1993 Volume 59 Issue 2 Pages 163-170
Histochemical reactions in root tissues of Japanese radish inoculated separately with two isolates of Peronospora parasitica were studied. The root tissues inoculated with Japanese radish downy mildew fungus (Jr isolate) showed little resistant reaction 24 hr after inoculation and well grown hyphae were found in the tissues. In the tissues inoculated with shepherd's purse downy mildew fungus (Sp isolate), developed haustorium formed by the Jr isolate was not found at all. Instead, small haustoria and many hypha-like structures (HLS) appeared in the tissues. Development of the haustorium and HLS stopped 24 hr after inoculation. Wall apposition at the site of HLS penetration by Sp isolate contained callose and lignin. The haustorial sheath of the Jr isolate contained callose but no lignin. The haustoria of the Jr isolate showed specific fluorescence of membrane-bound Ca2+ by chlorotetracycline staining. HLS of the Sp isolate, however, did not show any fluorescence. It is suggested that HLS cannot function as a haustorium. SITS (4-acetamido-4'-iso-thiocyanate-stilben-2, 2'-disulphonic acid) fluorescence was observed on the small haustorium of the Sp isolate but never on that of the Jr isolate. Haustorium formation of the Sp isolate in Japanese radish root tissues may be inhibited by some host defense reactions against the fungus. Differences in pathogenicity between Sp and Jr isolates seem to be due to the failure of haustorium formation by Sp isolate.
