Comparative Studies on Infection Behavior of Four Isolates of Peronospora parasitica Differing in Pathogenicity to Cotyledons of Cruciferous Plants

Pathogenicities of four isolates of the downy mildew fungus of cruciferous plants, Peronospora parasitica, obtained from Japanese radish, oil seed, broccoli and shepherd's purse, were compared by observing infected cotyledons. These isolates eventually sporulated on host plant cotyledons (compatible combination), but formed only small necrotic lesions on non-host cotyledons (incompatible combination). Sheath encasement of haustoria and host cell death frequently occurred in the incompatible combinations. Growth of intercellular hyphae stopped within 48hr after inoculation. The shepherd's purse isolate could only infect the host plant; haustorium formation in non-hosts was inhibited mainly by papillae. Cytochemical observation showed that these papillae contained callose and polyphenol compounds but not lignin. The significance of these results is discussed in relation to pathogenicities of P. parasitica isolates and plant defense reactions.