Abstract
Effect of light and darkness on hyphal growth of cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., in the host tissue was examined anatomically. The effect was estimated by counting the number of hyphae which was observed in the stained paraffin section of infected tissue by a microscope. Hyphae and haustoria in the infected tissue kept in both light and dark condition, began to increase in number abruptly from 2 to 3 days after inoculation and continued to increase until 5 days. The hyphae and haustoria in dark condition, however, were about a half in number compared with those in light condition. Some differences of hyphal diameter in average were also observed between the two treatments. It was 5.4μ in lesion kept in light and 4.6μ in darkness. It is speculated that such decrease in hyphal density and diameter in darkness may affect the suppression of lesion development by the dark treatment for the infected cucumber leaves as reported in the previous paper.
