Effect of Abamectin on Fungal Growth and Its Efficacy as a Miticide in the Laboratory

Abamectin was tested for use with solid agar media in the laboratory to eliminate or kill the common mold mite Tyrophagus spp. in fungal cultures of Phaeomoniella chlamydospora and Phaeoacremonium minimum, two important grape pathogens involved in grapevine trunk disease. Abamectin concentrations tested were at or below the recommended dosage for abamectin in greenhouse spray applications (≤625 µg/ml) to control mites and determine the following: (i) if fungal growth would be inhibited and (ii) if mites would be killed or their activity suppressed. Abamectin was either added to the media before autoclaving or filter-sterilized and added after autoclaving to test the effects of autoclaving on abamectin efficacy. Streptomycin (100 µg/ml) was also added to a set of treatments to determine whether this commonly used antibiotic would affect abamectin efficacy against mites or have an effect on fungal growth when combined with abamectin. Filter-sterilized abamectin in the range of 62.5 to 312 µg/ml, delivered to the media after autoclaving, provided the most effective control of mites while also showing limited inhibition of fungal growth on solid agar media in the absence of streptomycin. The addition of filter-sterilized streptomycin had no significant effect on fungal growth for Phaeomoniella chlamydospora, whereas for Phaeoacremonium minimum a small but significant reduction in growth with streptomycin occurred at abamectin concentrations >62.5 µg/ml.