Management

Abstract

Cyst nematodes that cause damage to cultivated crops are species of the genera Globodera, Heterodera, Punctodera and Cactodera. These nematodes present unique problems in their management. Many or all of the eggs are produced inside the female body that, upon death, becomes a cyst with a hardened protective wall. The cyst wall is resistant to invasion by potential parasites and protects the eggs on the inside from rapid desiccation. Eggs inside the cysts may remain dormant for many years (Oostenbrink, 1966). In some cases, for example Globodera pallida and G. rostochiensis, they only hatch in the presence of a hatching factor produced by a potential host (Rawsthorne and Brodie, 1986). These unique characteristics of cyst forming nematodes make necessary the use of special management regimes to prevent damage to their host crops where the nematodes are found. Another characteristic of cyst nematodes that increases the difficulty of controlling them is their variability within species. The most damaging species of cyst nematodes, such as potato cyst, cereal cyst, sugar beet cyst and soybean cyst nematodes, all have a number of infraspecific forms (Canto-Saenz and de Scurrah, 1977; Kort et al., 1977; Andersen and Andersen, 1982b; Riggs and Schmitt, 1988; Müller, 1992). The presence of mixtures of parasitic phenotypes in all of the above species adversely affects the use of resistant cultivars and reduces their longevity. If a resistant cultivar is planted each year for three or more years, a phenotype that parasitizes the resistant cultivar is likely to be selected (Seinhorst, 1995). In many areas growers resist rotation, usually because the alternate crops are not as profitable or because they require changes in machinery or practices that make them more expensive to grow. A third characteristic of cyst nematodes that makes management more difficult is their ease of spread. Even though a rotation is used to reduce the population level in a field, cysts may be transferred from other fields to maintain a population level higher than expected. Cysts can be blown by the wind as free cysts (Lauenstein, 1991), or in small aggregates of soil or on small roots that are attached to other parts of plants that are blown from one field to another. Soil adhering to tubers, such as potatoes, or fleshy roots, such as sugar beets (Schlang, 1993), may contain cysts, as well as small balls of soil called peds mixed with soybean seed (Epps, 1969). Flood water or surface runoff also may carry cysts from one field to another as will birds (Epps, 1971), other animals (Kontaxis et al., 1976) or machinery. Finally the characteristic of the cyst nematodes to become dormant or to undergo anabiosis hinders management of cyst nematodes. This characteristic aids in survival in the absence of a host. It necessitates rotations that are longer than would otherwise be needed to ensure good control.