Abstract
Haustorium formation of the downy mildew fungus Peronospora parasitica Pers. ex Fr. was suppressed in blocks of Japanese radish (Raphanus sativus L. var. hortensis Backer) root tissues (5×5×3mm) treated with 50°C for 30 to 60sec. The fungus penetrated directly into cells of the tissues by a hypha-like structure (HLS) instead of the haustorium. Fluorescence from chlortetracycline, specific for membranebound Ca2+, was found on the haustorium but not on the HLS. HLS formation was highest in the tissues treated with 50°C/60sec. In heat-treated root tissues, growth of the haustorium and the HLS stopped within 24hr after inoculation. The suppression of haustorium formation disappeared 24hr after the 50°C/30sec treatment but still remained 48hr after the 50°C for 60sec treatment. Lignin and callose did not accumulate at the HLS penetration site in the heated root tissues. Colletotrichum lagenarium, a nonpathogenic fungus, penetrated heated tissues and grew very well. Heat-treated root tissue homogenate did not suppress haustorium formation of the fungus. These results suggest that the suppression of haustorium formation of downy mildew fungus by heat treatment does not involve known host defense reactions against the fungus. The heat treatment probably disturbs processes within the host which are associated with haustorium formation of P. parasitica.
