Interaction of the mycoparasite Pythium oligandrum with other Pythium species.
The interactions of P. oligandrum and 4 plant-pathogenic Pythium spp. (P. ultimum, P. vexans, P. graminicola and P. aphanidermatum) were studied in vitro by (i) video microscopy of hyphal interactions on water agar films, (ii) counting of host and mycoparasite propagules in different regions of opposing colonies on sunflower-seed extract agar films and (iii) ability of P. oligandrum to overgrow plates of PDA previously colonized by Pythium spp. P. oligandrum typically coiled round the hyphae of Pythium hosts and penetrated the host hyphae after c. 50 min from the hyphal coils, causing disruption of host hyphal tips up to 1.2 mm ahead of the contact points. The relative growth rates of mycoparasite and host hyphae, timing of penetration and distance (sub-apical) at which penetration led to host tip 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 by P. oligandrum and in antagonizing the mycoparasite by coiling and penetrating the mycoparasite hyphae. Other host Pythium spp. differed in susceptibility, the most susceptible being P. vexans and P. ultimum, while P. graminicola was more resistant. The results are discussed in relation to the role of P. oligandrum as a biocontrol agent, especially for limiting the ability of other Pythium spp. to increase their propagule populations in crop residues.