Indigenous Populations of a Biological Control Agent in Agricultural Field Soils Predicted Suppression of a Plant Pathogen

The nematophagous fungus Hyalorbilia oviparasitica and
relatives (Hyalorbilia spp.) are known to parasitize several
endoparasitic nematodes. In this project, we hypothesized
that indigenous populations of this fungus could
be used to predict nematode suppression in agricultural
field soils. We quantified Hyalorbilia spp. in soil samples
from 44 different sugarbeet fields in the Imperial Valley
of California. Seven soils harboring Hyalorbilia spp. and
two that tested negative for the fungi were examined for
nematode suppressive activity. Untreated and autoclaved
portions of each soil were planted with cabbage and infested
with sugar beet cyst nematode (Heterodera schachtii)
juveniles. Females and cysts of H. schachtii were enumerated
after 12 weeks. In the seven soils harboring Hyalorbilia
spp., females and cysts in the untreated soils were
reduced by 61 to 82% compared with the autoclaved controls.
Soils with no detectable Hyalorbilia spp. exhibited
no nematode suppression. Two novel Hyalorbilia strains,
HsImV25 and HsImV27, were isolated from H. schachtii
females reared in field soil using an enrichment and double-
baiting cultivation technique. Both strains suppressed
H. schachtii populations by more than 80% in soil-based
assays, confirming that Hyalorbilia spp. are the likely
causal agents of the nematode suppression in these soils.
This study demonstrated that indigenous populations of a
hyperparasite (Hyalorbilia spp.) in agricultural field soils
predicted suppressive activity against a soilborne plant
pathogen (H. schachtii). To our knowledge, this is the first
report to demonstrate this capability. We anticipate that this
research will provide a blueprint for other similar studies,
thereby advancing the field of soilborne biological control.