One link of the causal chain of cause and effect is still missing. Many plant species, or resistant phyli, respectively, react towards the induction by fungi, viruses, and other causative agents, or towards mechanical damage with hypersensitivity. It is characterized by the withering of a local section of tissue. It cuts the parasite off the nutrient supply, its opportunity to spread is taken away, and often are phenolic compounds set free, that may also lead to its death. The dead, often brownish spots on the plant are called necroses .
Hypersensitivity is no general reaction.
Only certain plant species react towards certain parasites in this way. Genetic analyses showed, that both host and parasite have to have dominant alleles of certain resistance genes . Host and parasite are incompatible. This seemingly simple relation shows that the recognition can be analyzed genetically. This possibility is qualified by the fact that a whole number of independent R-loci exist.