POPULATION DYNAMICS OF PYTHIUM APHANIDERMATUM IN OIL CAKE AMENDED SOILS

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Abstract
Experiments were carried out to determine the effect of decomposition of oil cakes, used as organic manures in India, on the population of Pythium aphanider-matum in soil. Low quantities (0.1 to 1.0% w/w) of castor and peanut cakes stimulated the growth of the fungus at 20° to 30 °C but in presence of higher quantities of these cakes the growth of the fungus in soil was inhibited. Inhibition was more conspicuous and was caused by lesser amounts of these cakes at high temperatures than at low temperatures. On the other hand, margosa cake (Azadirachta indica) was highly stimulatory to the fungus in all concentrations of 0.1 to 5% at 20°–30 °C, but caused inhibition at 36 °C. Washing of margosa cake with alcohol made it as inhibitory as the other two cakes. It has been suggested that the three cakes affected P. aphanidermatum in soil differently because of their chemical nature and the nature of their decomposition products which may (i) directly suppress the fungus through toxic effect, (ii) stimulate it through nutritional effect, (iii) stimulate or inhibit actinomycetes lytic to the pathogen. Temperature and pH of soil during.decomposition, as well as C:N ratio of the cakes, do not play any major role in the suppression of the fungus.