Fungal parasites of cyst nematodes

Abstract

Cyst nematodes are major pests of a range of crops grown throughout the world. In the last decade it has been demonstrated that nematophagous fungi can effectively control some cyst-nematode pests on crops grown in monocultures. However, such natural control may be slow to establish in soil and difficult to exploit. Cropping history, summer rainfall and soil texture may affect the activity of these fungi, but their manipulation is severely restricted by a lack of detailed information on the factors that affect their growth and survival in soil. Populations of the cereal cyst nematode (CCN) may be controlled by two fungi, Nematophthora gynophila Kerry and Crump and Verticillium chlamydosporium Goddard when cereals are grown in monocultures. These fungi parasitise nematode females and eggs. The natural control of CCN has the characteristics of an “induced suppression”, in which it is essential for the host pest to be numerous in the early years of monoculture to support the multiplication of its parasites; during this time other control measures will be necessary to prevent large yield losses. Natural control caused by parasitic fungi may be difficult to measure but several techniques have been devised and in soils suppressive to CCN it has been estimated that 97% of females and eggs are parasited. Approximately 150 species of fungi have been found colonising cysts, females, and eggs of 8 species of cyst nematodes. The time of sampling and the method of isolation may affect the number and species of fungi collected. Few have been tested for their parasitic status and little is known about infection mechanisms. Some fungi have been added to soil and caused significant reductions in nematode populations but none have provided predictable control at rates of application that could be considered practical for an arable farmer. Fungi that attack eggs are facultative parasites, whereas some species are obligate parasites of females and survive in soil only as resting spores. A knowledge of the survival mechanism in soil is essential for choosing the appropriate method of applying a potential biological-control agent. The successful introduction of such an agent depends on whether a suitable niche for the organism exists or can be created and until we know much more about the factors that affect the activity of nematophagous fungi in soil, their full potentials as control agents for cyst nematodes will not be realised.