Comparative studies on infection behavior of four isolates of Peronospora parasitica differing in pathogenicity to cotyledons of cruciferous plants

摘要

The pathogenicity of 4 isolates of P. parasitica, obtained from Japanese radish, rape, broccoli and shepherd's purse [Capsella bursa-pastoris], 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. The C. bursa-pastoris isolate could only infect the host plant with haustorium formation in non-hosts 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 the pathogenicity of P. parasitica isolates and plant defense reactions.