Observations on the sporulation of cucumber downy mildew fungus.

Abstract

Sporulation process of cucumber downy mildew fungus (Pseudoperonospora cubensis) was studied, and microphotographed by use of the methods adopted by Toyoda and Suzuki (1952) for rice blast fungus.The diseased leaf was detached at 7.00-8.00 p.m. from the potted cucumber plants grown in the greenhouse, and its surface was washed with water to remove the existing conidiophores. The lesion was then cut off into a piece of 1×2cm. and placed in a moist cell under a microscope in a dark room, at 22-26°C. for periodical observation.After the leaf lesion was placed in the moist cell, conidiophores emerged through the stomata from 2 to 4 1/2 hours, and began to branch dichotomously from 6 1/2 to 9 hours. They continued to branch in the same manner until they had 3 to 16 ultimate branchlets. 20 to 50 minutes after the beginning of branching, a small hyaline projection was produced almost simultaneously at each ultimate branchlet of a conidiophore. This projection developed, to a mature conidium 50 to 70 minutes later, turning grey.When the conidiophore which had begun to branch was subjected to unfavorable conditions such as drying, it stopped to branch and turned to produce conidia.