GIGASPORA-GIGANTEA - PARASITISM OF SPORES BY FUNGI AND ACTINOMYCETES

Fungi and actinomycetes were isolated monthly for one year from spores of the arbuscular mycorrhizal fungus Gigaspora gigantea recovered fro m a maritime sand dune. Spores in four different stages of vigour (new ly formed, greenish-yellow healthy; yellow, moribund, mottled with bro wn spots; brown; and dead, blackened and collapsed) were used for isol ation. From 272 isolates cultured from crushed, surface-disinfected G. gigantea spores, 44 species of fungi and six actinomycete species wer e recovered. The five most frequently isolated organisms were Acremoni um sp., Chrysosporium parvum, Exophiala werneckii, Trichoderma sp. and Verticillium sp. The species lists derived from the four spore types differed significantly. Thirty-one of the isolated species were tested for their ability to parasitise healthy G. gigantea spores and to inv ade spores killed by a hot water treatment. Twenty-two species could f unction as pathogens, forming internal projections (IP), fine radial c anals (FRC) or both in the spore wall. IP were induced by Acremonium s p., Chrysosporium parvum, Cladosporium sp., Geomyces pannorum, Oidiode ndron sp., Sporothrix sp., Verticillium sp. and by the actinomycetes N ocardia sp. and Streptomyces sp. The IP length was positively correlat ed with duration of exposure to the parasites. In the bioassay, both I P and FRC were formed by live spores, while heat-killed spores possess ed only FRC after penetration by the test microorganisms. Pathogenicit y differed among the parasites and was greatest for Verticillium and A cremonium.