Quinone outside inhibitor-resistant Colletotrichum nymphaeae isolates from strawberry lack mutations in cytb gene

Anthracnose fruit rot is one of the main diseases affecting strawberries especially in warm and humid climates. In commercial settings, producers are relying on fungicides to control the disease. One of the most potent chemical classes of fungicides for anthracnose control are the quinone outside inhibitors (QoIs), but resistance in field isolates due to a point mutation in the target gene cytb has recently been reported in multiple states of the USA. During a routine screening, two Colletotrichum nymphaeae isolates (OH_S1 and OH_S2) obtained from strawberry plants of a commercial field in Marietta-Ohio were found to be resistant to QoI fungicide azoxystrobin. The EC50 values determined in poison agar tests were greater than 100 μg/L. Both isolates were not controlled on detached strawberry fruit treated with label rates of Abound fungicide (a.i. azoxystrobin) and revealed disease severity of 86% and 96% on detached fruit. A baseline isolate (Cn_ebel2) of the same species revealed an EC50 value of 1.2 μg/L and no disease developed on Abound-treated fruit. In contrast to other reports, sequencing of the cytb gene from all three isolates used in this study revealed no amino acid changes in cytb, indicating the existence of a different and still unknown resistance mechanism.