It is quite striking that fungal diseases are rather rare in nature due to the very effective defence mechanisms of plants that inhibit a spread of parasitic fungi. Some of them depend on an alternation of hosts, so that one stage of the parasite s development occurs in one host, the following in another that is phylogenetically not closely related to the first host. The annual loss of foodstuff caused by them corresponds to an amount large enough to provide 300 million people. They produce toxins that do either reduce the activity of the host cells or inhibit it completely. The literature about parasitic fungi is very extensive.
Many studies originate in economic considerations.
The majority deals with the classification of the fungi, their life cycles, the symptoms of the plants and their diagnostic, the host range, and the search for the host s factors of resistance. The plants molecular mechanism of action after infection with fungi has by now just been elucidated in a few cases, and these examples can only partially be generalized due to the variety of possible reactions. The resistance of the hosts is not only based on the general and unspecific defence mechanisms mentioned above, but also on the production of specific, genetically determined substances directed against specific fungi. Genetic analyses showed that the resistance is on one hand caused by the existence of dominant alleles of the respective genes, and on the other hand by resistance genes that are inherited independently of each other. The fungi are often only optionally parasitic, they can just as well proliferate in dead or withering plant material.