Strobilurin-resistant frogeye leaf spot (FLS) was confirmed on two fungal isolates collected from St. Francis and Phillips, Co., AR. This is the first time strobilurin-resistant FLS has been reported in AR, though resistant strains have been reported in neighboring states (TN, LA, and MO). Strobilurin fungicides or more properly, Quinoine outside inhibitor (QoI) fungicides, are one of the most common fungicide classes used to manage foliar soybean disease, thus it is very likely that other locations within the state have populations of strobilurin-resistant FLS.
Frogeye leaf spot is caused by the fungus Cercosproa sojina. Infected soybean leaves have small (1 to 5 mm diameter) leaf spots with a tan/brown center and reddish or purple margin (Fig. 1). The pathogen overwinters in soybean residue and is dispersed by splashing rain or wind onto soybean plants. Warm, wet (heavy dew) weather conditions favor fungal infection and increased disease development in the field.
Strobilurin-resistant isolates of C. sojina are less sensitive to ALL QoI fungicides (FRAC group 11), which includes azoxystrobin, fluoxastrobin, pyraclostrobin, and trifloxystrobin. The QoI fungicides are classified as being at high risk for having resistance develop in targeted fungi by the Fungicide Resistance Action Committee (FRAC). Selection pressure occurs anytime a fungicide is applied and individuals in the fungal population with reduced sensitivity to that fungicide are selected. Repeated use of the same fungicide increases the risk of selecting fungicide-resistant strains of the target fungi that increase in the target fungal population. The fields St. Francis and Phillips counties had a high FLS severity (90+ leaf spots per leaflet) and incidence (100 % of the field) two weeks after axozystrobin was applied (Fig. 1), which suggested repeated selection pressure for strobilurin-resistant strains of C. sojina. This was confirmed in an in-vitro laboratory assay that determined the EC50 (effective concentration to kill 50% of conidia) to be 280 times higher than a strobilurin-sensitive strain of C. sojina.
There are several management tactics to reduce selection of fungicide-resistant fungi. First, plant soybean varieties with high levels of resistance to frogeye leaf spot. Resistance is the most economical method to manage FLS, particularly in high risk areas. Soybean varieties are screened annually for resistance to FLS and published on the Arkansas variety testing website. Secondly, choose different fungicide classes when a susceptible variety is grown in a high-risk area. Strobilurin-resistant strains of FLS can be managed with triazoles and thiphanates class fungicides (FRAC group 3 and 1, respectively), see MP 154 for trade names that contain these fungicide classes. Finally, apply a foliar fungicide only to control plant disease based on disease risk and scouting observations will slow the development and spread of fungicide-resistant strains of plant pathogenic fungi.