Comparative Physiology and Behaviour of the Mycoparasites Pythium acanthophoron, P. oligandrum and P. mycoparasiticum

Pythium acanthophoron (two isolates) was as aggressive as P. oligandrum in hyphal interactions with Fusarium oxysporum on water agar films. It caused rapid post-contact lysis or cytoplasmic coagulation of the host, and often branched and penetrated the host at contact points. P. acanthophoron grew across a range of fungi on pre-colonized agar plates; the range was narrower than for P. oligandrum but broader than for P. mycoparasiticum. In chemically defined liquid media, P. acanthophoron was unique among the six known mycoparasitic Pythium spp. in requiring organic nitrogen but not thiamine. It also grew on mannitol as the sole carbon source in the presence of calcium or sterols or both. However, individual isolates of the three mycoparasitic species showed different responses to calcium and the sterols ergosterol, cholesterol and beta-sitosterol when grown on mannitol. All three mycoparasites required components of molasses, carrot extract or sunflower seed extract to produce oogonia in culture; they formed few oogonia on potato extract, with or without dextrose, even when supplied with sterols. The three mycoparasites were less tolerant of high NaCl levels in culture than were three phytopathogens (P. aphanidermatum, P. ultimum, P. graminicola), in contrast to a previous report for P. oligandrum and P. ultimum. These in vitro studies suggest that P. acanthophoron has potential for use as a biocontrol agent instead of, or in addition to, P. oligandrum.