Rapeseed Sclerotinia, there are so many symptoms

Rapeseed sclerotinia is a common disease that occurs specifically in rapeseed. The stems, leaves, flowers and siliques can all be affected, with the stems being the most seriously affected. When the stems are infected, light brown water-soaked spots appear at first, which later develop into long, ring-shaped spots with brown edges. When the humidity is high, cotton-like white hyphae grow on the surface, and black sclerotia are occasionally seen. The pith of the diseased stems rots into a cavity, with many black rat-dung-like sclerotia growing inside.

1. Symptoms

It occurs in both winter and spring rapeseed cultivation areas in my country, and winter rapeseed in the Yangtze River Basin and the southeast coast is severely affected. The disease can occur throughout the entire reproductive period, with the most severe occurrence during the fruiting period. The stems, leaves, flowers and siliques can all be affected, with the stems being the worst affected. When the stems are infected, light brown water-soaked spots appear at first, which later develop into long, ring-shaped spots with brown edges. When the humidity is high, cotton-like white hyphae grow on the surface, and black sclerotia are occasionally seen. The pith of the diseased stems rots into a cavity, with many black rat-dung-like sclerotia growing inside. After the epidermis of the diseased stem cracks, hemp-like fibers are exposed, the stem is easy to break, and the stems and branches above the diseased part wilt and die. When the leaves are infected, they initially appear as irregular water-soaked spots, which then form nearly circular to irregular spots. The center of the spots is yellow-brown, the outside is dark green, and the edges are light yellow. Sometimes there are obvious ring patterns on the spots. When the humidity is high, white woolly hyphae grow and the diseased leaves are easily pierced. The petals are initially water-soaked, gradually turn pale, and then rot. When the siliques are infected, water-soaked brown spots appear first, which then turn grayish white, and the seeds become shriveled and dull.

2. Disease pattern

The pathogen mainly overwinters or oversummers in the form of sclerotia mixed in the soil or attached to seed-collecting plants, or mixed among seeds. In the winter-sown rapeseed areas in southern my country, a small number of sclerotia germinate from October to December, causing the seedlings to become diseased. The vast majority of sclerotia germinate between March and April of the following year, producing ascocarps. In the rapeseed areas of northern my country, it germinates between March and May. When the ascospores mature, they pop out of the ascus and are spread by air currents, infecting aging leaves and petals, growing mycelium and causing the host tissue to rot and change color. The pathogen spreads from the leaves to the petioles and then invades the stems. It can also repeatedly infect through contact or attachment of diseased and healthy tissues. In the later stage of growth, sclerotia are formed to overwinter or oversummer. The suitable temperature for mycelium growth and development and sclerotium formation is 0-30℃, the optimum temperature is 20℃, and the optimum relative humidity is above 85%.

The sclerotia do not need to be dormant and can germinate at 5-20℃ and higher soil humidity, with 15℃ being the most suitable. The sclerotia can survive for 1 year in moist soil and 3 years in dry soil. Ascospores can germinate at 0-35℃, but 5-10℃ is the most suitable temperature, and germination is completed after 48 hours. In production, when the number of sclerotia is large, the occurrence and spread of the disease depends on the rainfall during the flowering period of rapeseed. If the rainfall in ten days exceeds 50 mm, the disease is severe, while if it is less than 30 mm, the disease is mild, and if it is less than 10 mm, it is difficult to cause the disease. In addition, the disease is more likely to occur in continuously cropped fields, when inadequately decomposed organic fertilizer is used, when seeds are sown too densely, or when excessive nitrogen fertilizer is applied. The terrain is low-lying, drainage is poor or moisture is trapped, plants fall over, early spring cold waves frequently attack or suffer from frost damage and severe disease.