ALTERED GROWTH OF FUSARIUM-OXYSPORUM F-SP CHRYSANTHEMI IN AN IN-VITRODUAL CULTURE SYSTEM WITH THE VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS INTRARADICES GROWING ON DAUCUS-CAROTA TRANSFORMED ROOTS

Vesicular arbuscular mycorrhizal fungi can reduce plant disease sympto ms and populations of pathogens through mechanisms that are not well u nderstood, Glomus intraradices was grown on Daucus carota transformed roots in a two-compartment in vitro system. One compartment contained mycorrhizal roots on a complete growth medium, while the other contain ed a medium lacking sugar on which only mycelial growth was allowed. T he direct interaction between G. intraradices and Fusarium oxysporum f . sp. chrysanthemi was studied in the compartment lacking sugar during a 5-day period. G. intraradices hyphal density and spore number were estimated along with F. o. chrysanthemi conidial germination, mycelial growth and sporulation. Five hours after inoculation, germination of F. o. chrysanthemi conidia doubled in the presence of G. intraradices. Radial growth of F. o. chrysanthemi colonies was always slightly but significantly enhanced in the presence of G. intraradices. No correlat ion was obtained be tween G. intraradices hyphae or spore densities an d F. o. chrysanthemi hyphal growth, Overall sporulation of the 5-day-o ld F. o. chrysanthemi colonies was not influenced by the presence of G . intraradices. However, significant negative correlations were found between F. o. chrysanthemi conidia production and G. intraradices hyph ae or spore concentrations. G. intraradices increased F. o. chrysanthe mi conidial germination and slightly stimulated its hyphal growth in d ual culture without any root influences. No antibiosis was observed be tween the fungi. The significance of the results and their potential i mplication for rhizosphere biology are discussed.