Molecular Pathogens, 2025, Vol.16, No.3, 121-133 http://microbescipublisher.com/index.php/mp 127 also synchronously regulate the occurrence of multiple diseases, making them tend to overlap or stagger in time and space. Temperature is an important variable that affects the disease spectrum. Generally speaking, warm winter and spring are beneficial to most rapeseed diseases overwintering and early onset. Precipitation and humidity have a more direct impact on the co-occurrence of multiple diseases. Continuous rainfall leads to saturation of the field humidity. Under this condition, diseases such as Sclerotinia sclerotiorum and downy mildew that require high humidity occur on a large scale; at the same time, excessive soil moisture will aggravate Plasmodiophora brassicae, and eventually the soil infectious diseases and gas-transmitted diseases occur at the same time. Wind also affects the disease pattern: strong winds are conducive to the long-distance transmission and spread of air-borne pathogens such as black shin disease, and have little impact on Plasmodiophora brassicae diseases transmitted by water and soil. Therefore, in the air-dry climate, the proportion of gas-borne diseases increases (Michael et al., 2023). In addition, extreme weather events can also induce special compound effects. Low temperature frost damage in winter will damage rape tissue. In the early spring of the following year, mixed infections of soft rot and Sclerotinia sclerotiorum are often prone to occur at the same time in the frost damage area. For example, after hail causes leaf trauma, trauma-infected pathogens such as black shin and gray mold may invade together. From a regional perspective, areas with similar climates often have similar disease spectrums. The climate in the winter rapeseed area of the Yangtze River Basin is warm, and Plasmodiophora brassicae, Sclerotinia sclerotiorum, and Hyaloperonospora parasitica can occur at the same time (Wang, 2024); the northwest plateau is cool and dry, and local occurrences such as virus diseases and root rot are mainly occurring, and there are few large-scale compound epidemics. But climate change is breaking some traditional patterns: the number of warm winters and warm springs in recent years has caused diseases such as black shin that were limited to local areas in the past to spread to a wider area, and its interaction with local epidemics is worth paying attention to. 5.2 Factors of agricultural cultivation Agricultural cultivation systems and field management measures are important human factors that affect the occurrence of various diseases of rapeseed. The crop rotation method is the first to be affected: long-term continuous cropping of rapeseed will accumulate soil and stubble pathogens, causing Plasmodiophora brassicae, Sclerotinia sclerotiorum, Leptosphaeria maculans, etc. to exist at a high level in the fields, forming a breeding ground for compound diseases. Planting density and cultivation methods also significantly affect the common ecological potential of the disease. Concentrated planting will cause congestion in the fields, high microclimate humidity and poor ventilation, making Sclerotinia sclerotiorum and downy mildew more likely to prevail; at the same time, dense roots of plants are intertwined, which is also conducive to the mutual transmission of soil-borne pathogens, so dense planting often leads to the occurrence of multiple diseases at the same time (Zhu, 2010). Fertilization and soil management factors should also not be ignored. Unreasonable application of nitrogen fertilizer will lead to greed and long-lasting plants, tender tissues, susceptibility to Sclerotinia sclerotiorum and soft rot, and will also aggravate the occurrence of Plasmodiophora brassicae (because nitrogen promotes acidification and root expansion) (Rabiee et al., 2021). Regulating the soil pH (such as lime application) can significantly inhibit Plasmodiophora brassicae, but the impact on other diseases is limited, so balanced application is required. In addition, treatment of diseased and disability affects the pathogen base, which in turn affects the disease combination spectrum. If the previous diseased and disability body is not thoroughly cleaned up, and the bacterial sources of Sclerotinia sclerotiorum and Leptosphaeria maculans are abundant, the new season will face the risk of concurrence between the two; and timely removal of diseased strains and deep cultivation to eliminate stubble can reduce the overwintering of various pathogens. Agricultural machinery operations and irrigation methods may also mediate the symbiosis of diseases. Shared agricultural machinery can bring soil-borne pathogens in one place to another, triggering new sources of complex infection; flooding of water can easily spread root tumours and bacterial spores and increase field humidity, thereby contributing to root tumours and downy mildew. In contrast, technologies such as submembrane drip irrigation can reduce surface humidity and reduce some leaf diseases but have little impact on root diseases, which need to be considered comprehensively.
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