Fungi Recyclers Pathogens Parasites and Plant Partners

About 300 million Africans in 25 countries are suffering because of the invasion of crops by witchweed (Striga), a parasitic flowering plant. This parasite has attacked more than two-thirds of the sorghum, maize, and millet crops in sub-Saharan Africa, doing damage estimated at U.S. $7 billion each year.

In 1991 a team of Canadian scientists began a search for a solution to the Striga problem. By 1995 they had begun fieldwork in Mali. What was their strategy? They had isolated a strain of a fungus, the mold Fusarium oxysporum, that has two outstanding properties. First, it grows on Striga, wiping out a high percentage of the parasites. Second, it is not toxic to humans, nor does it attack the crop plants on which Striga is growing. Now farmers apply the fungus to their crops and are rewarded by greatly increased crop yields as Striga is held in check.

It may be possible to repeat this story—using a fungus to wipe out a particular type of flowering plant—in a very different context. A different strain of F. oxysporum preferentially attacks coca plants (the source of cocaine). There is a controversial proposal to use F. oxysporum to wipe out the coca plantations of Andean South America and some countries in other parts of the world.

Some other fungi attack people, not plants. Every breath we take contains large numbers of fungal spores. Some of those spores can be dangerous, and fungal diseases of humans, some of which are as yet incurable, have become a major global threat. However, other fungi are of immense commercial importance to us. Fungi are essential to plants as well. They interact with roots, greatly enhancing the roots' ability to take up water and mineral nutrients. Fungi and plants probably invaded the land together in the Paleozoic era (see Table 22.1).

Earth would be a messy place without the fungi. They are constantly at work in forests, fields, and garbage dumps, breaking down the remains of dead organisms (and even manufactured substances, such as some plastics). For almost a billion years, the ability of fungi to decompose organic substances has been essential for life on Earth, chiefly because by breaking down carbon com-

Fungus Trumps Plant The fungus Fusarium oxysporum is a potent pathogen of witchweed (Striga), a parasitic plant that attacks crops. The fungus spores are shown in blue; the fungal filaments are in tan. Both colors were added to this electron micrograph.

Pathogens Parasites And Fungi

pounds, they return carbon and other elements to the environment, where they can be used again by other organisms.

In this chapter we will examine the general biology of the kingdom Fungi, which differs in interesting ways from the other kingdoms. We will also explore the diversity of body forms, reproductive structures, and life cycles among the four phyla of fungi, as well as the mutually beneficial associations of certain fungi with other organisms. As we begin our study, recall that the fungi and the animals are descended from a common ancestor—molds and mushrooms are more closely related to us than they are to the flowers we admired in the last chapter.

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