Frogeye leaf spot, caused by the fungus Cercospora sojina is a soybean disease that infects leaves, stems and pods. Although the disease is common worldwide, it is most serious in warm regions or during periods of warm, humid weather. In the U.S., the disease is most common in the mid-South, Mississippi Delta, and Southeastern soybean growing areas, but development of resistant varieties there has limited the impact of the disease. In individual fields, losses may be negligible with minor symptoms to as high 30% on susceptible varieties with severe leaf blighting.
Frogeye Leaf Spot in the Midwestern U.S.
Frogeye leaf spot was identified in Midwest U.S. soybean fields over 50 years ago, and was widespread in Illinois, Indiana and other states. But the disease decreased in importance long ago and had practically disappeared until recently. Then, in 1999, frogeye was detected in soybean fields in both Iowa and Missouri (Yang, 1999; Sweets, 2000), but its occurrence was limited and damage was minimal. In 2000, frogeye leaf spot occurred again in these states, and at high enough levels in central and northern Missouri to cause yield losses in some fields (Sweets, 2001; Yang, 2000.)
Infection and Symptoms
The Cercospora sojina fungus survives in soybean residue and seeds. Initial infection occurs as spores produced on infected residues or cotyledons are spread by splashing rain or winds. Secondary infection occurs as lesions on the soybean plant produce more spores. Disease development is favored by warm, humid conditions, and frequent rains following disease onset can lead to serious epidemics. On the other hand, dry weather severely limits disease development.
Young, expanding soybean leaves are most susceptible to the fungus, and older leaves tend to be resistant. Consequently, leaves expanding during warm, humid conditions may show symptoms, while leaves developing under dry conditions remain disease-free.
Initial symptoms appear about two weeks after infection as small, circular to somewhat irregular spots on the upper surface of the leaf. Initial dark, water-soaked spots develop into lesions with dark brown centers surrounded by red or dark reddish-brown margins. As lesions age, the center becomes light brown to light gray, and the border remains dark.
Minor leaf damage is not likely to affect soybean yield, but if high rainfall and humidity persist, the disease may escalate quickly. Leaf lesions may coalesce to form larger irregular spots on the leaf. Heavily diseased leaves may wilt and drop prematurely, or dead tissue may weather away, leaving tattered leaflets.
Stem, Pod and Seed Symptoms
Stems and pods may also become infected as the disease progresses. Stem lesions are reddish brown with a narrow, dark margin. The centers of the lesions become brown to gray with age.
Lesion development on pods is similar to that of the leaves. Symptoms begin as water-soaked spots that progress to dark reddish-brown lesions. Lesions are circular to elongate in shape, and may appear slightly sunken and lighter-colored in the center. The fungus can also grow through the pod wall to infect maturing seeds. These seeds may show cracking of the seed coat and discoloration ranging from small specks to large blotches.
Frogeye Leaf Spot on Soybeans
Management of Frogeye Leaf Spot
Soybean growers in the areas where damaging levels of frogeye leaf spot occur may want to consider management practices to limit the impact of the disease. This is most important in fields with known occurrence of the disease, especially where disease levels were high enough to impact crop yields. The following practices can help limit the spread of and damage from frogeye leaf spot (Sweets, 2001):
Plant tolerant varieties. Soybean varieties differ in their tolerance to frogeye leaf spot. In fields that had frogeye leaf spot in recent years, growers should consider tolerant varieties. However, other diseases may present a higher risk than frogeye. Growers shouldn't sacrifice resistance to soybean cyst nematode, Phytophthora root rot or other key traits for frogeye leaf spot tolerance. Your Pioneer representative can help you select suitable varieties for your farm.
Consider tillage. Survival of the frogeye leaf spot pathogen is higher when infested soybean residues are left on the soil surface. Plowing or turning residues under lowers the levels of pathogen inoculum. Reduced inoculum levels can delay the onset and spread of the disease, thus lowering its potential to damage the crop and reduce yields.
Rotate crops. A rotation of at least one year between soybean crops is necessary to try to reduce inoculum levels in the field. In reduced or no tillage situations where tillage cannot be used to reduce inoculum levels, crop rotation becomes even more important.
Plant high quality seed. Tests for frogeye leaf spot are not routinely performed on seed grown in the Midwest. Choose seed that has been cleaned and tested for germination. Only high quality seed with good germination should be used.
Consider a fungicide seed treatment. If there is a question or concern that the seed lot may not be disease-free, growers could consider the application of a fungicide seed treatment to their soybean seed. A broad spectrum seed treatment fungicide with an active ingredient such as captan, carboxin, fludioxinol, PCNB, thiabendazole or thiram should be used.
Foliar fungicides. Use of a foliar fungicide is seldom warranted in the Midwest U.S., because disease pressures high enough to justify the cost of the treatment are rarely encountered. Field trials with foliar fungicides such as benomyl and chlorothalonil applied at early- and mid-pod fill have been shown to reduce the incidence of frogeye leaf spot, but corresponding increases in yield with the fungicide treatments have not been consistent.
Sweets, L. 2000. Frogeye leaf spot on soybean. In Integrated Pest & Crop Management Newsletter Vol. 10, No.19 (8/4/2000). University of Missouri, Columbia.
Sweets, L. 2001. Frogeye leaf spot of soybean. In Integrated Pest & Crop Management Newsletter Vol. 11, No.1 (1/19/2001). University of Missouri, Columbia.
Yang, XB. 1999. Fall soybean disease scouting. In Integrated Crop Management IC-482(22) (9/13/1999). Iowa State U., Ames.
Yang, XB. 2000. Mid-summer soybean diseases. In Integrated Crop Management IC-484(20) (8/7/2000). Iowa State U., Ames.
Wrather, J.A. 2001. Soybean Disease Loss Estimates for the United States, 1996-2000. Report from the Missouri Ag Experiment Station, Delta Research Center, Portageville, MO.