Interactions between mycophagous nematodes, mycorrhizal and other soil fungi
The hypothesis that mycophagous nematodes feed on the mycelia of VAM fungi and consequently reduce the spread of vesicular-arbuscular mycorrhizal (VAM) infection in plant roots and the growth response of plants to this infection was investigated in experiments carried out under controlled environmental conditions. Preliminary experiments indicated that nematodes might be able to reduce VAM infection by Glomus clarum if they were added to soil at the same time as the fungal inoculum. However, nematodes did not affect the growth response of red clover to VAM infection even though there was an indication that nematodes may directly affect plant growth possibly by causing damage to root hairs which could facilitate the entry of pathogens. There seemed to be little effect of mycorrhizal status on the numbers of nematodes which could be recovered from the growth medium. The two standard methods used to extract nematodes in these preliminary experiments were found to give low and variable recoveries. A method which gave greater and less variable recoveries was developed and the conditions for its successful use were defined. The four species of nematodes used in this study differed in their ability to reduce the growth of a range of fungi and in their ability to increase in number as a result of feeding on these fungi. Both experimental data and a mathematical model, which was developed subsequently, showed that the suitability of a fungus as a source of nutrition could be measured by the increase in numbers of nematodes and the reduction in weight of mycelium as a result of grazing. The model therefore provided a rationale for ranking different fungi in terms of their suitability as hosts for a given species of nematode. Of the four species of nematodes, A phelenchus avenae was the most damaging to the growth of a range of fungi. However, when added at the same time as, or later than, the VAM inoculum, A. avenae had no effect on the spread of VAM infection or on the response of plants to VAM infection. The numbers of A. avenae recovered were similar in non-mycorrhizal and mycorrhizal treatments. A mathematical model was developed which simulated the spread of VAM infection and the change in numbers of nematodes with time. Close agreement between predictions of the model and experimental results was only achieved if it was assumed that the nematodes were not capable of feeding on the VAM fungus. Aphelencholdes composticola multiplied most when cultured in agar plates but grew poorly when cultured on its own in soil. When organic material (bran) colonized either by Agaricus bisporus (the nematodes preferred host) or by volunteer fungi was introduced into soil, the numbers of A. composticola increased by up to one hundred times. Neither VAM infection by G. clarum nor the mycorrhizal responses of the plant were affected by these high numbers of A. composticola, by the introduction of bran alone or by bran colonized by A. bisporus. In soil without added bran or saprotrophic fungi numbers of A. composticola were not affected by the presence of G. clarum, indicating that the available food sources in unamended soil were sparse. Mycorrhizal plants appeared to benefit more than non-mycorrhizal plants from the mineralization of phosphorus by the saprotrophic fungi. Factors which may determine the suitability of some fungi as food sources for mycophagous nematodes are discussed. The possibility that VAM fungi are immune to grazing by these nematodes is considered. Ways in which mycophagous nematodes may influence the growth of plants are also discussed.