Dactylella oviparasitica parasitism of the sugar beet cyst nematode observed in trixenic culture plates

Abstract

The nematophagous fungus Dactylella oviparasitica is considered the primary cause of a sugar beet cyst nematode (Heterodera schachtii) population suppression in a field at the Agricultural Operations, University of California, Riverside. Parasitism of H. schachtii by the ascomycete D. oviparasitica was studied using both Arabidopsis thaliana (type Landsberg erecta) and cabbage as host plants in gnotobiotic agar culture. Suitability of Arabidopsis as a host for H. schachtii was confirmed using seedlings grown with the nematode in axenic sand culture. Both developing males and females of H. schachtii broke through the Arabidopsis root surface during late juvenile stages and both were susceptible to D. oviparasitica parasitism. In contrast to Arabidopsis, developing juvenile males remained in nearly all observed cases enclosed within the cabbage root tissues while the larger body expansion of the female juveniles caused the root cortex to split; consequently only the latter ones were accessible to the fungus. In the presence of D. oviparasitica, the number of females with eggs was reduced by more than 95% and the number of eggs per female by almost 60% as compared to females developing on plates without the fungus. Viable eggs were not susceptible to parasitism while more than 90% of heat- or cold-killed eggs were rendered susceptible. These observations suggest that parasitism of developing juveniles may be the essential mode of action in the population suppression of H. schachtii.