INTERACTION OF THE MYCOPARASITE PYTHIUM-OLIGANDRUM WITH OTHER PYTHIUMSPECIES

Interactions of Pythium oligandrum and four plant-pathogenic Pythium s pp. (P. ultimum, P. vexans, P. graminicola and P. aphanidermatum) were studied in vitro by (i) video microscopy of hyphal interactions on wa ter agar films, (ii) counting of host and mycoparasite propagules in d ifferent regions of opposing colonies on sunflower-seed extract agar f ilms and (iii) ability of P. oligandrum to overgrow plates of potato-d extrose agar previously colonized by Pythium spp. Pythium oligandrum t ypically coiled round the hyphae of Pythium hosts and penetrated the h ost hyphae after approximately 50 min from the hyphal coils, causing d isruption of host hyphal tips up to 1.2 mm ahead of contact points. Th e relative growth rates of mycoparasite and host hyphae, timing of pen etration and distance (sub-apical) at which penetration led to host ti p disruption were used to assess the potential of mycoparasitism by P. oligandrum to prevent the growth of Pythium hosts. P. aphanidermatum was unique among the 'host' Pythium spp. in being largely unaffected b y P. oligandrum and in antagonizing the mycoparasite by coiling and pe netrating the mycoparasite hyphae. Other host Pythium spp. apparently differed in susceptibility, the most susceptible being P. vexans and P . ultimum, whereas P. graminicola was more resistant. The results are discussed in relation to the role of P. oligandrum as a biocontrol age nt, especially for limiting the ability of other Pythium spp. to incre ase their propagule populations in crop residues.