Biology Reference
In-Depth Information
species harming the other, or even beneficial to one or both species. Most
higher plants, for example, possess fungi, known as endophytes, that live
within their tissues without causing obvious harm (Palm 2001).This rela-
tionship appears to be a balanced antagonism, in which both plant and
endophyte have evolved to tolerate each other's offenses (Schulz et al.
1999). Mutualism is a further step in such relationships. The relationship
between nitrogen-fixing bacteria and their host plants, for example, is
mutualistic, with both members benefiting. The host plant receives fixed
nitrogen and the bacteria receive energy-rich photosynthate. Similar rela-
tionships occur between many animals and microorganisms. Gut-
dwelling endobionts occur in most higher animals, where they may be
harmless saprotrophs or mutualists that aid in digestion or produce
organic compounds beneficial to their hosts.
Benign or beneficial interactions may be altered, however, when
microorganisms that are in evolutionary balance with their native host
agents are transported to new areas and introduced to vulnerable new
hosts. In the modern world, long-distance transport of microbes is
becoming commonplace.The results of such events are some of the most
damaging outbreaks of plant and animal diseases (Wingfield et al. 2001).
Chestnut blight ( Cryphonectria parasitica ), a benign fungal associate of
Asian relatives of the American chestnut ( Castanea dentata ), is a classic
example of the result of introduction of a fungal pathogen to a new geo-
graphical area (Liebhold et al. 1996). Chestnut blight is the most destruc-
tive tree disease to strike North America.American chestnuts once ranged
from Georgia and Alabama north to Maine and west to the Mississippi
River. Chestnut was one of the dominant canopy trees in the eastern
forests and produced mast that was consumed by many native mammals
and birds. It was also one of the most economically important hardwoods,
supplying lumber for construction, rot-resistant ties for railroads, and fine
wood for manufacture of furniture.
Chestnut blight appeared in New York City prior to 1904, probably
entering North America on ornamental Japanese chestnut trees. Once
established, it spread rapidly by wind transport of spores and within 20 yr
had killed almost all mature chestnut trees in New England. In another
20 yr, it had spread throughout the range of the chestnut. Although the
blight kills the aboveground portion of the tree, the roots are resistant.
Thus, although adult chestnuts have been eliminated, sprouts continue to
be produced by root systems of former trees.These sprouts may grow into
small saplings, but they are eventually killed. Loss of the chestnut as a
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