Possible Role of Competition for Nutrients in Biocontrol of Pythium Damping-Off by Bacteria

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Abstract
Of the 130 bacteria isolated from the rhizospheres of plants infested with Pythium spp., six were found to be efficient biocontrol agents of this pathogen under greenhouse conditions. No lytic enzymes were involved in in vitro interactions between the bacteria and P. aphanidermatum. Substances inhibiting growth of the fungus were produced by both the biocontrol agents and other rhizobacteria. On the other hand, competition for nutrients between germinating oospores of P. aphanidermatum and bacteria, which was unique to the biocontrol agents, significantly correlated with suppression of disese in the greenhouse. Oospore germination in rhizospheres of wheat, tomato, cucumber, melon, bean, and cotton plants was decreased in the presence of these bacteria. Broadcast application or seed coating with these bacteria reduced disease incidence in cucumbers planted in peat-vermiculite mixture or in soil by 60-75% compared with the untreated control. Damping-off caused by P. ultimum in cucumbers was reduced by up to 94%. Combining low levels of Pseudomonas putida or P. cepacia with the fungicide prothiocarb resulted in an additive effect. Bacteria applied to cucumber seeds became established along the roots. Populations were 100-fold higher at root tips than at their upper parts. On the other hand, except for the root tips, the total population of bacteria was decreased in the presence of the beneficial ones. Disease control was achieved in bean, pepper, melon, tomato, and cotton plants as well.