Foliar Health Basics: Frogeye Leaf Spot Control
Frogeye leaf spot (Cercospora sojina) can cause severe soybean yield loss. In the United States, it is most common in the South and Mid-South, but can appear wherever warm, humid conditions occur. Frogeye leaf spot (FLS) has been observed as far north as Minnesota and appears to be expanding its range.
FLS causes disease on younger upper foliage of soybean plants. This makes the disease especially damaging to photosynthesis and resulting yield unless it is controlled early. Soybean pods also are often infected, presenting both a yield and quality challenge.
Agronomists at the University of Tennessee and other states estimate that FLS has caused an average 16 percent yield loss over the past five years, although losses as great as 60 percent have been observed in some fields. The disease is known to be resistant to strobilurin fungicides in at least eight states.
FLS survives on crop residue, often appearing in no-till and continuous soybean fields. Spore survival is favored by mild winters. The fungus produces spores under warm, humid conditions with heavy dews (77 to 86 degrees F; greater than 90 percent relative humidity). Dense soybean canopies and high plant populations favor these conditions.
Untreated FLS is polycyclic, meaning it will produce multiple generations of spores, which increases disease severity. Disease may spread rapidly within a field and to other fields.
The first signs of FLS are usually seen right after flowering, but infection can occur at any growth stage if conditions are right. Check both sides of leaves for tan/brown elliptical lesions with brown to purple borders. Under humid conditions, inspection with a hand lens may reveal long silver spore-bearing hyphae extending from black dots on the undersides of leaves.
Signs of Infection
- Diseased plants tend to have a layered appearance because infection is more severe on young leaves. As infection spreads, spots coalesce to create irregular patterns. Under severe disease pressure, lesions may coalesce and cause leaf drop.
- The most common initial signs are small yellow spots on leaves. These spots eventually enlarge to a diameter of about 0.25 inch. Lesion centers become gray to brown and have red-purple margins. Under humid conditions, lesions on any part of the plant may develop dark centers when the fungus is producing spores. Early signs are often mistaken for herbicide drift or other leaf diseases.
- Stem lesions are somewhat red when new and darken with age. They lack the characteristic tan center and reddish purple border of leaf lesions.
- The fungus may grow through pod walls and invade seed. Pod lesions are circular or oblong, reddish-brown, and slightly sunken. Infected seeds may appear shriveled and seed coats may crack.
High spore counts increase risk of FLS resistance to fungicides, since each spore presents another opportunity for resistance development. Application of fungicides with two modes of action is recommended for effective control and to combat resistance.
Most severe cases of resistant FLS have been found in soybean varieties without the Rcs3 gene. U.S. soybean breeders have added the gene to some soybean varieties, more commonly suited to conditions in the South.
- If possible, plant soybean varieties with the Rcs3 gene to convey resistance. Complete resistance to all strains of FLS is not yet available in soybean varieties, especially northern varieties.
- Crop rotation and tillage can reduce disease pressure.
- Timely application of a fungicide with multiple modes of action can preserve green leaf material and prevent disease spread by sporulation. DuPont™ Aproach® Prima fungicide combines a strobilurin with a triazole for better control of resistant FLS to protect yield and input investments. Apply a full rate (6.8 fluid ounces per acre) of Aproach® Prima to soybeans at R2 to R3.
DuPont™ Aproach® Prima fungicide is not registered in all states. See your DuPont retailer or representative for availability in your state.
Fortix® (FMC); Priaxor® (BASF); Quadris® (Syngenta); Stratego® (Bayer).
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