Genotypic Diversity and Resistance to Azoxystrobin of Cercospora beticola on Processing Table Beet in New York

Cercospora leaf spot (CLS), caused by Cercospora beticola, is one of the major diseases affecting productivity and profitability of beet production worldwide. Fungicides are critical for the control of this disease and one of the most commonly used products is the quinone outside inhibitor (Q_OI) azoxystrobin. In total, 150 C. beticola isolates were collected from two commercial processing table beet fields in Batavia, NY in 2014. The mating types of the entire population were determined, and genetic diversity of a subset of samples (n = 48) was assessed using five microsatellite loci. Sensitivity to azoxystrobin was tested using a spore germination assay. The cytochrome b gene was sequenced to check for the presence of point mutations known to confer Q_OI resistance in fungi. High allelic diversity (H_e = 0.50) and genotypic diversity (D* = 0.96), gametic equilibrium of the microsatellite loci, and equal ratios of mating types were suggestive of a mixed mode of reproduction for C. beticola. Resistance to azoxystrobin was prevalent because 41% of the isolates had values for effective concentrations reducing spore germination by 50% (EC₅₀) > 0.2 μg/ml. The G143A mutation, known to cause Q_OI resistance in C. beticola, was found in isolates with EC₅₀ values between 0.207 and 19.397 μg/ml. A single isolate with an EC₅₀ of 0.272 μg/ml carried the F129L mutation, known to be associated with low levels of Q_OI resistance in fungi. This is the first report of the F129L mutation in C. beticola. The implications of these findings for the epidemiology and control of CLS in table beet fields in New York are discussed.