intermediate humidity, which
encourage insect populations to
grow. Scientists are developing
methods to reduce losses before and
after harvest, either through
strategies to reduce the buildup of
borers, weevils, and other insect
pests that attack mature grain in
the field or in storage, or by seeking
to improve resistance to these
insects. Work on improving husk
cover (to present a physical barrier
to insects) and grain hardness (to
inhibit boring) is also underway,
and specialists are developing
better methods for handling,
treating, and storing grain at the
farm level after harvest.

Diseases
Maize in sub-Saharan Africa is
attacked by numerous fungal, bac-
terial, and viral diseases. Prelimi-
nary CIMMYT data show wide-
spread incidence of ear rots, leaf
blight, maize streak virus, and
stalk rot. Ear rot is probably the
most serious disease, because it
reduces the yield and the nutri-
tional value of infected grain and
may cause the formation of myco-
toxins, a health threat to humans
and animals. Leaf blight and maize
streak virus also are extremely
widespread. More localized diseases
include downy mildew, sugarcane
mosaic virus, maize chlorotic mottle
virus, maize dwarf mosaic virus,
curvularia leaf spot, and brown
spot.

A variety of chemical and cultural
practices can help control maize
diseases. However, using resistant
germplasm is the most cost-effec-
tive method for disease control, as
well as the least harmful to the
environment. Breeding for disease
resistance thus remains a primary
objective of national and interna-
tional maize research programs.


Developing materials resistant to
maize streak virus receives high
priority. Epidemics of maize streak
occur periodically in many coun-
tries and in all maize-growing
ecologies of Africa. The virus is
most serious in crops that are
planted late, sown during the
minor rainy season in bimodal rain-
fall zones, or grown during the cool
season on residual moisture in
swampy areas. The international
research centers have played a
leading role in combatting maize
streak. During the 1970s, scientists
at IITA pioneered methods for mass
rearing the leafhopper that is a
vector for the disease; with the help
of these methods, IITA, CIMMYT,
and national program breeders
developed and improved maize
populations and varieties with
streak resistance. Additional streak
research is done at the CIMMYT
Maize Research Station outside
Harare, Zimbabwe.


Labor Shortages
Shortages of agricultural labor
occur throughout many parts of
Africa. These shortages tend to be
particularly severe if the returns to
farm work are low compared to
returns to outside employment, for
example, in areas where mining
offers steady and relatively remu-
nerative work.

While agricultural labor shortages
are basically caused by the limited
supply of farm workers, two types
of competition on the demand side
exacerbate the problem. In zones
where growing seasons are re-
stricted by rainfall distribution,
labor-intensive cropping operations
(such as land preparation, planting,
weeding, and harvesting) fre-
quently overlap (Figure 17). When
this happens, farmers must choose,
for example, between planting a
larger area and weeding the area
that has already been planted.


Labor (h/ha)
120 -


0o-
Sep


SNo e Jn Fb MIr Apr
Oct Nov Dec Jan Feb Mar Apr


May Juy Jul Aug


Source: Malawi Ministry of Agriculture (1977).

Figure 17. Household labor use on maize cropping operations, Chisasa,
Malawi, 1972/73.


ILL