Control Methods for Sweet Potato Scab, a Must-Read for Growers
Sweet potato scab, also known as sweet potato bud-shrinking disease, is commonly called “leprosy disease”, “hard stem disease”, etc.
[Pathogen scientific name] Sphaceloma batatas Sawada.
[Host crop] Sweet potato.
[Disease diagnosis] It mainly damages young shoots, leaves and stems, and can also damage tubers. The disease mostly occurs on tender leaves, and the coarse and fine veins on the underside of the leaves first show slightly transparent small brownish-red spots, after which the lesions gradually expand, and the surface tissue of the lesions becomes suberized, rough and raised, like a scab, grayish-white to yellowish-white. The affected veins bend, and the leaves shrink and curl. When stems and petioles are affected, round or oblong scab-like lesions form, and in severe cases they merge into large scars. The cortex of diseased stems becomes rough and suberized, losing its flexibility, so that the tips of diseased vines harden and become rigid and no longer trail along the ground. When young shoots are affected, dense light purple lesions appear, and the young shoots shrink and cannot grow, which is called bud shrinking. When tubers are infected, the buds curl up, and dark brown to grayish-brown spots appear on the tuber surface; when dry, the scabs easily fall off, leaving rash-like spots or scars. Diseased tubers are small and mostly deformed. The pathogen commonly seen in the field is its asexual generation, Sphaceloma batatas. The conidiophores are borne on the acervulus, short, unbranched, cylindrical, with a somewhat pointed and slender tip, and bear conidia on them. The conidia are single-celled, colorless and long-elliptical.
[Occurrence pattern] The pathogen overwinters as mycelium on seed tubers or with diseased residues in the soil. Infected seed tubers and sweet potato slips can spread it. After the disease occurs in the field, the conidia produced on diseased parts spread through wind and rain, and human contact and field insects can also spread it. The pathogen mostly invades through lenticels and wounds. Reinfection is frequent, and under suitable conditions the disease develops very quickly in the field. The disease begins when the temperature is above 20°C, with the optimum temperature at 25-28°C. Rain and dew, especially continuous rainfall and storms, favor the spread and outbreak of the disease. Turning vines on rainy days makes the disease spread even faster.
[Control measures] (1) Choose disease-resistant varieties, such as Beibuqi, Honghong No. 1, Guangxuan 16, Bulunchun, Xindazi, etc. (2) Choose disease-free tubers (vines) for raising slips. Before planting, soak the sweet potato slips in a 1000-fold solution of 70% thiophanate-methyl for 5 minutes. (3) In severely diseased fields, carry out a 4-6 year rotation with grain crops. (4) Apply sufficient decomposed manure, avoid excessive nitrogen fertilizer, and increase phosphorus and potassium fertilizer. Irrigate reasonably. Drain after rain to reduce field humidity and inhibit the spread of the disease. (5) Detect diseased plants early and pull them out promptly. After harvest, thoroughly remove diseased residues from the field and plow the soil deeply. (6) Spray chemicals for control at the early stage of disease onset. The chemicals can be 70% thiophanate-methyl wettable powder 1000-fold solution, or 50% carbendazim wettable powder 500-fold solution, or 80% zineb wettable powder 600-fold solution, or 80% Xinwansheng wettable powder 600-fold solution, or 30% Luyedan wettable powder 600-fold solution.