Interactions between two arbuscular mycorrhizal fungi and fungivorous nematodes and control of the nematode with fenamifos

We investigated the control of fungal feeding nematodes (Aphelenchus avenae Bastian) inoculated at two densities (1000 and 2000 per pot) with the nema ticide fenamiphos in pot cultures of the arbuscular mycorrhizal (AM) fungi Gigaspora margarita Becker and Hall and Glomus coronatum Giovannetti grown with clover (Trifolium subterraneum L.). Nematode populations increased in mycorrhizal pots, with a concomitant decrease in percent mycorrhizal coloni sation of the clover roots, Total numbers of fungal spores were little affe cted by nematodes, but the percentage of spores with contents declined, par ticularly for G. coronatum. In non-mycorrhizal pots the nematode population s declined below the starting density. These results indicate that both spe cies of mycorrhizal fungi acted as food sources for the nematode and that G . coronatum is more susceptible to spore damage than Gi. margarita. This is probably related to the thinner walls of G. coronatum spores, which could be relatively easily penetrated by the nematode styler (similar to 15 mum i n length). Both species of mycorrhizal fungus produced a growth response in clover, which was often greater in the presence of nematodes than in the z ero nematode control. This suggests that (a) nematodes might be increasing the rate of mineralisation of nutrients, which were then absorbed by the my corrhizal fungi and transferred to the plant, and (b) the percentage of the root length colonised by AM fungi in control plants (not inoculated with n ematodes) was above that necessary to promote the growth response, in the s oil conditions used. Differences between the two nematode densities in thei r influence on final numbers of nematodes and their effects on mycorrhizal fungi and plant growth responses were minor, and we conclude that the respo nses are non-linear at least over the range of nematode densities we used. Application of fenamiphos at the recommended rate controlled numbers of nem atodes and restored the percentage of root colonisation by fungi and percen tage of spores with contents to the control values. We conclude that fenami phos is an effective nematicide for control of nematodes in pot-cultures, b ut more than one application may be required to achieve complete control in pot-cultures grown for several months. The results are discussed in terms of the effects of nematode grazing on the interactions between mycorrhizal fungi and plants and on the spore populations of the fungi themselves. (C) 2001 Elsevier Science B.V. All rights reserved.