THE SEVERITY OF ROOT ROT AND PERSISTENCE OF PYTHIUM SPLENDENS IN GERANIUM CUTTINGS GROWN IN SOILLESS MIXTURES

Abstract

Four soilless mixtures, Cornell peatlite A, Yoder mixture, turface–vermiculite, and turface–peat, were compared with unsterilized soil–sand–peat, steam-sterilized soil–sand–peat, and autoclaved soil–sand–peat to determine the effects of the growing medium on the severity of root rot and blackleg of geranium (Pelargonium hortorum Bailey), and on the persistence of the causal fungus, Pythium splendens Braun. Dilution plates demonstrated that species of Trichoderma, Penicillium, and Streptomyces were present in the soil mixtures that were antagonistic to P. splendens in vitro. No antagonistic organisms were isolated from the soilless mixture with the exception of Cornell peatlite A which contained an antagonistic species of Penicillium. Two bacterial types showing definite antagonism to P. splendens were isolated from the unsterilized soil. With the exception of Cornell peatlite A, the disease was severe in the soilless mixtures and the pathogen remained viable for 7 wk after the original infected plants were removed. In Cornell peatlite A, P. splendens failed to cause symptoms 7 wk after the original diseased plants were removed. The pathogen had the least effect on the host in unsterilized soil.