Disease control

Sweet potato mosaic disease

Sweet potato mosaic disease leaves	[Distribution and Damage] Sweet potato mosaic virus, also known as sweet potato virus disease, occurs in all sweet potato production areas, especially during the seedling stage (seedbed). It is caused by a virus. Severe cases affect the quantity and quality of seedlings. [Symptoms] During seedling cultivation, infected seed potatoes sprout many leaves, all leaves become infected, wrinkled and small, with pale yellow and transparent veins, eventually forming a mosaic pattern. After transplanting diseased seedlings to the field, the leaves are deformed and the tubers are small. [Disease Pattern] Infected tubers produce diseased seedlings during the seedling stage. When these seedlings are transplanted to the field, they produce small, infected tubers. It is most common in spring when temperatures are low, and does not manifest in summer when temperatures are high. Mosaic symptoms reappear in late autumn when temperatures drop. [Control Methods] First, strengthen seedbed inspection; if diseased seedlings are found, remove them along with the tubers to ensure that diseased seedlings are not planted in the field. Second, establish disease-free seed production areas and save seed potatoes from disease-free plants.

Sweet potato frost damage

Frost-damaged sweet potatoes	This is a physiological disorder caused by freezing of sweet potato tubers when harvested too late or when storage is poorly managed to prevent frost damage. It often results from infection by soft rot fungi or gray mold, leading to tuber rot and even cellar rot. [Symptoms] Frozen tubers lack luster. Scraping open a diseased tuber reveals that the flesh adjacent to the skin quickly turns brown. If the cut surface immediately turns dark brown, the frost damage is severe; if it takes 2-3 minutes to turn light brown, the damage is milder. There is no white sap oozing from the cut surface. Frozen tubers develop a hard core, which remains hard and uncooked after cooking. [Occurrence Patterns] One cause is late harvesting, where tubers are exposed to frost or snow in the field or drying yard at temperatures below -1.5 degrees Celsius. At this low temperature, the tubers quickly deteriorate, their viability is greatly reduced, and they become susceptible to pathogenic fungal infections. Another cause is poor winter storage management, where the temperature inside the cellar remains below 9 degrees Celsius for an extended period, causing the tubers to freeze. Low temperature and long duration lead to severe damage. [Control methods] Soak potatoes in a 500-fold dilution of 50% carbendazim wettable powder for 3-5 minutes, then dry them and store them in a cellar. Use 10,000 kg of seed potatoes per kg of carbendazim wettable powder solution.	Cross-section of Jianshu


Sweet potato blight

　　Also known as sweet potato bacterial wilt, head rot, and blast disease. It occurs mostly in sweet potato-growing areas south of the Yangtze River. It spreads rapidly and causes severe losses, making it a devastating disease for sweet potatoes. Mild cases result in a yield reduction of 30-40%, while severe cases can lead to a 70-80% reduction or even total crop failure.
[Symptoms] This disease is a stunting disease. The pathogen invades through wounds on the plant or the base of the rootlets of the tuber, damaging the vascular bundles of the tissue and obstructing the transport of water and nutrients, causing the leaves to wither and droop. It can cause damage throughout the entire growth period, but the symptoms differ at different stages.
　　(1) When seedlings are raised using diseased sweet potatoes, when the seedlings are about 15 cm tall, 1-3 leaves begin to wither, and the base of the seedling becomes water-soaked, gradually turning yellowish-brown or even dark brown. Severe cases result in withering and death.
　　(2) During the field growth period, diseased seedlings do not develop roots after planting and die a few days later. After the seedlings are planted, when the vines are about 30 cm long, the pathogens invade through the wounds. The leaves become dull and wilted at noon. The stem base and the part buried in the soil, especially where there are wounds, are yellowish-brown or blackish-brown and water-soaked, and eventually rot completely. In the later stage of growth, adventitious roots have grown from each node of the stem and vine. The leaves do not wilt. After the adventitious roots are broken off by lifting the vine, the plant quickly withers and dies. Most of the fibrous roots are water-soaked and easily peel off when pulled by hand, leaving only thread-like fibers.
　　(3) Potato tubers: Plants infected in the early stage generally do not produce tubers or produce a small number of root tubers. When infected in the later stage, they do not produce tubers at all. The symptoms of tubers with mild infection are not obvious, but the tuber shanks are blackish-brown and fibrous, and the root tips are water-soaked and easily peel off when pulled by hand. In tubers with moderate infection, the pathogens have invaded the tuber flesh. They do not soften when steamed or boiled and lose their edible value. The people call them "hard corpse tubers". Severely infected potatoes develop patchy, dark brown, water-soaked lesions on the skin, while the flesh turns yellowish-brown. In severe cases, the entire potato rots and emits a pungent odor.
　　The disease can reproduce at temperatures ranging from 20-40 degrees Celsius, but it grows and multiplies fastest and causes the most damage at temperatures of 27-35 degrees Celsius and relative humidity above 80%. In southern potato-growing areas, the high temperature and humidity season from late April to the end of October is the peak period for transmission and disease. June to September is the peak period for disease outbreaks. The disease spreads widely, readily infecting through cross-contamination via diseased seedlings, potatoes, contaminated soil, fertilizer, and running water. The blast fungus is an aerobic bacterium; it can survive for only about one year in paddy fields but up to three years in dry land.
[Control Methods]
　　First, strict quarantine measures should be implemented, including thorough disease investigation, delineation of diseased areas, and prohibition of the export and sale of potatoes (seedlings) from infected areas. Disease-free seed production sites should be established.
　　Secondly, proper crop rotation, including water-dry rotation or rotation with crops such as wheat, corn, and soybeans, is one of the best methods for disease prevention. However, avoid rotation with solanaceous crops such as potatoes, tobacco, and tomatoes.
　　Thirdly, select disease-resistant varieties, such as Huabei 48, Guangshu 62, Xiangshu 75-55, and Minkang 330. Varieties like Yubeibai, Xinzhonghua, and Shengli 100 are susceptible to disease and should not be planted in epidemic areas.

Sweet potato black spot disease

　　Sweet potato black scurf occurs in various regions and specifically affects the skin of sweet potato tubers.
　　(1) Symptoms: Small light brown spots initially form on the skin of the sweet potato tubers, which gradually expand into large, irregular gray and black lesions and develop a black mold layer. In severe cases, the tubers lose water, shrivel, and crack. The lesions are limited to the skin layer and do not penetrate into the internal tissues. They do not affect edibility or quality, and have no bitter taste, but they do affect sprouting and have poor marketability. Some farmers mistakenly believe that this is caused by the application of chemical fertilizers.
　　(2) Disease pattern and transmission route: It is suitable for transmission at 30-32 degrees Celsius. The disease is more severe in summer when there is a lot of rain and in heavy clay soil. The transmission routes are diseased tubers, seedlings carrying bacteria, manure
　　carrying bacteria, and water transmission. (3) Control measures: Select disease-free seed potatoes, cultivate disease-free strong seedlings, establish disease-free seed fields, implement a crop rotation system of more than 3 years, pay attention to drainage, and reduce soil moisture. When planting, soak the seedlings in a diluted solution of fungicides such as carbendazim.

Gardening