Causes of Soybean Sclerotinia Stem Rot and Its Control Measures

1. Symptoms

The first thing to catch the eye in the field is the upper leaves of the plants turning brown and dying. Brown lesions continue to develop on the stems of diseased plants, producing white cottony mycelium and white granular material, which later turns black and becomes sclerotia. When the stem of a diseased plant is cut lengthwise, black cylindrical sclerotia are seen arranged in rows inside. After the diseased plant dies, it turns grayish white, the hollow stem cortex often rots into a hemp-fiber-like state, and the sclerotia on the outside of the diseased plant readily fall off. The lesions on the pods are brown, and the pods die rapidly without setting seed, finally turning pale white; lightly affected pods may still set seeds, but the affected seeds are rotted or shriveled and wrinkled.

2. Causes of the Disease

(1) A warm winter with little snow in the previous winter favors the overwintering of sclerotia in the soil; the more severe the sclerotinia stem rot, the more important the role played by the many overcast and rainy days after entering June and the high soil moisture content, which are also important causes of severe sclerotinia stem rot occurrence.

(2) Fields with continuous soybean cropping show severe disease. According to field surveys, the disease incidence in continuously cropped fields is above 30%. In addition, fields following sunflower and rapeseed also show relatively severe disease, which is related to the large amount of sclerotia left in the field.

(3) Excessive application of nitrogen fertilizer leads to severe disease. With excessive nitrogen fertilizer application, soybean grows luxuriantly, the stems are weak, and lodged areas show severe disease.

(4) Fields that have not been plowed show severe disease. The sclerotia that fall into the field germinate under suitable conditions the following year to form apothecia and ascospores, becoming the primary infection source in the field. Fields with few hoeing and ridging operations show severe disease, and fields sown on the original ridges are more severely affected than sown fields.

3. Control Measures

(1) Select seeds carefully: remove sclerotia mixed among the seeds before sowing.

(2) Deep plowing: carry out one deep plowing after the soybean harvest to bury the sclerotia that have fallen into the field deep into the soil, with a soil cover of more than 10 cm, so that the sclerotia cannot form apothecia, thereby inhibiting sclerotial germination and reducing the sources of primary infection.

(3) Crop rotation: carry out a rotation of more than 3 years with gramineous crops and other non-host plants.

(4) Strengthen field management: drain standing water from the field in time to reduce field humidity. Do not apply too much nitrogen fertilizer, which can lessen the disease; hoeing and ridging during the sclerotial germination period can destroy the apothecia and lessen the disease.

(5) Field control: spray once every 7—10 days in the early stage of the disease, for 2 consecutive sprays. Commonly used chemicals and their application concentrations are as follows: 50% Sumilex wettable powder 1000—1500 times dilution, 50—75 liters per 667 sq m; 50% carbendazim wettable powder 5000 times dilution, 50—75 liters per 667 sq m; 50% vinclozolin wettable powder 5000 times dilution, 50—75 liters per 667 sq m; 40% dimethachlon wettable powder 1000—1500 times dilution, 50—75 liters per 667 sq m.