Molecular Pathogens, 2025, Vol.16, No.3, 87-99 http://microbescipublisher.com/index.php/mp 91 3.3 Mechanism of action of soil physical and chemical properties 3.3.1 Regulation of soil pH on pathogen growth and activity Soil pH significantly affects the growth rate and pathogenicity of sweet potato root rot pathogens. Generally speaking, Fusarium fungi grow most vigorously under neutral or slightly acidic conditions (pH 6~7), while their activity decreases in acidic or alkaline environments. Cruz et al. (2019) found in a study on soybean root rot that 14 strains of F. oxysporum grew fastest at pH 6.0~6.3 and 27 °C, while their growth was restricted at pH 4.0 and 8.0 (Figure 2). However, it is worth noting that even at the same pH, the growth rates of different strains are still different. This ecological adaptation differentiation suggests that under continuous cropping or long-term planting conditions, some strains may form dominant groups due to their stronger environmental adaptability, aggravating the severity of the disease. Acidic environment may reduce pathogenicity by inhibiting the activity of pathogen enzymes and limiting spore germination, while neutral environment promotes the action of pathogen cell wall degrading enzymes, making it easier to invade sweet potato tissues. 3.3.2 Organic matter regulation and changes in soil microbial community structure The content of soil organic matter affects the structure and function of microbial communities and is an important ecological factor in regulating sweet potato root rot. High organic matter soils often have higher microbial diversity, especially the increase in the proportion of antagonistic microorganisms (such as Streptomyces, Trichoderma, etc.), which helps to form "disease-suppressive soils", thereby inhibiting the colonization of pathogenic fungi such as Fusarium. In poor soils lacking organic matter, the microbial community structure is single, and harmful bacteria are prone to form dominant populations. Imperiali et al. (2017) 0found in a study of agricultural soils in Switzerland that the higher the level of organic carbon in the soil, the more active the expression of antibacterial metabolic genes in the extracted Pseudomonas, and the incidence of crop root rot was significantly reduced. Similarly, in the soil of sweet potato continuous cropping, the study found that the proportion of pathogenic fungi in healthy soil was low, while the richness of actinomycetes and antagonistic fungi was high, which was conducive to the stability of microecological balance. Even under the same pH conditions, there were obvious growth differences between Fusarium strains, suggesting that microbial diversity under organic matter regulation can not only compete for nutrient resources and produce antimicrobial metabolites, but also limit the expansion of pathogenic communities with strong ecological adaptability by changing the buffering properties of the soil environment (Cruz et al., 2019). 3.3.3 Pathogen accumulation and community succession mechanism driven by continuous cropping obstacles Continuous planting of sweet potatoes for many years is prone to cause continuous cropping obstacles. The core problem is the imbalance of soil ecology, which is manifested as the accumulation of pathogens and the reduction of beneficial bacteria, which in turn aggravates the occurrence of soil-borne diseases such as root rot. Gao et al. (2019) used high-throughput sequencing technology to compare the fungal community structure of soil in one-year and three-year continuous cropping sweet potato fields, and found that the relative abundance of Fusarium in continuous cropping soil was more than double that of new crops. At the same time, although the diversity of the microbial community increased on the surface, the proportion of harmful bacteria increased significantly. During the formation of continuous cropping obstacles, root secretions accumulate in the soil, especially toxic metabolites such as phenolic acids, which may inhibit the growth of beneficial microorganisms, promote the reproduction of pathogens, and aggravate the deterioration of soil microecology. In addition, the different Fusarium strains revealed in Figure 1 still show different growth abilities under similar pH environments, indicating that pathogen groups with stronger adaptability and higher infectivity may be gradually selected under continuous cropping conditions, leading to the "involution" of diseases. Effective strategies to deal with continuous cropping obstacles include rotating with gramineous crops (such as corn and wheat) for 2~3 years to break the life cycle of pathogens and reduce the initial infection source; deep
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