Frogeye Leaf Spot of Soybean
Soybean Disease Information Note 3
Stephen R. Koenning, Extension Plant Pathologist

Frogeye leaf spot of soybean has been detected in the Piedmont and the northeastern counties of North Carolina. This potentially destructive disease of soybean has not been common in North Carolina in the last 20 years, because the majority of varieties grown were resistant. Frogeye leaf spot caused severe soybean yield losses in the mid-South and Mississippi Delta regions in 1989. While it is still too early to assess the current importance of this disease in North Carolina, many varieties released in the last 5-10 years are susceptible. Frogeye leaf spot, as the name indicates, is primarily a foliar disease of soybean. The fungus, Cercospora sojina, which causes this disease, however, can be seedborne. Frogeye leaf spot is most likely to become a problem if infected seed is planted or if the disease occurred in the previous year's soybean crop and the land is not rotated. Extended periods of wet weather during the growing season will favor disease development.

Symptoms

Lesions on leaves are circular to angular spots which vary in size from less than 1 mm to 5 mm in diameter. Lesions are first visible on the upper surface of the leaf. These lesions are distinctive in that the brown spots are surrounded by a narrow red or dark reddish-brown margin. As lesions age, the central area becomes ash-grey to light brown. Older lesions are light to dark brown and frequently are translucent, having a grey to white center which may contain minute dark spots. Smaller lesions may coalesce to form larger, irregular spots on leaves. During a wet year, symptoms of Frogeye leaf spot may appear uniformly over the foliage. In years with intermittent wet periods, the symptoms may appear layered within the plant canopy since only young leaflets are susceptible to infection. When plants are heavily infected, leaves may die and fall prematurely. This can result in early defoliation of soybean plants. If high rainfall and humidity persist, stems and seeds also may become infected. Lesions on pods are reddish brown, may appear sunken, and are circular or elongate in shape. Older lesions on pods become brown to gray, usually with a narrow, dark-brown border.

Management

The most important means for managing this disease in soybean is avoidance. Planting resistant varieties or high quality seed of certified varieties free of this pest are the best packages for management. If you detected or think you detected frogeye leaf spot in your soybean crop last year, there are several actions you should take. The first step would be selection of a resistant variety. If you cannot plant a resistant variety (SOY006), fields which had frogeye leaf spot last year should not be planted to soybean for at least one year and preferably two. The fungus which causes this disease will survive in crop residue. By all means, avoid planting seed from fields which may have been infected with frogeye leaf spot the previous year. The seed-borne aspect of this disease is probably the most important form of spread over long distances. Soybean varieties differ in their resistance to this pathogen. Some public varieties and many varieties developed by private companies appear to be immune while many other varieties possess resistance to certain races of the pathogen. Since we don't know which races are prevalent in North Carolina, selection of a variety with a high level of resistance or immunity is preferable. If this disease was not present last year, it is not necessary to select a variety solely on the basis of frogeye leaf spot resistance. However, if the disease was detected in your crop last year a resistant variety (SOY006) would be an appropriate choice.

When scouting soybean fields for weeds and insects, take notes on the severity and prevalence of foliar diseases. If you suspect frogeye leaf spot in your crop, contact your County Extension Agent and have symptomatic plants sent to the Plant Disease and Insect Clinic, North Carolina State University, at the first opportunity. Should you encounter this disease or if you have fields that were suspect last year, control methods are:

1. plant-resistant varieties,
2. rotate fields to corn or cotton (1-2 years),
3. treat seed which may be infected with a fungicide labeled for seed treatment,
4. plow under any crop residues on the soil surface, and
5. the use of a foliar fungicide (Table 1) at the R2 and R5 soybean growth stage may be beneficial if the disease is present and environmental conditions that favor disease development occur. Fungicides, however, are not generally recommended in North Carolina since we rarely encounter disease pressures sufficient to reduce soybean yield below the cost of treatment.

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data.

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent.

Last update to information: May 2000
Last checked by author: May 2000
Web page last updated November 2000 by A.V. Lemay.