Biology Reference
In-Depth Information
considerably cheaper to produce. A corn plant produces female flowers
(containing ovules that will become the seeds) and male flowers (which
make pollen). The female flowers, which become the ''ears,'' form along
the stem, and the male flowers, called tassels, form at the top. In order to
produce the high-yielding, uniform plants demanded by modern
mechanized agriculture, corn breeders remove the tassels from the
desired female parent so its pollen does not compete with pollen from
the desired male parent. Because male-sterile corn produces infertile
pollen on its tassels, it is not necessary to detassel the female parent
plants by hand, resulting in immense savings in labor costs.
Unfortunately, the genetic similarity of these plants also made the crop
more vulnerable to disease, meaning that if one plant was susceptible to
it, they might all be—and they were.
The southern corn leaf blight infestation of 1970 was caused by a strain
of Cochliobolus heterostrophus called ''race T,'' which preferentially
infected T-cytoplasm corn. The fungus found fertile hunting grounds in
the nation's cornfields that year, and a spate of warm, moist weather
allowed it to spread with shocking rapidity. Blight spores were carried
by the wind and leapt from county to county and then from state to
state. Farmers were left with fields of rotten, useless plants. The situation
would have been even worse had the weather not turned cool and
dry, stemming the spread of the disease. Additional details on
this infestation can be found in Ullstrup (1972) and Hooker et al. (1970).
Another striking example of the danger of lack of diversity is the Irish
potato famine of the mid-1840s. In this case, the pathogen was the
water mold or oomycete Phytophthora infestans, the cause of late blight.
The late blight infects both the leafy portions of the plant, causing
reduced yields, and the potatoes themselves, causing them to rot in the
ground or in the root cellar. By the mid-1840s, Ireland's mixed
agricultural system marked by a variety of crops and livestock had been
gradually replaced by a potato-based economy.
As with the southern corn leaf blight in 1970, a period of warm, wet
weather in 1845 provided ideal conditions for the spread of this
pathogen. Some potatoes survived the winter and were planted in 1846,
but the blight returned, harbored in some of the seed potatoes or
possibly in piles of discarded infected potatoes. As the germ theory of
disease was unknown at the time, the importance of destroying infected
plant materials was likewise unknown. Most of the crop failed, and the
Irish population, who relied upon the potato for most of their caloric
intake, went hungry. There were no stockpiles to relieve the famine
because potatoes cannot be stored for more than a year. Hundreds of
thousands of people starved, and many more emigrated. 1
1. Detailed studies of the Irish Potato Famine can be found in Fraser (2003), Fry
and Goodwin (1997), Goodwin (1997), Ristaino et al. (2001), and Garrett and
Mundt (2000).
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