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There are various ways in which plant nutrient supply or concentration affects plant diseases and pests. The supply of nutrients changes the resistance of plants to pathogens and pests by altering growth and tissue composition (e.g., concentration of soluble compounds or defence compounds). Depending on the pathogen/pest and nutrient, nutrient supply for optimal plant growth may increase or decrease disease incidence, with differences in response between facultative and obligate parasites. This chapter provides examples of the effect of nutrient supply on foliar and soil-borne fungal and bacterial diseases as well as on pests. Via their stabilizing effect on cell walls and membranes, Ca and B inhibit pathogen invasion. Silicon and Mn play important roles in the defence reaction to infection or attack, whereas N and K exert their effect mainly via modulating the concentration of soluble compounds in plant tissues. Lastly, effects of management (e.g., timing of fertilizer application and liming) on disease incidence are outlined.