Image 1. Leaf lesions cause by frogeye leaf spot infection.
- Frogeye leaf spot is favored by extended periods of warm (77-86F), wet and humid weather (>90%HR) and frequent rains.
- Depending on environmental conditions, it can take 7-14 days for lesions to appear after infection.
- Frogeye leaf spot symptoms include oval or circular spots with distinct reddish-brown to dark purple margins.
- Foliar fungicide applications can help reduce the severity of frogeye leaf spot but be aware that resistance to strobilurin fungicides has been confirmed for this pathogen.
Pathogen biology
Frogeye Leaf Spot (FLS) is caused by the fungal pathogen Cercospora sojina and found across several U.S. States and Ontario, Canada. Extended periods of warm (77-86°F), wet/ humid weather (>90%HR) and frequent rains favor FLS development. The fungus can be seed-transmitted but the main source of FLS comes from infested soybean residue from the previous year. New infections get started when wind and splashing rain help disperse these spores to new plants and nearby fields. Young leaves are more susceptible to infection than older leaves. Depending on environmental conditions, it can take 7-14 days for lesions to appear after infection. Although FLS is known primarily as a leaf-spotting organism, it can also infect stems, pods and seeds. FLS is a polycyclic disease in which infection, lesion formation and spore production can happen several times during the growing season if the environment is favorable. Yield losses of 10-60% have been reported for this disease.
Symptoms
Early symptoms of FLS include dark spotting of leaves. The oval or circular dark spots later turn brown with distinct reddish-brown to dark purple margins (Image 1). The centers of the spots, or lesions, become lighter as they age. The lesions vary in size but are rarely larger than 3/16” across. Lesions are visible on both tops and undersides of leaves. With severe infection levels, leaf lesions may merge. When lesions cover about a third of the leaf, a blight phase may occur where leaves will shrivel up and fall from the plant. In highly susceptible cultivars, FLS lesions may develop on the pods and stems. Lesions on pods are circular to oval in shape and reddish brown to light gray with dark margins. When pods are infected, FLS can infect seeds and if these seeds are planted, the pathogen will be introduced in that new field. Frogeye leaf spot symptoms can be confused with those of herbicide injury or those caused by other pathogens (i.e. Phyllosticta leaf spot, bacterial blight). However, when FLS is sporulating, a gray-silver fuzzy mass may develop in the center of the lesion on the underside of the leaf (Image 2). That fuzzy mass won’t be visible in lesions caused by Phyllosticta or bacterial blight.
Image 2. Frogeye leaf spot lesion on the underside of the leaf. Note the grey-silver spore mass in the center of the lesion.
Management
- Soybean varieties display different levels of resistance to FLS and soybean varieties resistant or partially resistant to this pathogen are available. Yield loss in the most susceptible varieties may be 30% or more. When making seed selection decisions be sure to check the FLS rating for the variety you want to plant.
- Seed from infected fields should never be saved for future planting. Infected seed often has poor germination and this seed may be the source of new infections. Infected seed may be symptomless or have light-dark gray blotches and cracked seed coats. Fungicide seed treatments may help reduce seedling expression of the disease, but are no substitute for clean, disease-free seed.
- Where practical, corn rotation and tillage of infected residue will reduce the survival of the pathogen and the amount of local inoculum. Corn and small grains are not susceptible to FLS and are a good and practical rotation choice.
- Foliar fungicide applications can reduce the severity of FLS and minimize the yield losses associated with it. Research work across the U.S. has indicated that applications made from R2-R4 growth stages are the most efficacious for management of FLS and yield protection. Several commercial fungicides are effective against FLS. However, be aware that FLS resistant to strobilurin fungicides has been reported in the U.S. Therefore, using a fungicide that contains active ingredients from at least two different modes of action is recommended for resistance management.
Sources
APS Frogeye leaf spot: https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/FrogeyeLeafSpot.aspx
Crop Protection Network: https://soybeanresearchinfo.com/pdf_docs/CPN1017_frogeyeleafspot.pdf