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(Accepted 9 March 1988) Summary-A “dilution plate assay” was developed to quantify parasitism of C. xenoplux 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 10e6 streptomycin sulfate. After 5 days at 22 k Z’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 IO, I5 or 2o’C was 61 , 26 or I6 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 INTRODUCl’ION Criconemellu xenoplux Raski (Luc and Raski) is a soil-borne pathogen of stone fruit trees and is attacked by the endoparasitic fungus Hirsutella rhossiliensis Minter and Brady (Jaffee and Zehr, 1982). There are four juvenile stages (designated J I, J2, J3 and J4), the adult female and eggs. Except for the egg and Ji (which is in the egg), all stages are motile and live in the soil. H. rhossiliensis grows from nematodes that it has parasitized and produces non-motile spores on bottle-shaped phialides. Spores adhere to passing nematodes. After the nematode is infected and killed, the fungus converts the material within the body cavity into hyphae. Four to five days after infection of adult C. xenopla.r at 25’C, the hyphae grow out of the nematode and produce distinctive phialides and spores. Sporulation continues, and the hyphae within the cuticle contract until the reserves within the cuticle are depleted (Jaffee and Zehr, 1982, 1983). Hirsurella rhossiliensis is a’poor competitive saprophyte and appears to be specialized for |
