Isolates of Didymella bryoniae from South Carolina Remain Sensitive to DMI Fungicides Despite Multiyear Exposure

Abstract

Triazole fungicides, which are sterol demethylation inhibitors, have become the primary systemic fungicides applied to cucurbits to control gummy stem blight caused by Didymella bryoniae. Isolates of D. bryoniae from South Carolina that were never exposed to tebuconazole or exposed for several years were tested for sensitivity to tebuconazole and difenoconazole. Colony diameters, percentage germination of ascospores and conidia, and germ tube lengths were measured when isolates were grown on agar amended with 0.10–10.0 mg/l tebuconazole and 0.01–1.0 mg/l difenoconazole. All 147 isolates tested were sensitive to tebuconazole and difenoconazole with mean EC50 values of 0.41 and 0.054 mg/l, respectively. Ascospore germination was greater than conidia germination on fungicide‐amended agar. Although the length of germ tubes arising from both spore types was reduced by both fungicides, the reduction was greater for ascospore germ tubes than for conidia germ tubes. Because many watermelon growers rotate crops among fields every two years, local populations of D. bryoniae have not been exposed repeatedly to tebuconazole. In addition, growers often apply a rotation of systemic and contact fungicides. Thus, despite exposure to tebuconazole for up to nine years, isolates of D. bryoniae from South Carolina remain sensitive to triazole fungicides.

Baseline Sensitivity of Didymella bryoniae to Cyprodinil and Fludioxonil and Field Efficacy of these Fungicides Against Isolates Resistant to Pyraclostrobin and Boscalid

“…The relatively small ranges and resistance factors for cyprodinil and fludioxonil for isolates of D. bryoniae that never were exposed to these fungicides was similar to the ranges (0.03 to 0.25) and resistance factors (2.7 to 14.2) for D. bryoniae isolates never exposed to boscalid or difenoconazole (Keinath 2009;Keinath and Hansen 2013;Thomas et al 2012b). The ranges for baseline isolates from South Carolina and other states tested for sensitivity to azoxystrobin and tebuconazole were 10 and 19 times greater, respectively, than the range for cyprodinil (Keinath 2008;Keinath and Hansen 2013).…”

“…Isolates for the baseline study were collected between 1994 and 2008 to preclude exposure to cyprodinil and fludioxonil (Keinath 2009;Keinath and Hansen 2013). For this study, 146 isolates of D. bryoniae were selected from an extensive, existing culture collection.…”

“…Cyprodinil (99% technical grade) and fludioxonil (98% technical grade) (supplied by Syngenta Crop Protection) were dissolved in acetone and ethanol, respectively, to make stock solutions of 2.0 and 10.0 mg/liter (Baroffio et al 2003;Hilber and Schüepp 1996). Relative colony diameter was calculated by dividing the mean colony diameter on each fungicide-amended plate by the mean colony diameter across the two nonamended plates for each isolate (Keinath and Hansen 2013). Each liter of fungicide-amended medium received additional solvent to bring the final concentration to 1.0 ml/liter.…”

“…Rehm (= Stagonosporopsis cucurbitacearum (Fr.) In place of fungicides with these active ingredients, triazoles (FRAC group 3), anilinopyrimidines (FRAC group 9), and phenylpyrroles (FRAC group 12) have become the most important active ingredients used to manage gummy stem blight (Everts and Armentrout 2004;Everts and Zhou 2006;Keinath and Hansen 2013;Langston and Sanders 2010;Sanders and Langston 2013;Seebold 2005;Thomas et al 2012a; Thornton et al 2005). Gummy stem blight is particularly severe on watermelon (Citrullus lanatus (Thunb.)…”

Toxicity of copper hydroxide, dithianon, fluazinam, tebuconazole and pyraclostrobin toDidymella applanataisolates from Serbia

“…[27] Tebuconazole is a C-14 demethylation inhibitor (DMI) of a biosynthetic step that occurs during the conversion of lanosterol to ergosterol, the final product of fungal sterol synthesis. [28] According to Fuchs and Drandarevski, [29] development of DMI resistance under practical conditions would be unlikely since laboratory DMI resistant mutants had reduced pathogenicity.…”