Control Methods and Technical Key Points for Sweet Potato Brown Spot Disease
Sweet potato brown spot disease, also known as angular leaf spot.
[Pathogen scientific name] Cercospora batatac Zimm
[Host crops] Sweet potato, water spinach, morning glory, etc.
[Disease diagnosis] It mainly damages leaves. When leaves are affected, water-soaked small spots first appear, then the lesions gradually expand. Because they are restricted by leaf veins, the lesions are mostly angular, and some are irregular. The lesions are initially yellow-green, then turn yellow-brown or dark brown, with a light brown center. When humidity is high, sparse light gray mold-like growth appears on the lesions. When the disease is severe, the leaves are covered with lesions, causing the leaves to turn yellow, wither, and fall off. The pathogen is the sweet potato brown spot pathogen. The conidiophores of the pathogen emerge in bundles from the stomata, are light brown, and bear conidia at the tips. The conidia are whip-like, colorless to slightly pale yellow-brown, and have 4 to 6 septa.
[Occurrence pattern] The pathogen overwinters as mycelium or conidia with diseased residues. The following year, the overwintering pathogen produces conidia that spread to sweet potato leaves and infect them, causing disease. After disease onset, conidia produced on diseased leaves spread by airflow, causing secondary infection in the field, and the disease continuously expands and spreads. The pathogen favors high temperature and high humidity, and the suitable temperature for pathogen development is 25 to 28°C. In hot seasons, when there are cloudy and rainy days or heavy mist and clouds, the disease often occurs severely.
[Control measures] (1) Prepare the land carefully, cultivate on ridges, and be careful not to plant too densely. (2) Apply sufficient decomposed manure and pay attention to the application of phosphorus and potassium fertilizers. (3) Irrigate at the right time and in the right amount. After rain, drain field water promptly to reduce field humidity. (4) In severely diseased plots, carry out a 2-year rotation with non-host crops. (5) After harvesting from diseased fields, thoroughly remove diseased residues from the field and then deep-plow the soil. (6) Spray chemicals for control at the early stage of disease onset. The chemicals can be 70% thiophanate-methyl wettable powder at 800 times dilution, or 50% thiophanate-sulfur suspension concentrate at 600 times dilution, or 50% dimethirimol wettable powder at 1500 times dilution, or 40% baisuangjing suspension concentrate at 600 times dilution, or 40% iminoctadine wettable powder at 500 times dilution, or 80% mancozeb wettable powder at 800 times dilution.