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Oral: Fungicide Resistance


Survey for Quinone Outside Inhibitor Resistant Cercospora sojina in Virginia Soybeans
T. ZHOU (1) H. Mehl (1); (1) Virginia Tech Tidewater AREC, U.S.A.

Quinone outside inhibitor (QoI) fungicides control a wide spectrum of fungal diseases, but this class of fungicides has a high risk of developing resistance. QoI fungicides are used in the management of frogeye leaf spot (FLS) caused by Cercocspora sojina. In 2014, FLS isolates from four fields in VA were tested, and QoI resistance was confirmed from two fields. In 2015, a statewide survey was conducted to estimate the prevalence of QoI resistant FLS in VA. Leaflet samples with FLS symptoms were collected from 55 soybean fields throughout the VA soybean growing region. Ten leaflets were randomly selected per sample and conidia were isolated from one lesion per leaflet. A total of 116 isolates were successfully cultured from 25 locations, and DNA was extracted from monoconidial cultures. A PCR-based assay was used to screen isolates for the G143A mutation in the cytochrome b gene that confers QoI resistance. Of the 116 isolates, 24% were positive for the G143A mutation. At the 10 locations with QoI resistant isolates, frequencies of the mutation ranged from 50 to 100%. DNA was extracted directly from leaves collected at all 55 locations, and a pyrosequencing assay was designed to quantify the frequencies of the G143A mutation within the C. sojina populations in each field. The survey confirms the presence of QoI resistant FLS in Virginia, so growers should use a different or mixed mode of action fungicides, plant resistant varieties, and rotate to non-host crops.