A Comparative Study on the Pathogenicity of Some Species of Saprolegniaceae and Pythium on Rice Seedlings

The rot-disease of seedlings in nursery-beds is one of the most serious diseases of the rice plant in Japan. This disease had been recognized by some investigators to be caused by a water mould, Achlya prolifera (NEES) DEBARY (Achlya prolifera NEES). However, in 1931 and 1933, ITO and his coworkers reported that the disease is caused by some species of Saprolegniaceae, Pythiomorpha and Pythium.The present paper deals with the results of the writer's inoculation experiments of some aquatic fungi on rice seedlings, in which seven species belonging to Saprolegniaceae and nine species of Pythium were used. The names of these sixteen fungi, which were isolated by the writer from water in Kyoto, are as follows: Saprolegnia Thureti DEBARY, S. mixta DEBARY, S. monoica PRINGSH. var. glomerata TIESENHAUSEN, Achlya racemosa HILDEBRAND, A. imperfecta COKER, A. Oryzae ITO et NAGAI, Aphanomyces helicoides v. MINDEN, Pythium tenue GOBI, P. angustatum SPARROW, P. torulosum COKER et PATTERSON, P. aphanidermatum (EDSON) FITZPATRICK, P. helicum ITO, P. proliferum DEBARY, P. marsipium DRECHSLER, P. polypapillatum ITO, and P. pleroticum ITO.The writer's inoculation experiments were carried out using the seedlings grown on SACHS' solution containing 2% agar or the sterilized field soil covered with water. In the experiments each fungus to be tested were inoculated on the seeds as soon as they were sown.The pathogenicity of Pythium aphanidermatum and P. helicum was extremely strong and that of the other seven species of Pythium seemed to be moderate, but all the species tested, which belong to Saprolegniaceae, showed a very weak pathogenicity. The pathogenicity of these aquatic fungi was suggested to be more destructive when the rice plants were cultivated at lower temperatures.Judging from the writer's experiments Pythium species seem to be more important than the fungi belonging to Saprolegniaceae as the causal organisms of the rot-disease of rice-seedlings.