Abstract: Method for detecting response to damage or disease in a plant or crop comprising detecting Active Oxygen Species (AOS), in a quantitative assay; diagnostic reagent comprising an extract of damaged or diseased plant or crop tissue, or active component thereof, or synthetic equivalent or derivative thereof; use of the method or reagent in detecting response to damage or disease in a plant or crop selected from potato (seed, processor or consumer), apple, lychee, pear, apricot, peach, orange (juice), banana, leaf or stem browning in cut flowers, lettuce, pomegranate, grape, mushroom, logan fruit, loquat fruit, blueberry, carambola, dog rose, rambutan, coconut, avocado, plantain, pineapple, walnut, artichoke, sugarbeet and onion; apparatus and kit therefor; and associated methods for modifying and selecting varieties, growth or environmental conditions of a plant or crop; and use therein of a computer programmed with data relating to susceptibility of a plurality of plants and crops to damage or disease.