Prevalence of FRAC Group 11 Fungicide Resistance in Stemphylium vesicarium Isolates, but Not S. beticola Isolates, Causing Stemphylium Leaf Spot of Spinach (Spinacia oleracea).

ABSTRACT

Stemphylium leaf spot of spinach, caused by Stemphylium beticola and S. vesicarium, is a disease of economic importance in fresh market, processing, and seed production. There have been increasing reports of difficulty managing the disease in the southern United States using fungicides in Fungicide Resistance Action Committee (FRAC) group 11. Isolates of S. beticola and S. vesicarium obtained from spinach leaves and seed from 2001 to 2020 were screened for resistance to azoxystrobin and pyraclostrobin in vitro, in vivo, and using PCR assays to detect mutations in cytochrome b associated with resistance in other fungi (F129L, G137R, and G143A). EC50 values for mycelial growth and conidial germination of S. vesicarium isolates in vitro were significantly less (mean of 0.35 µg/ml) than that of S. vesicarium (mean of 14.17 µg/ml) with both fungicides. All isolates were slightly more sensitive to pyraclostrobin than azoxystrobin in both assays. In vivo assays of plants inoculated with the isolates of S. vesicarium demonstrated poor efficacy of fungicides with each of the two active ingredients. Only the G143A mutation was detected in all spinach isolates of S. vesicarium, including an isolate of S. vesicarium collected in 2003 and 82.9% of isolates from spinach seed lots harvested from crops grown in or after 2017 in Europe, New Zealand, and the United States. The FRAC 11 mutations were not detected in any isolates of S. beticola. The in vitro, in vivo, and DNA mutation assays suggest FRAC group 11 fungicide resistance is widespread in spinach isolates of S. vesicarium but not S. beticola.