Aggressiveness and fungal host ranges of mycoparasitic Pythium species

Abstract

As evidenced by the ability to reduce cellulolytic activities of host fungi on cellulose film or filter paper, Pythium oligandrum was a more aggressive mycoparasite than Pythium SWO which, in turn, was usually more aggressive than Pythium nunn. Both P. oligandrum and Pythium SWO produced oogonia in the presence of susceptible host fungi, deriving carbon, organic N and sterols from the hosts. P. nunn did not form oogonia with any host, and it was the only mycoparasite that utilized nitrate as sole N source. Host fungi differed in resistance, in the same ranking order with each mycoparasite. Phialophora sp. and Trichoderma aureoviride were most susceptible; Pythium graminicota and Rhizoctonia solani were most resistant, and Fusarium culmorum, F. oxysporum and Botryotrichum piluliferum showed intermediate susceptibility. Botrytis cinerea was susceptible to P. oligandrum but not usually to Pythium SWO or P. nunn, which it antagonized. The findings support previous evidence from hyphal interactions on water agar but with some differences, ascribed to ‘disease escape’ within host colonies. P. oligandrum grew across agar previously colonized by several host fungi, but Pythium SWO and P. nunn grew across few host colonies. Trichoderma harzianum grew across colonies of several hosts, some different from those that supported P. oligandrum. Gliocladium roseum grew across colonies of nearly all hosts. The implications of these findings for biocontrol arc discussed.