VIDEO MICROSCOPIC COMPARISON OF MYCOPARASITISM BY PYTHIUM-OLIGANDRUM, P-NUNN AND AN UNNAMED PYTHIUM SPECIES

Videomicrography was used to record interactions between hyphae of the mycoparasites Pythium oligandrum, Pythium sp. (termed Pythium SWO) and P. nunn and ten hosts on films of water agar. Assessed by incidence and timing of parasitic events in replicate interactions, P. oligandrum and Pythium SWO were significantly more aggressive than P. nunn across a range of hosts or to hosts. Hosts differed in susceptibility: Pythium graminicola and P. vexans were highly resistant, Trichoderma aureoviride, Fusarium oxysporum and a Phialophora sp. were susceptible, and Botrytis cinerea, F. culmorum, Rhizoctonia solani (two isolates) and Botryotrichum piluliferum showed intermediate responses. B. cinerea inhibited approaching tips of Pythium SWO and P. nunn but not of P. oligandrum. The mycoparasites had an identical mode of parasitism. There was no pre-contact inhibition or tropism, but susceptible host hyphae stopped soon after contact-faster (mean 253 sec) in contacts between host tips and sub-apical regions of parasitic hyphae than vice-versa (mean 439 sec). Stoppage was often followed by host lysis at the contact point (mean 324 sec post-contact) or by coagulation/vacuolation of host contents (mean 568 sec). Penetration of host hyphae occurred in 69 of the total 148 interactions, at a mean 594 sec post-contact, from a parasitic branch formed after mean 418 sec at the contact point. The relative timings of these events necessitate a reappraisal of the mechanisms of mycoparasitism by Pythium spp. In equivalent conditions, Trichoderma harzianum and Gliocladium roseum inhibited 'host' hyphae by production of diffusible compounds but did not penetrate them.