PARASITISM AND BIOLOGICAL-CONTROL OF VERTICILLIUM-DAHLIAE BY PYTHIUM-OLIGANDRUM

Pythium oligandrum was a parasite of Verticillium dahliae in dual cult ure, impeding the latter's ability to grow and form microsclerotia. Va riation occurred among isolates of P. oligandrum in their parasitism t oward isolates of V. dahliae, and isolates of V. dahliae expressed a r ange of susceptibilities to parasitism by P. oligandrum. Temperature a nd matric potential significantly affected interactions between these fungi. Populations of P. oligandrum in the rhizosphere of pepper (Caps icum annuum) were similar in a potting soil infested and noninfested w ith V. dahliae, but the populations of V. dahliae in combined samples of rhizosphere and nonrhizosphere soil of pepper were significantly lo wer in the presence of P. oligandrum than in its absence. In greenhous e experiments with pepper grown in soil infested with V. dahliae, shoo t and fruit weights were significantly higher in the presence of P. ol igandrum than in its absence. However, when soil was infested only wit h P. oligandrum, fresh weights of shoots and fruits were 40 to 50% hig her than when plants were grown in its absence, evidence that this fun gus significantly promoted growth of this crop. Therefore, processes r esponsible for improved plant health associated with soil treatment wi th P. oligandrum could be the result of complex interactions between p athogen, host, and mycoparasite.