Quantification of parasitism of the soil-borne nematode Criconemella xenoplax by the nematophagous fungus Hirsutella rhossiliensis

Abstract

A “dilution plate assay” was developed to quantify parasitism of C. xenoplax by H. rhossiliensis. Aliquots of C. xenoplax in suspension (treated with 0.5% NaOCl after extraction from soil by wet sieving and centrifugation) were spread onto 1.5% water agar plates supplemented with 200 parts 10 −6 streptomycin sulfate. After 5 days at 22 ± 2°C, the numbers of C. xenoplax with or without sporulating H. rhossiliensis were determined using a dissecting microscope. The assay was used to measure the rate of decay (disappearance) of parasitized C. xenoplax in field samples incubated in the laboratory. Time required for 50% reduction in number of parasitized nematodes (averaged across all life stages) incubated at −33 kPa soil water potential and at 10,15 or 20°C was 61, 26 or 16 days, respectively. Rate of decay was much faster for parasitized second- and third-stage juveniles than for parasitized fourth-stage juveniles and adults. Rate of infection can be estimated if the number of parasitized nematodes and rate of decay of such nematodes are known.