Plant physiology, microbial community, and risks of multiple fungal diseases along a soil nitrogen gradient

Nitrogen (N) is an essential element for plant growth and development. N levels in soil may also impact plant disease occurrence. However, the physiological and microbial mechanisms between N levels in soil and plant disease occurrence are not quite clear at present. In this study, we examined the impact of seven urea levels (0 to 800 mg kg(-1) soil) on the physiology of tomato (Lycopersicon esculentum) and fungal disease occurrence. Our results showed that the disease incidence and index caused by a tomato early blight pathogen (Alternaria alternata) increased with increasing N levels. The disease index and percentage of disease incidence were positively correlated with N content, indole-3-acetic acid (IAA) and salicylic acid (SA) in plants, but negatively correlated with fungal community diversity. In addition to pathogens (A. alternata) that cause known early blight, 39 other fungal taxa were also identified as plant pathogens in tomato roots, leaves, and soil, the dominant putative pathogens included Ceratobasidiaceae sp., Fusarium oxysporum, Macrophomina phaseolina, Nectriaceae sp., Podosphaera fusca and Trichoderma viride. N levels affected the distribution and dynamics of the fungal pathogen community, such as the abundance of Fusarium oxysporum increased by 33% on roots from days 25 to 35. Our results demonstrated that plants undergo complex disease risk from different pathogens under different N levels, highlighting a need for proper N management, and integration of nutrient management as a disease control approach for sustainable agricultural production.