Ecology of nematophagous fungi in agricultural soils

Abstract: Nematophagous fungi possess various mechanisms for infecting nematodes, and are either obligate parasites or facultative saprophytes. Nematophagous fungi have a potential as biological control agents against plant- and animal-parasitic nematodes. Generally there has been little success in using these fungi in such a way, and one reason might be that too little is known about the ecology of the nematophagous fungi in natural environments. The goal with the work of this thesis was to increase knowledge in this area. The densities and species composition of nematode-trapping fungi were investigated in agricultural soils and in the rhizosphere of crop plants. In root-free soils between 10-1000 propagules of nematode-trapping fungi g1 were found, and the most common one was Arthrobotrys oligospora. The density of nematode-trapping fungi showed strong seasonal variation, with optima during the late summer and autumn months. Furthermore, their densities were 5-20 times higher in the pea rhizosphere than in the root-free soil. Up to 0.3% of the nematodes were infected with endoparasitic fungi, and the most commonly detected one was Catenaria anguillulae. The relation between naturally occurring nematophagous fungi and nematodes were investigated by making population studies in agricultural soils. The occurrence of the more parasitic nematode-trapping fungi and the endoparasitic fungi both decreased during a three-year study, and at the same time the number of nematodes was reduced by 50%, indicating that a host threshold density exists for the parasitic nematophagous fungi. However, the occurrence of the more saprophytic nematode-trapping fungi was not affected by the decreased number of nematodes, indicating that they are not dependent on nematodes in the soil. Evidence that the nematophagous fungi regulated the number of nematodes in the soil was not obtained in the work of this thesis. For instance, the density of nematode-trapping fungi and the number of nematodes showed similar seasonal dynamics, but were not correlated to each other. Furthermore, compared to the root-free soil the increase of the densities of nematode-trapping fungi in the rhizosphere were not correlated to the increase of the number of nematodes. The transition between the saprophytic and parasitic phase in the nematode-trapping fungi in soil was investigated. Conidial traps were formed in most of the fungi tested, and the formation of conidial traps was partly a response to competition with other microorganisms for nutrients. Furthermore, the degree of competition required to induce conidial-trap formation was lower for more parasitic nematode-trapping fungi than for saprophytic ones.