Molecular Pathogens, 2025, Vol.16, No.3, 121-133 http://microbescipublisher.com/index.php/mp 125 that two or more diseases are symbiotic and prevalent in the same field at the same time. For example, in the middle reaches of the Yangtze River, the warm and humid climate in winter not only encourages the large-scale occurrence of Sclerotinia sclerotiorum, but also provides conditions for the spread of Plasmodiophora brassicae. Surveys show that some rape fields in Sichuan, Hubei and other provinces have also experienced Sclerotinia sclerotiorum and Plasmodiophora brassicae outbreaks at the same time. The superposition of the two diseases has led to more serious production reductions. In some newly-occurred areas of Leptosphaeria maculans, farmers also reported Sclerotinia sclerotiorum in fields in the same season, coexisting with Leptosphaeria maculans. These examples show that multiple diseases are increasingly showing a trend of synchronous occurrence and compound damage. The complex occurrence of diseases includes both simultaneous infection of different pathogens on the same host (compound infection), and also the interlaced epidemic of different diseases in groups during the same reproductive period. Compound occurrence often breaks the prevalence of a single disease, making the disease development more complicated and difficult to measure. For example, when rape is infected with both root edema and Sclerotinia sclerotiorum, root damage leads to premature aging of the plant, which may provide a susceptible infection window for Sclerotinia sclerotiorum in advance, resulting in an increase in stem incidence (Seifbarghi et al., 2017). In field investigations, it was also found that some originally secondary attenuated pathogens will also increase when coexist with strong virus pathogens, their detection rate and harm level may also increase, which may be because the main diseases weaken plant resistance and give other pathogens an opportunity. These phenomena suggest that different rapeseed diseases may have mutually promoting relationships with each other when symbiosis occurs, resulting in a situation where the overall disease level is higher than any single disease (Chittem et al., 2020). 4 Pathogen Symbiotic Interaction Relationship 4.1 Synergistic effects between pathogens When rapeseed is infected with multiple pathogens at the same time, a synergistic (assistive) effect may occur between different pathogens, resulting in further intensification of the disease. Synergistic effects are manifested as the presence of one pathogen improves the infective efficiency or pathogenicity of another pathogen, thus causing the overall harm to the plant to exceed the simple sum of each single disease. Studies have found that the mixed infection of white rust bacteria and downy mildew bacteria on Indian mustard greens will cause the symptoms of white rust to appear earlier and more severely. This phenomenon shows that the premature infection of downy mildew weakens the host and plays a "paving" role in subsequent infectious white rust bacteria (Wang et al., 2021). Similarly, it was observed in the fields that when the Plasmodiophora brassicae of rape plants occurs severely, the upper part of the ground is more susceptible to other fungal diseases, which may be due to damage to the roots, resulting in malnutrition and decreased defense of the plants. Synergies may also be achieved through biochemical reciprocity among pathogens. Enzymes or toxins secreted by some pathogens can destroy plant tissues and create a more suitable living environment for another pathogen. For example, pectinase produced by fungal pathogens degrades the cell wall, which not only facilitates its own invasion, but also makes other pathogens more likely to colonize in damaged tissues. In the complex infectious system, physiological changes induced by one pathogen may also be inadvertently exploited by another pathogen to form "cooperation". After rapeseed is infected by the cruciferous black spot bacteria (Alternaria genus), the content of mustard mustard is increased in the leaves, and the moderately increased mustard mustard may inhibit some attenuated pathogens, but some pathogens (such as Plasmodiophora brassicae) are not sensitive to it, but can tolerate this environment and take advantage of the situation (Poveda et al., 2022). The result is that the occurrence of black spot disease provides a prerequisite for another pathogen, and the combined effect of the two diseases makes the plant suffer more deeply. 4.2 Antagonistic effects between pathogens Not all pathogens coexist will aggravate the condition, and in some cases there is an antagonistic (inhibition) effect between different pathogens, thereby reducing the severity of one of the diseases. Antagonism refers to the competition or interference between two pathogens, causing the growth and invasion of at least one of them to be inhibited. Its mechanisms are diverse. A typical case is competing for the same niche or nutrient: When a pathogen
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