Fungicide sensitivity survey of pecan scab in Texas pecan orchards

Abstract

A fungicide sensitivity survey of pecan scab (Venturia effusa) from eight different locations in seven pecan orchards in Texas was conducted during the summer of 2021. Using a rapid fungicide sensitivity assay based on single discriminatory concentrations, sensitivities to five fungicides (dodine, fentin hydroxide, thiophanate-methyl, propiconazole, and tebuconazole) were evaluated. Pecan orchards with frequent fungicide use tended to have greater levels of reduced sensitivities to demethylation inhibitor (propiconazole and tebuconazole; >40% of conidial germination at 1 µg mL-1 of active ingredient) and fentin hydroxide fungicide (>64% of conidial germination at 3 µg mL-1 of active ingredient), compared to those with fewer fungicide applications in the past. All populations tested in this survey exhibited a high level of resistance to thiophanate-methyl fungicide but no resistance to dodine. Thiophanate-methyl did not inhibit conidial germination for six scab populations, while dodine consistently inhibited conidial germination for scab populations tested at the discriminatory concentration. The cytochrome b gene was sequenced for determining mutations associated with quinone outside inhibitor (QoI) fungicides. The mutation that results in a substitution of glycine 137 to serine (G137S) in the cytochrome b gene and is related to high resistance to QoI was not found in the representative isolates of scab populations tested in this survey. Our findings are the first report of fungicide resistance development in Texas pecan orchards. Information about fungicide sensitivities of scab populations in orchards will help pecan producers to prepare proper fungicide treatment plans.