What are the control methods for sweet potato black star disease?
Sweet potato black star disease, also known as black spot disease.
[Pathogen scientific name] Alternaria bataticola Ikata.
[Host crop] Sweet potato.
[Disease diagnosis] It damages leaves, petioles and stems. When leaves are affected, water-soaked brown spots first appear, and after expansion the lesions become round to elliptical, 3-5 mm in size, brown. In the later stage, the center of the lesion is grayish brown, the surrounding area is brown, and the outer layer is light brown, with the boundary with the healthy part being dark brown, forming concentric rings of alternating light and dark shades. When the disease is severe, the lesions merge, causing the leaves to turn yellow and even fall off. On petioles and stems, the lesions are spindle-shaped to elliptical, brown to dark brown, and slightly sunken. When infection is severe, the lesions surround the petiole, causing the connected leaves to turn yellow and dry up. When humidity is high, a light black mold-like growth appears on the lesions. The causal agent is sweet potato Alternaria. The conidiophores of the pathogen are borne singly or in bundles of 2-3, brown, uniform in color, with the basal cell slightly larger, unbranched, straight or slightly curved, without geniculate nodes, and with 1-4 septa. Conidia are borne singly, obclavate, light brown, with 5-12 transverse septa, 0-3 longitudinal septa, constricted at the septa, and the beak slightly long to long, unbranched, pale in color, with 0-3 septa.
[Occurrence pattern] The pathogen mainly overwinters as mycelium on diseased residues, and conidia can also overwinter attached to seed tubers in storage pits. Raising seedlings from infected seed tubers can cause the seedlings to become diseased, and diseased seedlings can spread the disease over long distances. After the disease occurs in the field, conidia are produced on the diseased parts and spread by wind and rain. The incubation period of the disease is 3-7 days, and reinfection is frequent. When temperature and humidity conditions are suitable, the disease develops very quickly. The disease prefers warm and humid conditions, with an optimum disease development temperature of 24-25°C. Heavy disease occurs with frequent rain, heavy fog, and heavy dew. The disease is more severe on poor soil, with a shallow plow layer, and with insufficient fertilizer and water.
[Control measures] (1) Disinfect seed tubers by soaking in a 1000-fold dilution of 50% carbendazim for 5 minutes. (2) Plow the soil and apply sufficient manure. Topdress and irrigate in a timely manner to make the plants grow vigorously. (3) Pay attention to field sanitation. Remove the initially diseased leaves in the early stage of the disease, thoroughly remove diseased residues in the field after harvest, and plow the soil deeply. (4) Chemical control. Spray early with a 500-fold dilution of 64% oxadixyl mancozeb wettable powder, or a 500-fold dilution of 70% mancozeb wettable powder, or a 600-fold dilution of 80% Daconil wettable powder, or an 800-fold dilution of 80% Pencozeb wettable powder, or a 500-fold dilution of 77% Kocide wettable micropowder, or 1:1:200 Bordeaux mixture.