Influences of certain fungicides on parasitism of the nematode Criconemella xenoplax by the fungus Hirsutella rhossiliensis

Seven fungicides commonly used in South Carolina peach orchards were tested in the laboratory and greenhouse for their effects on Hirsutella rhossiliensis, a fungal parasite of Criconemella xenoplax. Benomyl at 1, 5, and 10 microgram a.i./ml completely inhibited hyphal growth of the fungus. Chlorothalonil, dicloran, iprodione, and triforine significantly suppressed hyphal growth at 20 microgram a.i./ml, but captan inhibited growth only at 40 microgram a.i./ml. Sulfur had no effect at the concentrations studied. Conidial germination and germ tube elongation were more sensitive to benomyl than to iprodione or triforine. Captan and dicloran slightly inhibited spore germination and germ tube elongation, but chlorothalonil and sulfur had no effect. In the greenhouse, H. rhossiliensis suppressed populations of C. xenoplax, but fungicides did not affect nematode populations, reproduction, or development. The frequency of parasitism of C. xenoplax was not measurably affected by any fungicide. However, fewer conidia were observed on nematodes exposed to some fungicides, and nematodes infested with conidia in chlorothalonil-treated soil were more likely to be parasitized. Soil residues after foliar application of benomyl or chlorothalonil in peach orchards were low relative to the concentrations studied in the greenhouse. Routine applications of fungicides commonly used in South Carolina peach orchards should not suppress activity of H. rhossiliensis as a parasite of C. xenoplax