Occurrence patterns and control methods of soybean sclerotinia stem rot

Soybean sclerotinia stem rot occurs commonly in Heilongjiang Province. In 2001, the disease was relatively severe in Raohe County. In continuous soybean cropping fields, the disease incidence rate was 3%-30%, and in individual fields following American sunflower, the incidence rate reached 50%. In severely affected fields, the crop was even completely lost.

1 Causes of the disease

1.1 Climatic conditions In mid-to-late July, continuous low temperature and scant sunshine, with more overcast and rainy weather, and temperatures between 18-22 degrees Celsius, favored the germination of the pathogenic fungus's apothecia to form ascospores, which became the primary infection source.

1.2 Cultivation conditions First, crop rotation conditions. In fields where American sunflower was previously planted or where soybeans had been continuously cropped for many years, sclerotia accumulated in the soil year by year, and their numbers kept increasing. When climatic conditions were suitable, they could form primary infection sources and infect host plants such as sunflower, Solanaceae crops, and soybean.

Second, variety selection. Through field surveys of soybean sclerotinia stem rot, it was found that different soybean varieties showed certain differences in disease resistance. For example, the incidence rate of Hefeng 25 in continuous cropping for 2 years was about 8.2%, while the incidence rate of Hefeng 35 in continuous cropping for more than 2 years was 3.3%.

Third, fertilizer application. In fields with excessive nitrogen fertilizer application, soybean plants grew luxuriantly, the field canopy was dense, and ventilation and light penetration conditions were poor, causing excessive soil moisture, which favored the occurrence of soybean sclerotinia stem rot. In the survey, it was found that severely affected fields generally received 4-5 kg of urea per mu, 10-12 kg of diammonium phosphate, and 3-4 kg of potassium sulfate; soybean plant height was 99-108 cm, and the incidence rate reached about 43.5%.

 2 Damage symptoms

The pathogenic fungus of soybean sclerotinia stem rot mainly infects the stem of soybean. In the field, the upper leaves of plants turn brown and die, and brown lesions appear on the stem or branches. On lesions with high humidity, white cottony mycelium grows and gradually forms white irregular granular mycelial masses, which later turn gray, until finally forming black sclerotia scattered inside and outside the stem of diseased plants. In the later stage, the lesion tissue breaks apart, exposing the xylem, and in severe cases the entire plant dies. When leaves are damaged, they appear dark bluish-green, water-soaked, and rotten, sometimes with cottony mycelium. It can also infect pods, forming sclerotia inside and outside the pods, which can cause seeds within the pods to rot, shrivel, and lose luster; in severe cases, no seeds can form in the pods.

3 Control measures

3.1 Agricultural measures First, carefully select seeds. Choose high-quality, early-maturing, disease-resistant varieties. Before sowing, sieve the seeds to remove sclerotia mixed among them.

Second, deep plowing. In severely affected fields, carry out deep plowing in autumn to bury sclerotia that have fallen into the field deep into the soil, so that diseased plant residues rot and die.

Third, crop rotation. Carry out crop rotation for more than 3 years with non-host crops or gramineous crops.

Fourth, drainage and fertilization. Promptly drain standing water from the field to reduce field humidity; applying appropriately less nitrogen fertilizer can reduce disease occurrence.

Fifth, hoeing and cultivation. Hoeing and cultivation during the sclerotium germination period can destroy apothecia and reduce disease occurrence.

3.2 Chemical control Use 40% Wenkulili wettable powder at 800-1200 times dilution or 50% Sumilex wettable powder at 2000 times dilution, and spray on the leaves at the initial stage of sclerotinia stem rot, with one supplementary spray after 7 days. During the peak germination period of the pathogenic fungus's apothecia, 40% Junhejing wettable powder at 1000 times dilution can be sprayed. All the above methods have good disease control effects.