Biology Reference
In-Depth Information
reproduced asexually and showed little genetic variability. This genetic
uniformity contributed significantly to the success of disease control. In
the early 1980s, however, resurgence of late blight disease began to occur
in the Old World, and by the late 1980s and early 1990s, severe outbreaks
of the disease also affected potato crops in Canada and the United States
(Fry and Goodwin 1997). These outbreaks were traced to new strains of
late blight fungus that apparently originated in Mexico. The new strains
were much more virulent and were resistant to one of the primary fungi-
cides previously used to control late blight. These strains have already
caused production losses measured in millions of dollars in parts of the
Pacific Northwest.
Even more serious is the evolutionary potential created by these new
forms of the late blight fungus. The new strains belong to the comple-
mentary mating type of the earlier fungus, so their arrival now makes pos-
sible sexual reproduction and resulting genetic recombination. Sexual
reproduction has already been confirmed in several locations (Goodwin
et al. 1998). New fungal strains that appear to be the result of recombina-
tion between different mating types have also been found on tomatoes in
North Carolina (Wangsomboondee et al. 2002).Thus, the stage has been
set for the rapid evolution of new genetic races of a fungus that is known
to affect two of the world's most important crops, potatoes and tomatoes,
both of which are members of a plant family containing a host of other
cultivated plants.
The Economic and Ecological Impact of
Alien Species
Invasive alien species are now recognized throughout the world as one of
the most serious ecological and economic threats of the new millennium
(Pimentel 2002).Alien plants are reducing the productivity of agricultural
crops, pastures, and rangelands and are disrupting many natural terrestrial
ecosystems. In addition, alien plants are choking waterways and altering
the function of freshwater and marine ecosystems. Many of these plants
are now legally designated as noxious weeds. Alien animals are also alter-
ing the biotic structure of land, freshwater, and marine ecosystems and are
pushing many native species toward extinction. Introduced disease agents
are infecting crops, livestock, fish and game animals, timber trees, and hor-
ticultural plants. Increasingly, introduced diseases and their vectors are
posing new threats to human health as well. The worldwide total of
species introduced to new geographical regions by human agency prob-
ably approaches half a million species (Pimentel et al. 2001).
