Ecologic and Taxonomic Studies on Pythium as Pathogenic Soil Fungi : IX. Effect of Inorganic Nitrogen on Vertical Distribution and Pathogenicity of Several Soil-borne Plant Pathogens

摘要

1. The present paper deals with the comparative study on the effects of inorganic nitrogen on vertical distribution and pathogenicity of soil-borne disease fungi such as Pythium aphanidermatum, Rhizoctonia solani and Sclerotium rolfsii, and on the effects of nitrate and nitrite on the vertical distribution in the soil of different isolates of R. solani. 2. The mycelial growth intensity of R. solani at different depths of soil was enhanced by addition of KNO_3 and KNO_2 unlike when (NH_4)_2SO_4 was added. Similar enhancement of mycelial intensity and vertical distribution at different depths of the soil when 5 to 40 g of KNO_3 was added. Both mycelial growth intensity and vertical distribution have shown similar curves to those obtained with KNO_3 when 5 to 20 g of KNO_2 was added. When 40 g of KNO_2 was added the growth of the fungus was suppressed probably due to the excess supply of KNO_2. 3. The addition of KNO_3 and KNO_2 increased mycelial growth intensity of subterraneantype of Rhizoctonia in the deeper layer of the soil but had no effect on P. sasakii occurring in aerial and surface environment. 4. The early stage of disease development caused by R. solani was significantly promoted and acute symptom of damping-off appeared when any form of nitrogen was supplied. Disease development was the most severe when ammonium sulphate was added. Ammonium and urea nitrogen stimulated mycelial extension towards the upper layer of the soil while the KNO_3 and KNO_2 promoted the downward growth in the deeper layer. 5. The disease incidence caused by P. aphanidermatum was enhanced by the application of ammonium or nitrate nitrogen. When KNO_3, NH_4NO_3 and (NH_4)_2SO_4 were added, mycelial intensity was remarkably increased in the deeper layer of the soil but it was inhibited by KNO_2 and NH_2CONH_2. 6. The mycelial growth and pathogenicity of S. rolfsii were equally unaffected by all nitrogen forms except KNO_2. Abundant mycelium was observed only on soil surface and scarcely observed in the lower layer of the soil.