STUDIES ON BROWNING ROOT ROT OF CEREALS: II. SOME PARASITIC SPECIES OF PYTHIUM AND THEIR RELATION TO THE DISEASE

Abstract

Evidence is presented which shows that browning root rot of cereals is caused primarily by species of Pythium the most important of which are P. arrhenomanes Drechsler var. canadensis n. var. and P. volutum n. sp. Soil conditions, especially those following summerfallow, and seasonal climatic factors also play a necessary role. Under experimental conditions Pythium injury to cereals manifests itself as an embryo rot or as pre-emergence killing of the seedlings, as post-emergence blighting, or as retarded development throughout the life of the plant, due to the impairment of the root system especially during the seedling stage. Both the spring and winter wheats are susceptible.The isolation and inoculation methods found convenient in the study of the problem are outlined. P. arrhenomanes var. canadensis is widely distributed over the province, whereas P. volutum appears to be more limited in its range. Specific diagnoses of these two species and a discussion of their taxonomy are given. Other less aggressive species of Pythium undoubtedly contribute to the disease complex.Comparative experiments show that Pythium injury to wheat may be as severe as that caused by Ophiobolus graminis or by Helminthosporium sativum. In general, P. arrhenomanes var. canadensis, the Louisiana sugar-cane Pythium and P. arrhenomanes are similar in their degree and range of parasitism, whereas P. volutum shows marked differences. Experimental evidence obtained under controlled conditions indicates that the damage caused to young wheat plants by P. arrhenomanes var. canadensis increases with both increasing soil temperatures and soil moistures. No correlation has been found between the hydrogen ion concentration of the soil and the distribution of the disease. Both P. arrhenomanes var. canadensis and P. volutum will grow in nutrient solutions with a lower pH value than that of any prairie soil tested, but optimum growth for both species occurs at neutrality. No conclusive results have as yet been obtained as to the effects of various fertilizers on the disease under artificial conditions.