UW Soybean Plant Health
Frogeye leaf spot is a common fungal disease of soybean in the southern USA. Only recently has it become more severe in North Central States, probably due to a combination of warmer winter temperatures and susceptible soybean varieties.
Yield losses up to 30% have been reported for susceptible cultivars grown in the southern regions of the US, while losses in the Midwest have generally been in the range of 10-20%.
Frogeye leaf spot is caused by the fungus Cercospora sojina. The fungus survives as mycelia (the vegetative body of the fungus) in infected seed and in soybean debris.
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Figure 1. Lesions caused by frog-eye leaf spot. Photo credit: A. Wrather, University of Missouri |
Seed infection is common. Infected seed germinates poorly and the resulting seedlings are usually weak. Spores produced on the cotyledons of infected seedlings are the main source of inoculum for the leaf phase of the disease.
Infected seeds are a means of long distance spread of the fungus. Use high quality, pathogen-free seed for planting.
Disease Cycle and Management
The fungus produces abundant spores in warm, humid weather, creating cycles of leaf infection throughout the season. Extended periods of wet weather will favor disease development. 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 field is not rotated.
When scouting soybean fields for weeds and insects, check for foliar diseases. If you suspect frogeye leaf spot, contact your County Extension Agent and have symptomatic plants sent to the UW Plant Diagnostic Clinic.
The most effective control for frogeye leaf spot is avoidance. Avoid introducing the pathogen by planting high quality, pathogen-free seed. If the disease was detected in your crop last year, planting a resistant variety is the most effective management strategy.
C. sojina is an extremely variable pathogen with multiple physiologic races. Since we do not know which races are present in Wisconsin; choose a variety with a high level of resistance. Three major dominant genes (Rcs1, Rcs2, and Rcs3) confer resistance to frogeye leaf spot.
Tillage practices that keep plant debris on the soil surface provide a conducive environment for pathogen survival and growth. Therefore, deep plowing of crop residues and 2-year rotations with crops other than soybean can lower inoculum levels and reduce disease.
How to Recognize Frogeye Leaf Spot
The most diagnostic symptom of frogeye leaf spot are angular spots with light gray centers and distinct purple to red-brown margins (Figure 1). No chlorotic halos surrounding lesions are present. Leaf spots can be single or merge to form larger lesions. This can result in premature leaf drop.
Lesions on stems and pod can occur later in the season, but they are less common and distinctive than lesions on the leaves. Seeds near pod lesions can be infected and develop conspcuous light to dark gray or brown areas.
References
First Report of Frogeye Leaf Spot (Cercospora sojina) in Wisconsin.
Alemu Mengistu, N. C. Kurtzweil and C.R. Grau. Plant Disease 86:1272, 2002;
published on-line as D-2002-0912-01N, 2002.
Yang, X.B, M.D. Uphoff, and S. Sanogo. 2001. Outbreaks of soybean frogeye leaf spot in Iowa. Plant Disease. 85:443.
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
Last update 3/30/07