Detailed Guide to Identifying Symptoms of Major Potato Diseases and Their Control Methods

I. Late blight

1. Symptom identification. This disease is a fungal disease that affects stems, leaves and tubers. Affected parts of stems and leaves develop brown water-soaked spots, which later expand into black patches. When humid, a ring of white mold-like growth surrounds the lesions, very obvious on the underside of leaves. After tubers are affected, brown or blue lesions initially form, then expand, and the surface gradually becomes sunken, while the potato flesh beneath the lesion turns brown.

2. Control methods. Selecting disease-resistant varieties is the most economical and effective way to control late blight: such as Wushu No. 8, No. 9, No. 11, Longshu No. 3, No. 5 and others with high resistance to late blight; choose disease-free seed potatoes to eliminate sources of infection; eliminate central diseased plants and spray promptly to prevent spread. When central diseased plants are found, they should be eliminated promptly, and chemical control should be adopted. Generally, 100 g of 58% Baodasen per mu is diluted with 3 times its volume of water, and a sprayer is used for foliar spraying, once every 7-10 days, and 2-3 consecutive sprays can achieve the desired control effect.

II. Early blight

1. Symptoms. It mainly affects leaves, and tubers can also be affected. This disease occurs earlier than late blight, generally starting from the lower old leaves first, initially forming small brown spots on the leaves, which later expand into nearly round dark brown lesions with concentric rings, hence also called ring-pattern lesions. The lesions have a clear boundary with healthy tissue, and in severe cases the leaves dry up.

2. After tubers are infected, the lesions are round or irregular, dark brown, slightly sunken, and the potato flesh turns brown and dry-rots, with a layer of suberized tissue beneath.

3. Control methods. Select disease-resistant varieties, apply more fertilizer, strengthen field fertilizer and water management, and improve plant disease resistance; remove diseased plant debris; rotate with non-solanaceous crops; at the early stage of disease, spray 58% Baodasen or 0.8-1% Bordeaux mixture 2-3 times.

III. Ring rot

1. Symptoms. This disease is a bacterial disease, transmitted by bacteria carried on seed potatoes. After diseased seed potatoes are planted, severe cases rot and fail to sprout; mild cases sprout, but the internodes are shortened and the plants are stunted. Chlorosis begins at the edges of the lower leaves and gradually spreads upward, turning yellow and withered, and finally the plants wilt, but the leaves do not fall off. When infected tubers are squeezed by hand, yellow bacterial fluid flows out, and the potato skin separates from the flesh; in severe cases the skin cracks, the flesh rots and deteriorates, giving off an unbearable stench. Lightly infected tubers show a distinct ring when cut open.

2. Control methods. Introduce seed potatoes from disease-free areas, promote ring rot-resistant varieties, and wipe the knife with 75% alcohol or salt water to disinfect it after cutting diseased potatoes before cutting again, to prevent transmission; sprout planting: remove diseased sprouts and weak inferior sprouts, and select disease-free strong sprouts. This can achieve the purpose of improving seed characteristics and increasing yield; establish disease-free seed production fields to provide seed potatoes with high purity, freedom from disease and mild degeneration.

IV. Viral diseases

Common mosaic disease. The typical symptom of this disease is that the leaves show mild mosaic mottling of dark green alternating with pale yellow along the veins. The leaves are slightly reduced in size and become wrinkled to a certain degree. The mottling is obvious under shade or on cloudy days, and appears or is masked under strong sunlight.

Streak mosaic disease, or severe mosaic disease. In the early stage, infected plants show mottled mosaic or necrotic spots on the leaves, and later develop vein necrosis. In severe cases, brown streaks spread along the petioles to the main stem. The leaves completely necrose and wilt, and the lower leaves show drooping leaf necrosis but do not fall off. After some varieties are infected, although there is no necrosis, the plants are stunted, the stems and leaves become brittle, the leaves show common mosaic symptoms, and they cluster into rosettes.

After complex infection by streak mosaic and common mosaic viruses, the plants show wrinkled mosaic symptoms, the leaves become smaller, the top leaves are severely wrinkled, the tips and edges of the leaflets curve downward, and the plants are markedly stunted, forming a hydrangea-like shape, do not flower, and mostly die early, with extremely small tubers.

Control methods: breed disease-resistant and disease-tolerant varieties; according to the occurrence pattern of virus-transmitting aphids, use the insecticide dimethoate emulsifiable concentrate at 800-1000 times dilution for foliar spraying to control aphid virus transmission.