root tissue. The mycorrhizal fungus suppressed the R. similis population by almost 50% (Table 3.24) and thus increased protection of the root against the nematode.

In contrast to the interactions among mycorrhizae and fungal pathogens, Salonen et al. (2001) investigated the effect of arbuscular mycorrhizae on a plant parasite of clover and a grass. In greenhouse experiments, the grass Poa annua or clover (Trifolium pratense) was grown in the presence or absence of an arbuscular mycorrhizal fungus and in the presence or absence of the root hemiparasitic plants Odontites vulgaris (for Poa ) and Rhinanthus serotinus (for clover). Mycorrhizal colonization of roots of P. annua had little effect on plant growth, whereas the hemiparasite infection caused a significant reduction in host biomass. The mycorrhizal status of P. annua did not affect the biomass or the number of flowers produced by the parasitic O. vulgaris plants. In contrast, root colonization by arbuscular mycorrhizae of clover greatly increased the host plant biomass, but the hemiparasite infection had no effect. The effect of the mycorrhizae on the hemiparasitic R. serotinus plants attached to clover was to increase the parasitic plant biomass and induce the production of more flowers than plants growing with nonmycorrhizal hosts. Salonen et al. (2001) caution, however, that improvement of the performance of the parasitic plant when attached to a mycorrhizal host depends on the degree of growth promotion afforded to the host plant by the mycorrhizae.

There appears to be a fine balance between the beneficial effects of a mycorrhizal fungus and the detrimental effects of a plant pathogenic fungus. In an experimental manipulation of the fungal communities associated with the annual grass Vulpia ciliata by the application of two fungicides, Newsham et al. (1994) showed that there was an interaction among the pathogenic and mycorrhizal fungal associates of the target plant. The fungicide perchloraz did not affect the abundance of arbuscular mycorrhizae on the plant, nor did it affect

Table 3.24 The Influence of Arbuscular Mycorrhizal Colonization of Carrot Root Tissue on the Invasion of Roots by the Parasitic Nematode Radopholus similes and Number of Nematodes in the Growth Medium

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