The Dual Role of Nitrogen in Plant Health: A Critical Review of Crop Growth, Disease Resistance and Susceptibility

Nitrogen is essential for crop growth and yield. However, its influence on plant health extends beyond its role as a nutrient source. Nitrogen availability can affect plant physiology, defence metabolism, tissue composition and interactions with pathogenic and beneficial microorganisms. Thus, nitrogen fertilisation may increase resistance to some diseases while making plants more susceptible to others. These contrasting effects complicate the optimisation of nitrogen management for sustainable agricultural production. This critical analysis examines the relationships among nitrogen nutrition, crop development, plant defence mechanisms and disease progression. It focuses on nitrogen availability, chemical form, application rate and environmental conditions. It evaluates the physiological, biochemical and ecological processes by which nitrogen influences plant–pathogen interactions and distinguishes between soil-borne and foliar diseases. The analysis also explores why reported disease responses to nitrogen fertilisation vary, highlighting the roles of host physiology, pathogen behaviour, soil characteristics, microbial populations and experimental design. A central conclusion is that nitrogen cannot be categorically classified as either disease-promoting or disease-suppressing. This is because its effects depend on interactions among the crop, pathogen and cultivation environment. Current studies are further limited by inconsistent disease assessments and insufficient integration of nitrogen use efficiency, disease intensity, crop productivity and environmental impacts within individual experiments. Future research should prioritize mechanistic field studies, standardised disease evaluations and forecasting models that account for the interaction between nitrogen management and disease susceptibility. Integrating plant nutrition with disease control may improve crop yields while minimising excessive fertiliser application and its associated environmental impacts.