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Transcript
Fungi 4 - Parasitic fungi Parasitic fungi are the principal disease-causing organisms in plants. Fungal attacks can result in devastating agricultural losses. Phytophthora infestans is not strictly a fungus although it was classified as a fungus for many years. It is, in fact, a colourless, filamentous alga and its walls contain some cellulose, unlike the true fungi. It is described here because its parasitic life style closely resembles that of the pathogenic fungi which infest plants and also because it causes serious plant diseases such as tomato blight and the potato blight which caused the devastating Irish potato famine in 1845. The hyphae of this parasite spread internally through the leaves. Short branches from the hyphae penetrate the cell walls, with the aid of enzymes, and absorb nutriment from the cell contents. The cells are eventually killed and then the leaves and finally the whole shoot die. Before this happens, branching hyphae grow out of the stomata and produce sporangia at their tips. The tips of the hyphae constrict to cut off individual sporangia which are blown away in air currents. If a sporangium lands on a leaf of a healthy potato plant in warm moist conditions, a new hypha grows out from it and penetrates the leaf. When sporangia fall on the ground, they may be washed into the soil by rain, so reaching and infecting the potato tubers, causing them to rot. The close proximity of the plants in the potato field allows very rapid spread of the disease from one individual to the next. Agricultural research is constantly trying to find varieties of food plants which are resistant to this infection and to other types of fungus disease. Researchers also aim to develop sprays which destroy the fungus without causing harmful side effects on the crop or on the other organisms in the area. Vertical section through a leaf infested with Phytophthora hypha absorbing nutriment from cell 1mm reproductive hyphae emerging from stoma sporangium sporangium released © D.G. Mackean