Download Parasitic fungi - Biology Resources

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