How to prevent and control soybean black spot disease? Growers must read

Soybean black spot disease is a commonly occurring disease. The disease damages pods and stems, causing stems to wither and die, pods to fail to set seed, and in severe cases plants to die in patches. Seeds in diseased pods are small and shriveled, affecting yield and quality, and diseased seeds have reduced commercial value and oil extraction rate.

1) Symptoms: Soybean black spot disease first appears on the stem base and lower branches, especially near the branching points, as grayish-brown, elongated or elliptical lesions with red edges; later the lesions become sunken and reddish-brown. In the later stage the lesions expand and can cover the entire stem, bearing rows of black dots, which are the fungus's pycnidia; this is the obvious characteristic of the disease. Diseased plants wither early, and the stems rot and break easily. Pod lesions are initially brown, nearly circular or irregular, gradually merging into large patches. In the later stage the lesions turn grayish-white, the diseased pods dry up, and slightly protruding small black dots are scattered on them, which are the fungus's pycnidia. White mycelium is present inside diseased pods. Seeds in diseased pods can all be damaged; damaged seeds are shriveled and hard, with white mycelium attached. Infected seeds generally cannot germinate, and even if they germinate, the seedlings that grow are very weak.

2) Pathogen: Soybean black spot disease is caused by a fungus. The fungus's pycnidia initially form under the epidermis, later breaking through the epidermis to form small black dots visible to the naked eye, spherical or flattened-spherical in shape, with an opening at the top and dark brown walls. The conidiophores are flask-shaped and colorless. There are two types of conidia: one elliptical or long-oval, single-celled, colorless and transparent; the other slender and hook-shaped, slightly curved, colorless and transparent, single-celled.

3) Prevention and control methods: ① Carefully select seeds and remove diseased seeds; treat seeds with 50% thiram wettable powder at 0.3% of the seed amount. ② Rotate with gramineous crops. Increase application of phosphorus and potassium fertilizers to improve the plant's disease resistance. After soybean harvest, clear diseased residues from the field to reduce sources of the pathogen. ③ In years when the disease occurs, spray for control; the agents include 65% mancozeb wettable powder at 500 times dilution or 50% benomyl wettable powder at 800 times dilution, and spray once more after 10 days.