Plant parasitic fungi may be broadly classified as necrotrophic or biotrophic fungi based on their modes of nutrition. Necrotrophic fungi are destructive parasites and derive their organic nutrients from the dead cells they have killed. Their effects vary from local discrete lesions, as seen in some leaf spot diseases, to massive tissue destruction, as seen in fruit rots. Upon infection, the hyphae produce polygalacturonases and pectate lyase enzymes which act on pectic substances of the middle lamella and release galacturonic acid residues that form the main source of organic carbon used by these fungi for growth. The dissolution of middle lamella leads to partial degradation of the host's cell walls, separation of cells, loss of turgor and their death. Several necrotrophic fungi, such as species of Polyporus, Fomes and Ganoderma (bracket fungi, Basidiomycotina), kill and then live on dead trees as decomposer saprophytes producing ligninases, cellulases and hemicellulases. With the structural cellulose microfibrils in cell walls weakened, the tree topples under high winds. The fungus continues to decompose the trunk as a saprophyte, producing its basidiocarps on the fallen and standing parts of the trunk for a number of years, blurring the distinction between a parasite and a saprophyte. The biotrophic fungal parasites, in contrast to the necrotrophic parasites, can only grow on living hosts. They are termed obligate parasites that include the downy mildews (e.g., Peronospora, Straminipila), the powdery mildews (e.g., Erysiphe, Ascomycotina) and the rust fungi (e.g., Puccinia, Basidiomycotina).
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