Biological Suppression of the Sugarbeet Cyst Nematode in Broccoli Fields Along California's Central Coast

The Central Coast of California is a large geographic region that includes some of the state's most fertile agricultural lands. The sugarbeet cyst nematode (Heterodera schachtii) was introduced to this region soon after sugar beet production was initiated, possibly as early as the late 1800s. Several susceptible host crops are currently grown in these coastal fields, including members of the Brassicaceae, such as broccoli, cabbage, and cauliflower, as well as members of the Chenopodiaceae, such as spinach and Swiss chard. Over the last several decades, H. schachtii populations in this region have declined considerably, resulting in less crop damage and reduced nematicide use. Prior baiting experiments identified Hyalorbilia oviparasitica as the most abundant fungus associated with H. schachtii females, suggesting that it may contribute to the nematode population decline. The goal of this study was to determine whether soils from this region biologically suppress H. schachtii. Soils were collected from an area of high broccoli production near Santa Maria, California. All 11 broccoli field soils suppressed H. schachtii, with the percent nematode suppression ranging from 32 to 95%. At the end of the suppression experiments, H. oviparasitica was the most abundant fungus associated with H. schachtii females and cysts, and its populations in soil increased substantially. These results suggest that H. oviparasitica is the primary factor driving H. schachtii suppression in this region, and that its ability to rapidly increase its population is one of the key features that enable this fungus to create effective nematode-suppressive soils.