Leaf and stem scab of sweet potato

Leaf and stem scab, caused by the fungus Sphaceloma batatas, is a destructive disease affecting sweet potato crops globally. This fungal pathogen specifically targets the aerial parts of the plant, leading to significant physiological stress. In severe cases, the infection can disrupt the plant's ability to undergo photosynthesis, thereby reducing both the yield and the overall health of the crop.

The primary host for this pathogen is the sweet potato. The fungus attacks leaves, petioles, and stems. By colonizing the epidermal tissues of the plant, the pathogen creates localized lesions that expand over time. When left unmanaged, the infection can lead to severe stunting, as the plant diverts energy from root tuber development to fight off the fungal colonization.

Symptoms initially manifest as small, brown, or grayish lesions on the leaf surface. As the infection progresses, these spots evolve into raised, corky, or scabby lesions. On stems and petioles, the fungus causes distinctive twisting and distortion of tissue. In later stages, the lesions can coalesce, resulting in widespread necrotic areas, which significantly impacts the aesthetic and biological value of the plant.

The development and spread of Sphaceloma batatas are heavily dependent on environmental conditions. High humidity, frequent rainfall, and warm temperatures provide the optimal microclimate for spore germination and infection. The pathogen is disseminated through splashing water, wind-borne spores, and contaminated agricultural machinery, making early detection vital for preventing field-wide outbreaks.

Management of leaf and stem scab requires a multifaceted strategy. Growers should prioritize the use of certified disease-free cuttings for propagation to prevent initial introduction into the field. Proper crop rotation cycles, ensuring good canopy ventilation, and the timely application of approved copper-based or systemic fungicides are essential practices. Additionally, removing crop debris after harvest helps to reduce the primary inoculum load for the following season.