Soybean Disease Update
In 2017 there was an array of weather conditions leading to several soybean diseases showing up. Excessive early season moisture and heavy rains during the season resulted in many fields being affected by Pythium and Phytophthora. Yet another year with cool conditions during flowering resulted in white mold being a common problem in the northern half of the state (fourth year of significant effects). Sudden death syndrome was also present in several fields. Frogeye leaf spot continues to build in the state and more fields are being affected. Additional information on disease identification can be found at CropWatch.unl.edu in the Soybean Disease Management section.
Early Season Seedling Diseases
Several pathogens are involved in damping off seedling diseases. The most common in Nebraska are Fusarium, Phytophthora, Pythium, and Rhizoctonia. All four are capable of killing the developing soybean seedling or causing damage that affects the plant’s ability to achieve its full yield potential. In 2017, Pythium was the most common seedling disease due to cooler soil temperatures.
Criteria for assessing the use of seed treatment fungicides to manage seedling disease problems: (If these conditions are part of your production system, your risk is greater.)
- History of a stand problem
- Early planting date when soils are cool
- Poor seed quality
Phytophthora Root and Stem Rot
Phytophthora root and stem rot of soybean is caused by a soilborne fungus that is present in many Nebraska soybean fields. The pathogen survives primarily as “resting” spores in the soil or in association with infested crop debris. Disease development is favored at soil temperatures above 60oF and high soil moisture. We have observed in the past that dry conditions followed by heavy rain events can result in higher amounts of Phytophthora. This is most likely due to the plants being slightly stressed and the higher soil temperature. It is most common in low areas of a field, in poorly drained or compacted soils, and in soils with high clay content, although it is not limited to these sites or conditions. It also may occur on well-drained hillsides during wet growing seasons.
Symptoms associated with Phytophthora sojae infections include seed rots, pre- and post-emergence damping off of seedlings, and stem rot of plants at various growth stages. The stem rot phase is easily identified by the dark brown color on the exterior surface of the stem and lower branches. Discoloration of the stem extends from below the soil to 6 inches or more above the soil line. The taproot turns dark brown and the entire root system may be rotted. Leaves on older infected plants become chlorotic between the veins followed by general wilting and death. Leaves will remain attached.
Management of Phytophthora Root and Stem Rot
- Genetic Resistance. Using resistant varieties. A combination of good partial resistance and an Rps gene are recommended. Partial resistance alone will not be as effective during early growth stages or under high disease pressure
- Cultural Practices. Anything that can be done to improve soil drainage
- Seed Treatment Fungicide application. Seed treatment fungicides containing mefenoxam or metalaxyl should be used
Sudden Death Syndrome
(Fusarium virguliforme syn. Fusarium solani f. sp. glycines)
The sudden death syndrome (SDS) pathogen is spread with soil; thus, the methods used to prevent soybean cyst nematode spread are also applicable to preventing spread of SDS. For symptoms to develop there needs to be high soil moisture at flowering. As this is a soilborne disease, it will not spread rapidly across the field from individual spots that show up. Infected areas in a field can also have an oblong distribution in the direction of tillage or equipment traffic.
The first symptoms of SDS appear as scattered yellow or white spots on the leaves in the upper portion of the canopy. In the intermediate stage, these spots eventually coalesce to form brown streaks between the veins (interveinal necrosis). On these leaves only the midvein and major lateral veins remain green. As the disease reaches the more advanced stages, premature defoliation occurs with petioles (leaf stems) remaining on the plant. The progression from early symptoms to defoliation will occur rapidly (less than 14 days in most cases). Symptoms of SDS can be confused with brown stem rot symptoms. To differentiate the two, split the stems of infected plants and check for discoloration. If the pith (center stem) is discolored, this is a symptom of brown stem rot. Stem discoloration will be confined to the outer stem layers (vascular tissue) with SDS and can extend up the stem of infected plants.
Management of Sudden Death Syndrome
- Resistance. Varieties will vary in their susceptibility.
- Cultural Practices. Avoid early planting as it favors SDS infection with cool soil temperatures.
- Fungicide Application. A seed treatment can help if the field is severely affected.
Sclerotinia Stem Rot (White Mold)
Sclerotinia stem rot, also referred to as white mold, is caused by a fungal pathogen that can reside in soybean fields an indefinite amount of time. 2017 was the fourth year in a row that white mold has shown up due to cool conditions during flowering. Saturated soils and a full canopy favor the emergence of apothecia from the sclerotia, which are mushroom-like bodies that produce millions of airborne spores almost daily over a 7- to 10-day period. These spores are released during favorable weather conditions and can travel to other fields in air currents.
Spores infect plants like soybean primarily through colonized blossoms that are senescing, but they also can infect through injured plant tissue. Free moisture must be present on the plant surface for infection to occur. Flowers on the tips of small pods provide a common entrance for the fungus. Invasion of the pod and eventually the stem may lead to lesions covered with sclerotia. During harvest these survival structures are scattered back onto the soil. Thus, inoculum for the next three or more seasons has been distributed.
Initial symptoms are visible during pod development. Leaves will wilt and turn gray-green before turning brown, curling, and dying. It is important to observe stems and pods for white mycelium and sclerotia to differentiate Sclerotinia stem rot from other stem and root rot diseases. Since blossoms are infected first, early stem or pod water-soaked symptoms often initiate near colonized flowers. In a few days diseased stem areas are killed, become tan, and eventually bleached. This bleached stem will have a pithy texture and will shred easily. Infected plant parts generally will have signs of the fungal pathogen as white, fluffy mycelium during humid conditions and sclerotia on the surface of, or embedded in the stem tissue. Although stem and pod infection usually occurs about 6 to 14 inches above the soil line, some basal infection also may be found. Infections will occur after flowering has initiated in the crop.
Management of Sclerotinia Stem Rot
- Resistance. Soybean varieties vary in their response to Sclerotinia but this will not fully remedy the situation.
- Cultural Practices. Row spacing has been shown to influence this disease, with narrow rows resulting in more Sclerotinia stem rot. Avoid irrigation during flowering. Using a longer rotation with corn and wheat has been shown to reduce pathogen buildup and disease risk.
- Fungicide application. Foliar fungicide applications at the R1 growth stage (beginning bloom) have been shown to provide better control than applications at R3 (beginning pod).
Frogeye Leaf Spot(Cercospora sojina)
Frogeye leaf spot is a fungal disease that is becoming more common in Nebraska. Yield loss estimates due to frogeye leaf spot have been reported to be as high as 30% nationally with extensive leaf blighting, but for Nebraska I would estimate less than 20% in highly susceptible varieties. The disease is most severe when soybean is grown continuously in the same field, particularly in fields where tillage is reduced, since this is a residue-borne disease. The primary source for this disease is infested residue, infected seed, and airborne spores. In areas where this disease has been observed in past years it will typically show up again if weather conditions are favorable.
Infection can occur at any stage of soybean development, but most often occurs after flowering and is typically in the upper canopy. Initial symptoms are small, dark spots on the leaves. Spots eventually enlarge to a diameter of about ¼ inch and the centers of the lesions become gray to brown and have a reddish purple margin. Individual leaf spots can coalesce to create irregular patterns of blighting on the leaf. In addition, stems and pods can also be affected. Stem infections appear later in the season and will be long, narrow, dark lesions with flattened centers. Pod lesions will be circular to elongate, slightly sunken and reddish brown in color.
Management of Frogeye Leaf Spot
- Resistance. Soybean varieties vary in their resistance to Frogeye leaf spot.
- Cultural Practices. More severe in continuously cropped soybean fields.
- Fungicide Application. Application of fungicides to manage frogeye leaf spot in Nebraska is typically not warranted in most fields. Fields with a history of frogeye should be watched carefully and if disease develops, a strobilurin fungicide should be applied at the R3 (pod set) – early R4 growth.