Frogeye leaf spot (FLS) is one of the most common foliar diseases of soybean in Arkansas. In 2012, the first detection of strobilurin-resistant (S-R) FLS was confirmed in St. Francis Co., and since then it has been reported in eleven different counties across the state. This article is intended to provide a brief review of S-R FLS that includes disease identification, risk factors for disease development, distribution, and management options.
Frogeye leaf spot is caused by the fungus Cercosproa sojina. Immature lesions look similar to mature lesions; however, they lack the pronounced maroon or purple edge surrounding the lesion (Fig. 1). Mature lesions are similar in size (1 to 5 mm diameter) with a tan/brown center and reddish or purple margin (Fig. 2). Identical lesion symptoms are caused by both S-R and strobilurin-sensitive strains of the fungus. Therefore, a lab assay is often needed to confirm S-R, but this is not the only method, see below.
Early detection is beneficial to determine management options, and scouting high-risk fields increases the likelihood of early detection in an area. A high-risk field would be planted in a highly susceptible soybean variety in a field that has a history of FLS. These fields are often in continuous soybean production. A low-risk field would be planted in a moderately resistant to resistant variety along with pathogen-reducing tillage and rotation practices.
The pathogen overwinters in soybean residue and is dispersed by splashing rain onto nearby plants or disseminated by wind to another field. Though FLS can occur at any stage of growth, the young emerging leaves are the most susceptible. FLS is often observed on susceptible soybean varieties at or near canopy closure when weather conditions favor disease development (cool temperatures (77 to 86ºF) and prolonged dew periods).
Generally, all fungicide resistance develops in a similar manner. First, a mutant strain (i.e. resistant to the fungicide) develops in the field. Initially, these strains are at a very low percentage of the total pathogen population and overs several years build up with continued selection pressure (i.e. using the same fungicide chemistry). These strains (S-R FLS strains) build up faster and are more common in high-risk fields where low rates of strobilurin fungicides are used to manage FLS.
Though FLS is commonly found in all soybean producing counties, S-R FLS has only been confirmed in eleven counties (Fig. 3). A lab test was used to confirm resistance based on suspicious isolates collected in one or two fields per county (Fig. 3). Suspicious isolates were collected where growers or consultants suspected poor control of FLS by a strobilurin fungicide. Based on samples collected from the past two years, growers and consultants are very good at detecting S-R FLS when these new strains make up 15-25% of the field population. In the future, new fields with S-R FLS will likely increase, especially along the Mississippi River Delta, but good management practices will minimize yield loss.
Host-plant resistance continues to be the most effective method to manage both S-R and strobilurin-sensitive strains of FLS. In 2013, field trials conducted at the Newport Extension Station confirmed the durability of host-plant resistance on these new strains. Several hundred varieties were inoculated with S-R FLS and evaluated for susceptibility. Basically, there was no change in host-plant resistance between S-R and strobilurin-sensitive isolates of FLS. In fields where S-R was confirmed, triazole fungicides were effective at managing these new fungal stains. A fungicide efficacy table can be found in the 2014 MP154 Soybean along with other recommended fungicides; however, the table does not separate S-R and strobilurin-sensitive strains. In areas where S-R FLS was confirmed (Fig. 3), it is recommended that strobilurin fungicides are mixed or alternated with triazoles fungicides in high risk fields rather than relying on a strobilurin-alone program to manage FLS. Strobilurin fungicides continue to be effective on many of the other soybean disease; for example, anthracnose, brown spot, and pod and stem blight to mention a few. Additionally, tillage and one year crop rotation can reduce the overwinter pathogen survival on crop residue.