Parasitism and biological control of Verticillium dahliae by Pythium oligandrum

Pythium oligandrum was a parasite of Verticillium dahliae in dual culture, impeding the latter's ability to grow and form microsclerotia. Variation occurred among isolates of P. oligandrum in their parasitism toward isolates of V. dahliae, and isolates of V. dahliae expressed a range of susceptibilities to parasitism by P. oligandrum. Temperature and matric potential significantly affected interactions between these fungi. Populations of P. oligandrum in the rhizosphere of pepper (Capsicum annuum) were similar in a potting soil infested and noninfested with V. dahliae, but the populations of V. dahliae in combined samples of rhizosphere and nonrhizosphere soil of pepper were significantly lower in the presence of P. oligandrum than in its absence. In greenhouse experiments with pepper grown in soil infested with V. dahliae, shoot and fruit weights were significantly higher in the presence of P. oligandrum than in its absence. However, when soil was infested only with P. oligandrum, fresh weights of shoots and fruits were 40 to 50% higher than when plants were grown in its absence, evidence that this fungus significantly promoted growth of this crop. Therefore, processes responsible for improved plant health associated with soil treatment with P. oligandrum could be the result of complex interactions between pathogen, host, and mycoparasite.