Boyd Padgett, Ray Schneider and Clayton A. Hollier

Soybeans are plagued by numerous diseases affecting the leaves, stems, pods and roots. Diseases of major concern are Cercospora foliar blight, purple seed stain, aerial blight, soybean rust, pod and stem blight, and anthracnose. Diseases that occur less frequently are downy mildew, bacterial pustule, Phytophthora rot, frogeye leaf spot, red crown rot and stem canker. If not properly managed, some of these diseases can reduce yields and seed quality. Losses to diseases in Louisiana soybeans were estimated at 13.5 percent in 2009 and 5.6 percent in 2010.

View Photos of Soybean Diseases.

An effective disease management strategy consists of disease identification, production practices, genetic resistance and fungicide use. Proper disease identification is crucial for effective management. This will determine the production practices to be implemented, the varieties to select and the choice and timing of fungicides.

Each year LSU AgCenter scientists conduct research to develop and improve disease management strategies and provide unbiased information to Louisiana producers. Research is conducted on research stations and in producer fields located strategically in the state. Programs target identifying and deploying genetic resistance in soybean varieties, evaluating fungicides for efficacy and monitoring them for resistance, disease epidemiology, and evaluating the effects of production practices on disease development. LSU AgCenter scientists and other professionals also monitor Louisiana soybean production regions for new diseases.

Disease identification and development 

Seedling disease can be caused by a number of soil-borne fungi (species of Rhizoctonia, Fusarium, Pythium, Phytophthora). Symptoms include pre-emergent seed rot and post-emergent damping-off. Pythium and Phytophthora prefer wet soils and cause a wet rot. Pythium usually develops best when soil temperatures are cool; however, Phytophthora develops best when soil temperatures are warm. Rhizoctonia solani develops best when soil temperatures are warm, and infected plants usually have a dry reddish-brown lesion at the base of the stem.

Cercospora blight/purple seed stain is the No. 1 soybean disease in Louisiana. The disease is caused by the fungus Cercospora kikuchii. The fungus can infect seedlings and either cause plant death or remain latent. Foliar symptoms of this disease are usually not evident until soybeans are in the mid- to late reproductive stages of growth. Initial symptoms are small, chocolate-brown lesions on the leaf stems near the leaflet. As the disease progresses, foliar symptoms are expressed as a reddish-brown to tan discoloration on the upper leaf surface in the upper canopy. Leaves have a leathery appearance. The fungus can emit spores in older lesions. The spore masses resemble ashes. Advanced stages of this disease result in premature defoliation, discolored pods and reduced seed quality. The seed phase of this disease is evidenced by purple-stained seed at harvest.

Seedling infection is favored by moderate temperatures (70 to 80 degrees Fahrenheit) and extended periods of leaf wetness (up to 16 hours). However, stem and leaf symptoms during late reproductive growth stages, including defoliation, are favored by hot, dry conditions. The pathogen may be seedborne and survives on plant debris in the soil. The fungus also has been isolated from some weeds. 

Aerial blight can spread rapidly in soybeans if not properly managed. This disease is caused by the fungus Rhizoctonia solani and favors warm, overcast days and extended periods of leaf wetness. This is the same fungus that causes sheath blight in rice. Initial symptoms appear as water-soaked, greasy blotches on leaves (usually in the lower to mid-canopy). As the disease progresses, white, cottony fungal fibers may cause adjacent leaflets to adhere together. If favorable conditions persist, the leaves become brown and blighted, and pods may have reddish-brown lesions. If the disease continues to progress, pods may be aborted. The disease is usually evident during early reproductive stages of growth and later. The potential for risk is increased when soybeans are rotated with rice. This disease can spread rapidly within the crop and should be managed immediately upon detection if the crop is in the late vegetative or reproductive growth stages. 

Frogeye leaf spot is caused by the fungus Cercospora sojina. Symptoms are found predominately on the leaves but may also appear on the leaf stems, stems and pods. Initially, small, chocolate-brown spots can be found on the leaflets. If the disease continues to develop, mature lesions have light brown to gray centers with reddish-brown margins. Stem lesions are rare and are elliptical with red centers and dark brown to black margins. Pod lesions are circular to elliptical, sunken and light gray to brown. Disease development favors warm, humid weather. The pathogen can survive on seeds and in infected plant debris. 

Soybean rust is caused by the fungus Phakopsora pachyrhizi. Symptoms begin in the lower canopy as small, brown-to-tan, raised pustules (volcano-like) on the lower leaf surface. Spores produced in these pustules resemble grains of sand and are tan when young. Older spores are darker in color. As the disease progresses, pustules may merge into blighted leaflets, causing them to defoliate. Symptoms are usually evident when soybeans are in the mid- to late reproductive growth stages. Pustules can also be present on leaf stems and pods when the disease is severe. Kudzu is another host for this fungus. The disease develops rapidly when temperatures are between 59 and 77 degrees Fahrenheit and when leaves remain wet for six to 10 hours. 

Downy mildew is caused by the fungus Peronospora manshurica. Currently, this disease is not a major threat to soybeans produced in Louisiana. Symptoms may be confused with those produced by soybean rust. Symptoms begin as small, pale green-to-yellow spots on the upper leaf surface. Older lesions or spots may turn gray to dark brown. When the disease is active, grayish tufts of fungal mycelium (similar to dryer lint) can be found on the underside of the leaf opposite the yellow spot. The disease develops rapidly when temperatures are between 50 and 77 degrees Fahrenheit during extended periods of high relative humidity.

Bacterial pustule is caused by the bacterium Xanthomonas axonopodis pv. glycines. Symptoms of this disease are similar to soybean rust. Although present in most years, this is not a major disease of Louisiana soybeans. Symptoms begin as small, pale green, water-soaked spots with elevated centers on the upper and lower leaf surfaces. As the spots mature, they turn brown with elevated, volcano-like pustules on the lower leaf surface and are easily confused with rust. Careful examination with a dissecting microscope or a 20X hand lens is required to distinguish between these two diseases. Pustules are dry in appearance and may be found on pods in susceptible varieties.

Disease development favors temperatures between 85 and 90 degrees Fahrenheit and wet weather. The bacterium may be seedborne. It survives on surface crop residue, on wheat roots and in weed hosts such as red-vine. The bacteria may be dispersed by splashing water or wind-blown rain.

Pod and stem blight occur most frequently on the stems and pods. The disease is caused by the fungus Diaporthe phaseolorum var. sojae. Infection may occur early in the season; however, signs of the disease are not evident until late in the season. Pycnidia (fruiting bodies that resemble black specks) occur in rows on the stems and pods. If favorable conditions persist, seed quality will be compromised. This disease favors warm, wet weather. The fungus overwinters in crop residue or infected seed. 

Anthracnose is caused by the fungus Colletotrichum truncatum. Early infections by the fungus can result in pre- and postemergence damping-off. Foliar symptoms include leaf stem cankers, leaf rolling, vein death and premature defo liation. The fungus can produce acervulli (fruiting bodies that resemble black specks) on the stems and pods. These bodies occur randomly on the stems and not in rows as in pod and stem blight. If the disease continues to develop on the pods, seed quality will be compromised.

The disease favors periods of high relative humidity. Infection occurs throughout the growing season. The fungus overwinters in crop debris and infected seed. 

Charcoal rot is caused by the fungus Macrophomina phaseolina. Infected seed may not germinate, or seedlings may die soon after emergence. Symptoms from plants with latent infections or mid- to late-season infections die prematurely during hot, dry weather. Symptoms are associated with dry spots (sandy areas) in the field. The roots and lower stems deteriorate. The epidermal and sub-epidermal tissue will be silvery in color and dotted with black pepper-like sclerotia (survival structures).

Disease development favors hot, dry weather (82 to 95 degrees). The fungus can survive in the seed coat, in host residue or in the soil. 

Red crown rot is caused by the fungus Calonectria ilicicola. Root infections may occur soon after planting, but initial symptoms are usually not evident until soybeans are in mid- to late reproductive growth stages. Roots become black  with rotted segments, and the base of the stem at the soil line may be covered with brick-red reproductive structures (usually most evident during periods of high soil moisture). Foliar symptoms are characterized by interveinal yellow or brown blotches.

Moderate temperature and wet soil conditions at planting promote disease development. Maximum root infections occur when soil temperatures are 77 to 86 degrees. The fungus may overwinter in the soil and in infested plant debris on and in the soil.

Production practices, genetic resistance and fungicides
A healthy plant is the first step toward optimizing yields and preventing disease. These production practices can be used to manage many diseases.

• Plant when conditions favor rapid germination and seedling establishment. This recommendation is supported by AgCenter planting date studies conducted in 2009 and 2010.

• Improve drainage within the field. This will help reduce the risk of soil-borne disease such as phytophthora rot, pythium seedling disease and red crown rot.

• Promote air movement within the canopy to reduce periods of leaf wetness and lessen the risk from some foliar diseases.

• When possible, genetic resistance should be the foundation of any disease-management strategy. Always use high-yielding, disease-resistant varieties when available. Every year, AgCenter scientists evaluate commercial varieties on several research stations. Disease reactions for each variety can be found on the LSU AgCenter soybean website.

• When genetic resistance is not available, fungicides can be used for managing diseases. Fungicides are monitored for their performance in experiments conducted on research stations and in producer fields every year. In addition, the effects of single versus multiple applications and application timing also are evaluated. There are several fungicide classes to choose from. The disease present will dictate which class is needed. Three classes of chemistries are available to producers: strobilurins, benzimidazoles and triazoles (Table 1).

In LSU AgCenter tests, the strobilurins have broad-spectrum activity. This class is effective against aerial blight and pod diseases, slightly effective against Cercospora blight and suppressive against soybean rust. Topsin M and TM-85 are moderately effective against Cercospora blight and pod diseases but not effective against soybean rust. Triazole fungicides are used for managing soybean rust, and some evidence suggests they may be effective against Cercospora blight. Some of these fungicides may confer activity against other diseases, but more evaluations are needed to make this determination.

Other factors to consider when making a fungicide application are timing and sprayer setup. Applications should be made when soybeans are between pod-initiation and seed-initiation growth stages. Early applications will provide better efficacy against Cercospora foliar blight, and later applications will provide better efficacy against pod diseases. Ideally, spraying should be done with 15 gallons of solution per acre by ground or 5 gallons of solution per acre by air. For more information concerning soybean disease management, consult your LSU AgCenter county agent.

Boyd Padgett, Professor, Macon Ridge Research Station, Winnsboro, La.; Ray Schneider, Professor, and Clayton A. Hollier, Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2011 issue of Louisiana Agriculture Magazine.)