Soybean leaf diseases affect plant health and reduce crop yields. These diseases, caused by various pathogens like fungi, bacteria, and viruses, manifest through distinct symptoms on soybean leaves. Understanding these visual cues is important for timely identification and effective management. In 2021, diseases were estimated to cause a 5.8% loss in potential soybean yield in the US.
Common Fungal Leaf Diseases
Frogeye Leaf Spot, caused by the fungus Cercospora sojina, appears as small, irregular or circular lesions on soybean leaves. These spots have a gray center and are surrounded by a reddish-brown or dark purple border. Initially, lesions may look like dark, water-soaked spots on the upper leaf surface. As the disease progresses, lesion centers can become lighter, turning gray to light brown, and may merge, leading to defoliation. This disease occurs in warm, humid conditions, with symptoms appearing during the reproductive growth stages.
Septoria Brown Spot, caused by the fungus Septoria glycines, is a common foliar disease of soybeans. Symptoms begin on the lower leaves as small, irregular, dark brown spots on both leaf surfaces. These lesions can merge to form larger blotches, especially along leaf edges or veins, and infected leaves often turn yellow and drop prematurely. The disease thrives in warm, wet conditions and progresses upwards from the lower canopy during grain fill.
Soybean Rust, caused by the fungi Phakopsora pachyrhizi or P. meibomiae, forms small, irregular spots that are initially gray and then turn tan or reddish-brown. These lesions appear translucent when held up to light and contain small pustules, usually on the underside of the leaves, which release cream-colored or tan spores. Symptoms are most common after flowering, starting on lower leaves, and can lead to premature defoliation and reduced pod set. This disease is favored by temperatures between 54°F and 84°F, with high humidity and continuous leaf wetness.
Anthracnose, caused by the fungus Colletotrichum truncatum, affects soybean stems, pods, and petioles, though leaf symptoms also occur. On leaves, symptoms include vein reddening and leaf rolling. Irregular, brown lesions develop on other plant parts, covered with tiny black dots or spines visible with magnification. The fungus overwinters in crop debris and can be seedborne, with infections favored by warm, moist weather and extended periods of leaf wetness.
Bacterial and Other Leaf Diseases
Bacterial Blight, caused by Pseudomonas savastanoi pv. glycinea, causes distinct symptoms on soybean leaves. Initial signs are small, angular, reddish-brown lesions with water-soaked margins, surrounded by a yellow halo. These lesions can expand and merge, creating larger, irregular dead areas on the leaf surface. This disease occurs in cool, wet conditions, and the bacteria can survive in residue and seeds, spreading through rain and wind. It can occur at any growth stage, but young leaves are particularly susceptible.
Soybean Mosaic Virus (SMV) manifests symptoms on soybean leaves, and is not caused by a bacterium or fungus. Common leaf symptoms include mosaic patterns, mottling, crinkling, and distortion. Leaves may appear stunted or develop a darker green color along the veins with lighter green areas between them. The severity of these symptoms can vary depending on the soybean variety and environmental conditions.
Identifying and Managing Leaf Diseases
Accurately identifying soybean leaf diseases begins with careful observation of symptoms, noting the color, shape, and distribution of lesions on the leaves. Consulting local agricultural extension services or plant diagnostic labs can provide confirmation, especially when symptoms are ambiguous or multiple diseases appear concurrently. Early and precise identification allows for timely intervention, potentially reducing the impact on yield.
Management strategies for soybean leaf diseases include preventive and control measures. Planting resistant soybean varieties is a preventive step, as some varieties offer natural defenses. Crop rotation with non-host crops helps break disease cycles by reducing the buildup of pathogens in the soil and residue. Proper sanitation, such as managing crop residue through tillage or removal, also limits overwintering sites for many disease-causing organisms.
Optimizing planting dates can help plants avoid periods of high disease pressure, especially for diseases favored by specific environmental conditions. When diseases are present, fungicides are an option for fungal infections, but their application requires careful timing and consideration of fungicide resistance. Integrated Pest Management (IPM) principles, which combine various strategies, are recommended for a holistic approach to disease control. This includes regular scouting for symptoms, understanding disease cycles, and making informed decisions about interventions.