In nature, organisms can be classified as producers, the green plants; consumers, all organisms that exploit producers or other organisms; and decomposers, organisms that use dead organisms. The green plants are used by a multitude of primary consumers of practically all classes of the living world, from various types of herbivores to typical parasites. Avoidance is mainly active against animal parasites and includes such diverse mechanisms as volatile repellents, mimicry, and morphological features like hairs, thorns, and resin ducts. Resistance is usually of a chemical nature. Parasites classified as fungi, bacteria, mycoplasma, viruses, or viroids are collectively indicated as pathogens, disease-inciting parasites. In potatoes the accumulation of viruses and the dramatic late blight epidemic in the middle of the 19th century were the major forces leading to the selection of new varieties derived from cross breeding. Breeders have to produce varieties that are superior in many traits, and resistance to disease is just one of the many aims.