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.

Antioxidants and iron chelators inhibit oxygen radical generation in fungal cultures of plant pathogenic fungi.

Eutypa dieback and Esca are serious fungal grapevine trunk diseases (GTDs). Eutypa dieback is caused by Eutypa lata (Elata), and is often associated Phaeoacremonium minimum (Pmin), and Phaeomoniella chlamydospora (Pch) which are also important contributors to Esca disease. Understanding the complex pathogenesis mechanisms used by these causative fungi may potentially lead targeted treatments for GTDs in the future. Elata has been reported as a wood decay "soft rot" fungus and understanding of Elata's pathogenesis chemistries can aid in controlling GTDs. Recent work that suggests that Pmin and Pch may contribute to pathogenesis by stimulating hydroxyl radical generation via secretion of low molecular weight phenolic metabolites. Building on these findings, we tested a hypothesis that antioxidants and chelators, and biocontrol agents that have been reported to secrete antioxidants and low molecular weight chelators, may inhibit the growth and activity of these fungi. Butylated hydroxy anisole (BHA) and butylated hydroxytoluene (BHT) were tested as antioxidant/chelators. BHA was found to be a highly effective control measure for the three pathogenic fungi tested at concentrations >0.5 mM. The biocontrol species Bacillus subtilis and Hypocrea (Trichoderma) atroviride were also tested, with both H. atroviride and B. subtilis effectively inhibiting growth of the three GTD fungi.

Oxygen Radical-Generating Metabolites Secreted by Eutypa and Esca Fungal Consortia: Understanding the Mechanisms Behind Grapevine Wood Deterioration and Pathogenesis.

Eutypa dieback and Esca complex are fungal diseases of grape that cause large economic losses in vineyards. These diseases require, or are enhanced by, fungal consortia growth which leads to the deterioration of the wood tissue in the grapevine trunk; however, pathogenesis and the underlying mechanisms involved in the woody tissue degradation are not understood. We examined the role that the consortia fungal metabolome have in generating oxygen radicals that could potentially play a role in trunk decay and pathogenesis. Unique metabolites were isolated from the consortia fungi with some metabolites preferentially reducing iron whereas others were involved in redox cycling to generate hydrogen peroxide. Metabolite suites with different functions were produced when fungi were grown separately vs. when grown in consortia. Chelator-mediated Fenton (CMF) chemistry promoted by metabolites from these fungi allowed for the generation of highly reactive hydroxyl radicals. We hypothesize that this mechanism may be involved in pathogenicity in grapevine tissue as a causal mechanism associated with trunk wood deterioration/necrosis in these two diseases of grape.