Detection of Nematophagous Fungi from Heterodera schachtii Females Using a Baiting Experiment with Soils Cropped to Brassica Species from California’s Central Coast

Until the early 1990s, cyst nematodes were abundant pathogens in fields where hosts of Heterodera spp. were frequent members of crop rotations along California’s Central Coast. To mitigate damage caused by Heterodera schachtii and H. cruciferae, the soil fumigant 1,3-dichloropropene (1,3-D) was used by more than 43% of surveyed broccoli growers. Over the last few decades, use of 1,3-D and other nematicides has dramatically diminished, suggesting a decline in nematode disease pressure. The goal of this study was to examine the hypothesis that increased population densities of nematophagous fungi contribute to the low populations of Heterodera spp. in fields frequently cropped to their hosts. In 2016, soil samples were collected from 152 Brassica fields with a broad geographical distribution, from Santa Barbara County to Santa Cruz County. The average number of Heterodera cysts per 250 cm3 of soil ranged from 0.5 to 27.5, with 62% of the soils harboring no detectable cyst nematodes and only a few samples reaching a potentially damaging threshold level. A baiting experiment with H. schachtii and cabbage was performed in a greenhouse to detect nematophagous fungi associated with nematode females as their posterior end emerged and became exposed to the soil’s rhizosphere. An Illumina-based sequence analysis of these H. schachtii females identified several known nematophagous fungi, including members of the Hyalorbilia oviparasitica clade, Pochonia chlamydosporia, certain Fusarium spp., and others. These soils clearly harbor a diverse population of hyperparasitic fungi that could be biologically suppressing cyst nematodes below a damaging threshold. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .