Fungal parasites are among the most destructive diseases of plants. Parasitic fungi occur in each of the fungal divisions. They can have tremendous economic impacts. For instance, rust fungi, members of the Basidiomycetes, infect every species of cereal grain and can reduce yields by as much as 25 percent.
In addition to their direct economic impact, parasitic fungi have historically had profound social effects. In the 1800's outbreaks of Phytophthora infestans, late blight, decimated the potato crops of Europe just prior to harvest. This oomycete first turned the potato leaves and stems to mush and then infected the rubbers. As a result, millions of people who depended on potatoes as their primary source of food starved to death. Ireland was particularly hardhit, and millions emigrated from there.
Some fungal parasites have a mixed impact on humans. The genus Claviceps, an ascomycete, infects cereal grains, producing structures called ergots. A variety of alkaloids are produced in the ergot tissue that causes ergotism in animals that eat infected grain. Symptoms include convulsions, psychotic delusions, and gangrene. It has been suggested that the Salem "witches" executed in colonial Massachusetts or their "victims" were suffering from the effects of eating ergotted grain. Today ergot is the source of vasoconstricting drugs used in medicine to control bleeding and relieve migraine headaches.
See also: Haustorium, Angiosperm Parasites, Origin of Parasitism
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