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. The parasite takes nutrients and growth-regulating substances from the plant, but does not kill it.
Most biotrophic fungi are obligatory parasites.
One of the economically most important representatives of the parasitic myxomycetes is Plasmodium brassicae , the causative agent of clubroot, a disease whose symptoms can be seen in the roots of numerous species of cruciferes. Phytophthora infestans is the causative agent of late blight of potato. The early destruction of the foliage and the reduction of the rate of photosynthesis caused by it leads to heavy crop failure. A huge migration into the USA was the consequence. Four kinds of Ascus exist.