Spore Transmission and Epidemiology of Verticillium balanoides, an Endozoic Fungal Parasite of Nematodes in Soil

Abstract

The number of spores of the nematophagous fungus Verticillium balanoides in aqueous suspension binding to the nematode Ditylenchus dipsaci was proportional to time over 0-4 hr and influenced by nematode activity. Spore binding to this nematode fell exponentially with storage over 1-7 days at 20°C to an estimated 11% of the initial value. Some bound spores were also lost when D. dipsaci migrated through wet sand. Spore production per cadaver was 15,905 ± 107 for D. dipsaci, 11,670 ± 98 for a second plant parasitic nematode, Globodera rostochiensis, but only 840 ± 86 for the microbivorous species Panagrellus redivivus . This lowest mean was increased about threefold following either pretreatment of the nematode with antibiotics or its repeated subculture to brewer's yeast, which reduced the prevalence of other microbes associated with this nematode. Plant parasitic nematodes may favor V. balanoides in comparison to P. redivivus by lacking an extensive range of microorganisms able to compete with this fungus for the cadaver. A simple epidemiological model suggests that the ability of V. balanoides to produce a high spore density when D. dipsaci enters soil in high numbers from host plants may result in a transient density-dependent epizootic. This may explain the well described overwintering decline of this nematode.