Influence of Nonhosts, Crucifers, and Fungal Parasites on Field Populations of Heterodera schachtii.

Heterodera schaehtii egg number decline under nonhosts was surveyed for 3-4 years at soil depths of 0-30 cm and 30-60 cm in three fields in the Imperial Valley, California. In the two fields continously cropped to alfalfa, annual decline rates were 49 and 63%, respectively, and did not differ (P = 0.05) between depths. In the third field, cropped to annual nonhosts and fallowed, decline rates of 56 and 80% at 0-30-cm and 30-60-cm depths, respectively, were significantly different (P = 0.05). Egg hatch is the major cause of decline. Soil moisture in relation to type of cropping sequence apparently influenced egg hatch and activity of fungal parasites. An interaction matrix is used to assess the importance of biological, environmental, and management factors affecting decline of H. schachtii egg numbers. The required rotation length to non-hosts for various egg densities can be predicted. In coastal California, inclusion of a winter crucifer crop in the rotation increased H. schachtii egg density up to threefold.