Comparative studies on mycoparasitic Pythium species

Pythium mycoparasiticum is described as a new species. Its mycoparasitic behaviour was compared with those of P. oligandrum and P. nunn against nine species of potential host fungi P. graminicola, P. vexans, Rhizoctonia solani (two isolates), Fusarium culmorum, F; oxysporum f sp lycopersici, Botrytis cinerea, Botryotrichum piluliferum, Trichoderma aureoviride and Phialophora sp. In interactions on filter paper or cellulose film, the degree of aggressiveness of each mycoparasite and the degree of resistance of host furgi were assessed by the difference in cellulose breakdown caused by the hosts alone or in dual inoculations with mycoparasites. P. oligandrum was the most aggressive mycoparasite, active against most hosts; P. nunn was least aggressive, with a narrow host range, and P. mycoparasiticum was intermediate in these respects. Of the hosts, Pythium spp and R. solani were least susceptible to parasitism and T. aureoviride and Phialophora were most susceptible. B. cinerea, and some other hosts to a lesser degree, was susceptible to P. oligandrum but not to P. nunn. Some of these findings were confirmed by the abilities of mycoparasites to grow on potato-dextrose agar precolonized by the host fungi; this assay method was not as sensitive as others, but it could be used to compare potential host ranges of mycoparasitic Pythium spp and the cellulolytic mycoparasites, Trichoderma harzianum and Gliocladium roseum. Interactions between individual hyphae were studied by videomicroscopy on films of water agar. The incidence and timing of different events were assessed from replicate interactions on videotape and statistically analysed. The results generally confirmed those of interactions on cellulosic substrates, but several new details of interactions were observed. All three mycoparasitic Pythium spp had similar modes of parasitism, different from those of T. harzianum and G. roseum, used in a small comparative study. They did not affect host hyphae before contact, and none showed tropism to host hyphae. Soon after contacts involving parasite or host tips, the susceptible host hyphae stopped growing and generally either lysed at the point of contact or exhibited vacuolation/coagulation of their cytoplasm, starting at this point; this was often followed by penetration by a mycoparasite. In some cases the parasites penetrated directly, sometimes after proliferating on the host surface. The parasites branched at points of contact and penetrated from these branches. Lysis and cytoplasmic coagulation typically preceded penetration or even the origin of a penetrating branch from the mycoparasites. Based on the timing of these events and the number of hosts affected, P. nunn was significantly the least aggressive mycoparasite, whereas P. oligandrum and P. mycoparasiticum were equally aggressive. The hosts differed in resistance as on cellulosic substrates. Mycoparasitism by P. oligandrum was investigated by inactivating the parasite or host hyphae with fine beams of intense light prior to contact on water agar. The results demonstrated an essential role of mycoparasitic activity in host stoppage, lysis and cytoplasmic coagulation. However, lysis also required activity by the host. The findings are discussed in relation to postulated surface recognition events and the respective roles of hostand mycoparasite-derived wall-lytic enzymes. In limited tests, P. nunn was found to be unique among mycoparasitic Pythium spp in its ability to utilize inorganic (nitrate) nitrogen. Germination of oospores of P. oligandrum was found to depend on length of storage in culture and on conditions of aeration and illumination. Germination was stimulated by peptone and, to a lesser degree, malt extract, and also by acetaldehyde among the volatile metabolites that were tested. The results of all these studies are discussed in relation to the roles or potential roles of the mycoparasites in biocontrol of plant