Soybean scab

Soybean scab (caused by Sphaceloma glycines) is a destructive fungal disease affecting soybean crops globally. The pathogen is classified within the Coelomycetes group.

The fungus reproduces through conidia produced in acervuli, which are saucer-shaped fruiting bodies formed beneath the epidermis of the host plant tissues.

Sphaceloma glycines thrives in high-humidity environments and moderate temperatures, making it particularly problematic in tropical and subtropical growing regions.

The pathogen survives primarily on infected crop debris and can also be seed-borne, allowing it to persist in a field even after rotating to other crops.

Identification often requires careful inspection of the stem and leaf lesions to observe the characteristic fungal fruiting structures using a magnifying lens.

The disease causes significant damage to all aerial parts of the soybean plant, including leaves, stems, petioles, and pods throughout the growing season.

Stem infection is particularly severe, as it causes lesions that weaken the plant structure, leading to lodging and premature senescence.

When pods are infected, the fungus penetrates the shells and directly affects seed development, resulting in shriveled, discolored, or non-viable seeds.

Yield losses are significant, often manifesting as a reduction in seed quality, weight, and overall volume, sometimes causing total failure in highly susceptible fields.

Crop loss is exacerbated by the pathogen's ability to spread rapidly via rain splashes and wind during the flowering and pod-filling stages of soybean growth.

Initial symptoms include small, reddish-brown circular to irregular spots on leaves that may enlarge and coalesce, leading to tissue necrosis.

Stems and petioles exhibit sunken, elongated lesions that often feature a light center surrounded by a distinct dark, necrotic margin.

The appearance of numerous minute, dark-colored black dots (acervuli) within the lesions is a definitive characteristic of soybean scab infection.

Under humid conditions, a whitish or salmon-colored mass of conidia may be visible on the surface of the lesions, indicating active sporulation.

Severe infestations lead to defoliation, stunted growth, and in some cases, the death of the entire plant before physiological maturity.

Management focuses on an integrated approach, starting with the use of certified, disease-free seed and treating seeds with recommended fungicides.

Cultural practices such as deep tillage to bury crop residues are effective in reducing the amount of primary inoculum available at the start of the season.

Crop rotation remains a critical strategy to break the life cycle of the pathogen, preventing the build-up of inoculum in the soil and debris.

Foliar application of fungicides during the critical growth stages (R1–R3) is recommended in areas with a history of soybean scab or high-risk weather patterns.

• Selecting resistant or tolerant soybean varieties
• Ensuring adequate spacing to improve canopy airflow
• Removing volunteer soybean plants that act as inoculum sources