Frogeye Leaf Spot of Soybean

• Warm, humid weather encourages the development of frogeye leaf spot. The fungus that causes the disease survives in soybean residue and in infected seeds.
• Frogeye leaf spot lesions reduce the photosynthetic leaf area and can cause premature leaf drop when severe.
• Management options include product resistance, tillage, crop rotation, and fungicides.

Frogeye leaf spot has been an important disease in the southeastern United States for many years, but has recently become more common in more northern regions such as the central Midwestern states. Milder winters and the use of susceptible soybean products seem to be encouraging the incidence of the disease in these regions.

Frogeye leaf spot is caused by the fungus Cercospora sojina, which survives in soybean residue left on the soil surface and in infected soybean seeds. Warm, humid weather encourages the development of spores on infected residue and/or on seedlings arising from infected seeds. The spores are spread by wind and splashing rain. Young soybean leaves are more susceptible to infection than older, mature leaves. More spores will be produced from infected leaves and secondary infections will continue as long as weather conditions are conducive.

Frogeye leaf spot is most common during the soybean reproductive growth stages (blooming through maturity) but may develop earlier in continuous soybean fields and/or under optimal environmental conditions. Initial symptoms appear as small, yellow or gray spots on the leaves. As the lesions mature, they expand (up to 5 mm in diameter) and the centers of the lesions become gray to tan with reddish-brown to purple boarders (Figures 1 and 2), which are characteristic of the disease. The lesions may appear fuzzy on the underside of the leaf, which are the spore-containing structures of the fungus. When plants are heavily infected, the lesions may coalesce. If the lesions cover 30% or greater of the leaf surface, this often triggers premature leaf drop. Lesions may also appear on the stem and pods, though this is less common. Lesions on pods and stems appear reddish brown and darken as they mature, but lack the characteristic gray colored center. Lesions on pods may appear sunken and the seeds inside the pods may turn brown with cracked seed coats.

Frogeye leaf spot lesions reduce the photosynthetic leaf area, which results in less production of carbohydrates to sustain plant growth and fill the seeds, and consequently, reduced yield potential. Infected seeds will have reduced quality and poor germination. Infection can occur at any stage of soybean development. Infections occurring prior to or at flowering can result in substantial amounts of disease and loss in yield potential; however, infections that occur at or after the beginning seed (R5) growth stage will have limited impact on yield potential. Once an infection starts and conditions remain conducive for further disease development, infections can spread very rapidly throughout the crop.

Frogeye leaf spot lesions may be confused with a different soybean disease, Phyllosticta leaf spot. The two diseases have similar symptoms but are caused by different species of fungus. The different reproductive structures of the two species are used to distinguish between the two diseases. Under magnification, small dark specks may be seen in the center of a Phyllosticta leaf spot lesion, which are the spore-producing pycnidia; whereas frogeye leaf spot lesions have small dark hairs, which are the conidiophores that produce the conidia (spores). If in question, leaf samples can be sent to a plant diagnostics lab for confirmation.

Management is aimed at protecting plants from infection and reducing the amount of inoculum available.

Product resistance is generally the best option for managing a disease. Several races of Cercospora sojina exist in the U.S. and a resistant soybean product may confer resistance to one or multiple races depending on which resistance gene(s) it contains. Talk to your local agronomist or seed representative about resistant varieties that are available in your area and which races exist in your area (if known).

Plant high-quality, certified disease-free seeds to avoid introducing the fungus into your field.

Fungicidal seed treatments reduce the risk of early-season infection.

Crop rotation allows more time for inoculum in the field to degrade before soybean is planted again. Even small amounts of soybean residue can contain enough inoculum to begin an epidemic. In fields where frogeye leaf spot has been severe, a two-year rotation away from soybean may be needed to thoroughly degrade the residue from the previous soybean crop.

Tillage that completely buries soybean residue may be necessary in high disease pressure fields to kill the overwintering fungal structures. Tillage that is performed early, after a soybean harvest, will be more effective at killing the fungus in soybean residue than when tillage is delayed until the next season.

Fungicide applications can protect yield potential and may be warranted on susceptible products when lesions are found prior to the beginning pod (R3) growth stage and conditions remain conducive for further disease development. If the weather forecast predicts a prolonged dry period in the near future, it may be more economical to take a wait-and-see approach as dry weather substantially limits disease development.

The R2-R5 growth stages are the recommended period to apply a fungicide to protect plants from infection and preserve yield potential. The threshold of disease severity (number of spots or percent affected leaf area) at which a fungicide application is justified has not been established for frogeye leaf spot and control recommendations vary by region. Some regions rarely encounter disease pressures sufficient to reduce soybean yield enough to justify treatment. In other regions, fungicide applications may be recommended at very early signs of infection (1-2 lesions every 25 feet of row at soybean growth stage R2) when environmental conditions are conducive to disease development, a susceptible product is grown, and the field is planted to continuous soybean. In addition, new strobilurinresistant strains of the frogeye leaf spot fungus have recently been reported in several states, rendering this type of fungicide ineffective at controlling these strains. Therefore, your local Extension agent is a good resource for information on application timing and fungicide recommendations in your region.

Remember that the development of frogeye leaf spot is highly dependent on environmental conditions. Infection can spread rapidly under warm, humid conditions; however, dry conditions may hinder disease development to the point that treatment may not be necessary. If lesions cover 30% or greater of the leaf surface, this will likely stimulate leaf drop and lead to a greater potential for yield losses. The Ohio State University has developed a scale to estimate the percent leaf area affected. See the publication Frogeye Leaf Spot of Soybean at http://ohioline.osu.edu for a copy of this leaf injury scale. As no economic threshold has been developed, it is important to discuss the best management approach and timing with local experts.

Dorrance, A.E. and Mills, D. 2011. Frogeye leaf spot of soybean. AC-53-10. Ohio State University Extension. http://ohioline.osu.edu

Westphal, A., Abney, T.S., and Shaner, G. 2006. Frogeye leaf spot. BP-131-W. Purdue University Extension. www.btny.purdue.edu

Faske, T. and Kirkpatrick, T. Frogeye leaf spot of soybean. University of Arkansas Extension. https://www.uaex.edu/

Koenning, S. and Dunphy, J. 2013. Pest alert: Frogeye leaf spot resistance alert. North Carolina State University Extension. 

Bradley, C.A. 2007. Similar symptoms on soybean: Phyllosticta leaf spot vs. frogeye leaf spot.

University of Illinois Extension. http://bulletin.ipm.illinois.edu/print.php?id=818

Web sources verified 7/14/16. 160713072417