Behaviour of zoospore cysts of the mycoparasite Pythium oligandrum in relation to their potential for biocontrol of plant pathogens

Abstract

Several in vitro features of zoospore cysts of Pythium oligandrum suggest their potential value for biocontrol, in place of oospores which germinate slowly and erratically. Cysts germinated rapidly (20–30 min) in water at 20–25 °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 could be stored for at least 3 wk in PEG at 4–7°, and germinated when the PEG was diluted and temperature was raised to 20°. Germinating cysts rapidly (<1 h) parasitized hyphae of Fusarium oxysporum and F. culmorum in the absence of exogenous nutrients. When cysts of a metalaxyl-tolerant mutant of P. oligandrum were added to moist, unsterile soil at 20°, the fungus was recovered at one-third of the original level at 28 d, and was still detected at 80 d. Cysts and motile zoospores were studied for mycoparasitic activity on water-agar films in vitro . Cysts germinated readily on colonies of F. oxysporum and Pythium aphanidermatum , and in the presence of spores of Idriella bolleyi , but poorly on colonies of F. culmorum , which seemed to produce an inhibitor. Motile zoospores were not attracted to hyphae but if they encysted on hyphae they showed significant germ-tube emergence towards the host; pre-encysted spores germinated with random orientation. A higher incidence of parasitism from motile spores than from cysts indicated a need for anchorage (adhesion during encystment) in the conditions of the assays, implicating mechanical pressure in mycoparasitism. Sporelings needed to grow some distance (about 30 m) before they parasitized efficiently; this might involve transition from a germ-tube to a vegetative hypha, because it was similar to the maximum growth (26–27 m) from cysts at water potentials that inhibited hyphal growth. These findings extend previous work on zoospore behaviour in Pythium and Phytophthora spp.