Agriculture Reference
In-Depth Information
Figure 3.18. Movement of YR9 virulent race of wheat rust from Africa to Asia.
varieties, and plowing under straw after harvest help control this insect without use of
expensive insecticides.
Insects cause extensive damage in stored grains of both wheat and barley. Typically,
if only 5 to 10 percent of grains in a storage container may be damaged by insects, the
entire lot is either destroyed or fed to animals. 14 Because these grains are destined for
human consumption, they cannot contain residual chemicals. Some fumigation chemi-
cals are used to clean storage bins. In some developing countries certain low-toxicity
chemicals are used to prevent insect damage. The best control practices are to dry
grain to 12.5 percent moisture and thoroughly clean grain bins before filling each year.
Diseases are a major problem for both wheat and barley. Leaf rust is especially viru-
lent, resulting in 80 to 90 percent loss under favorable weather conditions. Resistant var-
ieties have been developed, but the fungus is genetically variable and readily mutates to
form new races that overcome resistance. Once a new race is able to infect resistant var-
ieties, the spores follow wind currents to infect new areas. A worldwide network of
wheat pathologists has been established to identify and trace movement of these virulent
races. An example of such a movement from Africa to Asia is shown in Figure 3.18.
Movement over the 12-year period followed the prevailing wind patterns. The rate of
mutation of the fungus is such that new races develop and spread every 10 to 15
years. The development of new, resistant wheat and barley varieties is a constant battle.
3.3.9 Harvest and Storage
Mechanical harvesting of wheat and barley grain must wait until the grain has dried to
around 15 percent moisture. If the grain is too moist, there is danger of incomplete
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