Mycorrhiza (1995) 5:431-438 9 Springer-Verlag 1995 M. St-Arnaud 9 C. Hamel 9 B. Vimard M. Caron 9 J.A. Fortin Altered growth of Fusarium oxysporum f. sp. chrysanthemi in an in vitro dual culture system with the vesicular arbuscular mycorrhizal fungus Glomus intraradices growing on Daucus carota transformed roots Abstract Vesicular arbuscular mycorrhizal fungi can reduce plant disease symptoms and populations of pa- thogens through mechanisms that are not well under- stood. Glomus intraradices was grown on Daucus caro- ta transformed roots in a two-compartment in vitro sys- tem. One compartment contained mycorrhizal roots on a complete growth medium, while the other contained a medium lacking sugar on which only mycelial growth was allowed. The direct interaction between G. intrara- dices 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. chry- santhemi conidia doubled in the presence of G. intrara- dices. Radial growth of F. o. chrysanthemi colonies was always slightly but significantly enhanced in the pres- ence of G. intraradices. No correlation was obtained be- tween G. intraradices hyphae or spore densities and F. o. chrysanthemi hyphal growth. Overall sporulation of the 5-day-old F. o. chrysanthemi colonies was not in- fluenced by the presence of G. intraradices. However, significant negative correlations were found between F. o. chrysantherni conidia production and G. intraradices hyphae or spore concentrations. G. intraradices in- creased F. o. chrysanthemi conidial germination and slightly stimulated its hyphal growth in dual culture without any root influences. No antibiosis was observed between the fungi. The significance of the results and their potential implication for rhizosphere biology are discussed. M. St-Arnaud (1~) 9 C. Hamel 9 B. Vimard - M. Caron J.A. Fortin Institut de recherche en biologie v6g6tale, Universit6 de Montrdal, and Jardin botanique de Montrdal, 4101 est, rue Sherbrooke, Montr6al, Qudbec HtX 2B2, Canada Fax: (514) 872-9406; e-mail: starnaum@ere.umontreal.ca Key words Pathogen 9 Biological control 9 Spore 9 Conidia 9 Germination 9 VAM Introduction Vesicular arbuscular mycorrhizal (VAM) fungi can re- duce plant root disease symptoms and pathogen popu- lations in soil through mechanisms that are not well un- derstood (Sch6nbeck 1979; Dehne 1982; Graham 1986; Caron 1989; Perrin 1990; Linderman 1994). Their ac- tion has been attributed to improvement of plant nutri- tion, to stimulation of host plant disease resistance mechanisms, to a direct interaction with pathogens and to an indirect effect through changes in soil microflo- ra. Improvement of phosphorus assimilation alone can- not always explain the beneficial effects of VAM fungi on host plants which lead to reduction of disease symp- toms or pathogen populations in soil (Caron et al. 1986; St-Arnaud et al. 1994). Induction of disease resistance mechanisms by the mycorrhizal fungus in host plants is controversial. The synthesis of compounds implicated in physical or chemical resistance to pathogen infection has been observed in response to mycorrhizal coloniza- tion (Baltruschat and Schbnbeck 1975; Dehne et al. 1978; Dehne and Sch6nbeck 1979; Krishna and Bagya- raj 1983; Morandi et al. 1983; Lieberei and Feldmann 1989; Grandmaison et al. 1993; Harrison and Dixon 1993), but the absence of any stimulation of defence mechanisms in mycorrhizal plants has also been re- ported (Spanu and Bonfante-Fasolo 1988; Codignola et al. 1989; Dumas et al. 1989; Wyss et al. 1989; Dumas et al. 1990 ; Gianinazzi-Pearson et al. 1992). A direct or an indirect competitive effect could exist near the extraradical phase of the VAM fungus. Shifts in the presence or abundance of microbial species are known to occur in the rhizosphere of mycorrhizal plants (Bagyaraj and Menge 1978; Ames et al. 1984; Meyer and Linderman 1986b; Secilia and Bagyaraj 1987, 1988; Linderman 1992), and other significant pa-