BEHAVIOR OF ZOOSPORE CYSTS OF THE MYCOPARASITE PYTHIUM-OLIGANDRUM IN RELATION TO THEIR POTENTIAL FOR BIOCONTROL OF PLANT-PATHOGENS

Several in vitro features of zoospore cysts of Pythium oligandrum sugg est their potential value for biocontrol, in place of oospores which g erminate slowly and erratically. Cysts germinated rapidly (20-30 min) in water at 20-25 degrees C. They also germinated, but more slowly, at water potentials down to -6.6 MPa (PEG 8000 solutions) or -2.65 MPa ( mannitol solutions), whereas mycelia did not grow at -2.1 MPa. Cysts c ould be stored for at least 3 wk in PEG at 4-7 degrees, and germinated when the PEG was diluted and temperature was raised to 20 degrees. Ge rminating cysts rapidly (< 1 h) parasitized hyphae of Fusarium oxyspor um and F. culmorum in the absence of exogenous nutrients. When cysts o f a metalaxyl-tolerant mutant of P. oligandrum were added to moist, un sterile soil at 20 degrees, the fungus was recovered at one-third of t he original level at 28 d, and was still detected at 80 d. Cysts and m otile zoospores were studied for mycoparasitic activity on water-agar films in vitro. Cysts germinated readily on colonies of F. oxysporum a nd Pythium aphanidermatum, and in the presence of spores of Idriella b olleyi, but poorly on colonies of F. culmorum, which seemed to produce an inhibitor. Motile zoospores were not attracted to hyphae but if th ey encysted on hyphae they showed significant germ-tube emergence towa rds the host; pre-encysted spores germinated with random orientation. A higher incidence of parasitism from motile spores than from cysts in dicated a need for anchorage (adhesion during encystment) in the condi tions of the assays, implicating mechanical pressure in mycoparasitism . Sporelings needed to grow some distance (about 30 mu m) before they parasitized efficiently; this might involve transition from a germ-tub e to a vegetative hypha, because it was similar to the maximum growth (26-27 mu m) from cysts at water potentials that inhibited hyphal grow th. These findings extend previous work on zoospore behaviour in Pythi um and Phytophthora spp.