected with Peronospora effusa (Grev. ex Desm.) Ces.

Four isolates of the spinach downy mildew pathogen, Peronospora effusa were used. Two isolates, K1 and K2, belonged to the mating type P1, and two isolates, M1 and A1, to the mating type P2. Seedlings of spinach at the cotyledon stage were inoculated with conidia of each isolate or a mixture of conidia in paired combinations of two mating types (isolates K1 and M1, or isolates K2 and A1) in which each conidial suspension was mixed in equal volume before inoculation. The leaves infected with conidia of either one of the mating types, P1 or P2, produced only conidia from the 3rd day after inoculation. The leaves infected with a mixture of conidia of the two mating types, P1 and P2, produced oogonia and oospores from the 4th and 5th day after inoculation, respectively, and in those leaves conidium production was delayed and suppressed. Cotyledons were inoculated with a mixture of conidia in paired combinations (isolates K1 and M1, or isolates K2 and A1) in the mixing ratios of 100:1, 1:1, 1:100(v/v), and the incidence of oospore production as well as intensity of conidium production in the infected leaves was observed. In the infected leaves which produced oospores at a high level, conidium production was reduced; whereas in the infected leaves which produced oospores at a low level, abundant conidia were formed. Moreover, the site of conidium production was completely different from that of oospore production. From these results, it was demonstrated that the leaves infected with a mixture of conidia of two mating types formed oospores in the early days after infection without producing conidia, and in those leaves conidium production was suppressed because the site where oospores were produced was different from that where conidia were formed.