Scab, a plant disease; also, the roughened crustlike area on the plant surface resulting from the disease. The disease is caused by various parasitic fungi, each of which usually infects only one kind of plant. Agricultural commodities susceptible to the disease include apples, avocados, barley, cucumbers, peaches, pecans, potatoes, and wheat. Spraying with fungicides and the use of disease-resistant varieties are common methods of prevention and control, but scab is difficult to control once it has taken hold.