Molecular Pathogens, 2025, Vol.16, No.3, 121-133 http://microbescipublisher.com/index.php/mp 124 3 Characteristics and Prevalence Laws of Disease Occurrence 3.1 Periods and symptoms of different diseases The main diseases of rapeseed have their own characteristics in their occurrence period and symptoms. Sclerotinia sclerotiorum often begins to develop after the peak of rapeseed flowering. Because the infection comes from the fall of petals, the symptoms often appear in the middle and upper parts of the stem. Within 23 weeks after flowering, the peak period of the development of Sclerotinia sclerotiorum symptoms. The stems of the onset plant become soft and white, and rat fecal-like Sclerotinia sclerotiorum grows inside. In the later stage, the plant has turned yellow prematurely. Plasmodiophora brassicae can begin to occur in the seedling stage. As early as 34 true leaves, small tumors form at the roots, and the growth of the diseased plant is blocked. As the plant grows, the nodules continue to expand. In severe cases, the main roots are all swollen and deformed, and the plant dies before and after flowering. The symptoms of this disease are hidden underground and are not easy to detect early, but during the bolting and flowering period, the diseased plant can be observed to wilt in the diseased plant and have obvious tumors at the root after being pulled up (Stotz et al., 2018). The initial symptoms of Leptosphaeria maculans are more common in the seedling leaves and inferior tibia, and round hazel spots appear on cotyledons or real leaves, and small black dots (conidia) in the center. The bacteria invade the stem base along the petiole, forming black dry shrinkage lesions near the surface, which can easily break from the lesions in the later stage. Leptosphaeria maculans generally has obvious symptoms from the rebirth to the bolting stage, and the diseased area gradually expands and necrosis after flowering. It should be noted that different diseases occur in sequence: when the temperature and humidity are low in early spring, the symptoms of Plasmodiophora brassicae are likely to appear first; Sclerotinia sclerotiorum begins to prevail in spring and rainy periods; if there are bacteria-carried residues in the previous seedling stage, Leptosphaeria maculans can infect earlier, but the symptoms of stem-based ulcers often appear before maturity. 3.2 Environmental and regional characteristics of disease prevalence The occurrence and prevalence of rapeseed diseases is deeply affected by environmental conditions and regional factors. The epidemic of Sclerotinia sclerotiorum is closely related to the climate during the flowering period: when the relative humidity of the air continues to be above 80%, the temperature is 15 ℃~20 ℃, and the rainy weather is frequent, it is very conducive to the major outbreak of Sclerotinia sclerotiorum. The middle and lower reaches of the Yangtze River and the south of the Yangtze River are rainy in spring and the fields are very humid, so Sclerotinia sclerotiorum occurs almost every year in these areas. Among them, the recurrence areas in Sichuan, Hubei, Hunan and other provinces have been widely prevalent for many years (Xu et al., 2014). The prevalence of Plasmodiophora brassicae is closely related to the physical and chemical properties of the soil and the farming system. This disease is severely ill in areas with acidic soil (pH<7), sticky and poor drainage. For example, the red soil areas in the mountains and hills in the southwest and the rapeseed areas in the rice fields in central China are often severe. In addition, continuous cropping and repeated vegetation are important reasons for the high incidence of root disease. The accumulated dormant spores in the soil can repeatedly infect the host once appropriate conditions. Therefore, the incidence of Plasmodiophora brassicae in continuous cropping areas of winter rapeseed in the Yangtze River Basin has rapidly expanded in recent years. Leptosphaeria maculans is mainly transmitted through air, and its prevalence is affected by the number of previous ill-affected bodies and the rainfall in the current season. Leptosphaeria maculans is prone to occur in plots with a large number of mushroomed rape straw residues. If it is rainy in autumn and winter, the release and initial infection of ascospores will be high. If it is rainy in the seedling stage in spring, it will be beneficial to the spread of lesions and secondary infection. Due to the large amount of squatter left after harvesting in the autumn-sowed rapeseed area in the Yangtze River Basin of China every year and the humidity in winter and spring, a certain degree of Leptosphaeria maculans has been detected. In the northern spring-sowed rapeseed area, due to the long crop rotation period and a dry climate, black shin disease has not been seriously harmful. 3.3 Trends of the compound occurrence of multiple diseases In recent years, the phenomenon of multiple diseases in densely planted rape fields has become increasingly common. Traditionally, a certain area may be dominated by one dominant disease, but now it is often observed
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