Fungal community structure, irrigation water chemistry, and antioxidant responses in Capsicum annuum and C. chinense under contrasting field conditions

Understanding how fungal communities, nutrient environments, and plant antioxidant responses interact under field conditions is essential for interpreting decline symptoms in agricultural systems. This study characterized fungal diversity (ITS2 amplicon sequencing), irrigation and soil nutrient profiles, and antioxidant activity in Capsicum annuum and C. chinense cultivated in two contrasting agroecosystems in southeastern Mexico. Plants at one site exhibited naturally occurring symptoms of root and stem decline, while plants at the second site remained asymptomatic. Fungal communities differed markedly between sites, with symptomatic tissues dominated by sequences assigned to Fusarium in both species. Irrigation water from the symptomatic site contained significantly higher concentrations of K⁺ and total phosphorus, and these variables were more strongly associated with variation in fungal community structure in redundancy analyses. In contrast, soil nutrients showed limited association with fungal composition. Antioxidant responses also differed between species. Peroxidase activity increased in symptomatic C. annuum, whereas catalase and ascorbic acid responses were more pronounced in C. chinense. Lycopene decreased in symptomatic plants of both species. These physiological patterns indicate species-specific strategies for managing oxidative stress under naturally occurring decline conditions. Because symptoms developed in the field, the relationships observed represent ecological associations rather than controlled pathogenicity effects. Together, the results show that nutrient-enriched irrigation water, fungal community restructuring, and differential antioxidant responses were consistently associated with symptom expression across contrasting agroecosystems. These findings contribute to a broader understanding of how environmental gradients influence plant–fungus interactions in heterogeneous agroecosystems.