The influence of Alternaria porri Ell.(Ciferri) associated with purple blotch disease in onion on the proximate and mineral composition of onion leaves was investigated. Samples of both healthy and purple blotch-infected onion leaves were dried and analyzed for the nutritional components except moisture content. The mineral contents (Na, K, Ca, P, Mg, Fe, Zn) were determined by flame photometry and atomic absorption spectrophotometry. The infected leaf showed a significant (p< 0.05) decrease in the quantity of the crude protein, fat, fibre and ash. The moisture content was. significantly (p< 0.05) lower in healthy onion leaves (88.0%) than in the infected leaves (94.7%). Similarly, carbohydrate content was lower in diseased leaves than in healthy onion leaves. The levels of minerals were significantly (p< 0.05) lower in diseased leaves compared to the healthy (control) leaves indicating that the infection of onion leaves by purple blotch pathogen (Alternaria porri) had a significant impact in reducing the nutritional value of the onion leaves. It is important to avoid fungal contamination with a view to enhance nutrient bioavailability for human consumption.