DeVries and Toenniessen (3)
have analyzed in detail both the biotic and abiotic constraints that
currently limit yield on many of the major crops of Africa, a list that
overlaps considerably with other very poor regions of the world.
Examples of major constraints are given below; succeeding sections will
suggest strategies for translating current basic research efforts into
downstream solutions for some of these constraints.
Abiotic Stresses.
Nutrient-poor, degraded, and often acidic, soils limit crop production
in many tropical regions. When coupled with the high cost of inorganic
fertilizer, especially in Africa, much small-scale agriculture occurs
under conditions of nutrient deprivation and/or metal ion toxicity.
Limiting amounts of phosphorous and excessive levels of aluminum are
characteristic problems of acidic soils (7).
The unintended consequence of trying to do good by drilling large
numbers of wells in Bangladesh and parts of India has resulted in
extremely high rates of arsenic poisonings in humans, but the problem
also extends to agriculture, where these wells have served as
irrigation sources, resulting in high levels of arsenic in the food (8).
Saline soils are found naturally in many locales and have been created
in others by poorly managed irrigation. Most of the extreme poor depend
upon rain-fed agriculture and, according to World Watch, drought is
perhaps the biggest constraint to agricultural productivity worldwide.
As recognized by the International Rice Research Institute, in many
countries of Asia, depletion of ground-water resources, rising soil
salinity, and the competing demands for water by agriculture and a
growing urban sector are likely to result in a shift in cropping
systems away from traditional paddy rice toward growth under aerobic
conditions. This, in turn, calls for more drought-tolerant varieties,
new strategies for weed control, and a much better understanding of how
large-scale changes in cropping systems for major crops like rice will
affect the global balance of C and N.
Biotic Stresses. Fungal diseases are a huge problem worldwide. The fungal stem rust (Puccinia gravinis) of wheat was effectively controlled through introgression, decades ago, of the Sr31 resistance gene by Norman Borlaug and
colleagues (9)
and has been remarkably durable, but a resistant strain of the rust has
recently emerged in Africa and, in this age of globalization,
represents a potential worldwide threat if not addressed in a timely
fashion (9). Soybeans of Africa, Asia, and Latin America are heavily affected by rust (Phakospora), and North American varieties have had good resistance until recently as the pathogen has emerged in some areas. According
to the International Potato Center, the late blight of potato (Phytophthera)
is the single most costly biotic constraint to global food production.
Powdery mildews affect a wide range of crops, including major cereals
like wheat, sorghum, and millet; fungal anthracnose affects crops such
as sorghum, beans, and cassava; leaf spot and root rots also plague
beans and other important crops; turcicum and gray leaf spot diseases
are serious pests of maize in Africa; blast disease is serious for
rice; and Black Sigatoka limits banana production worldwide. Small farm
environments seriously promote the development of fungi that lead to
mycotoxin accumulations; better statistics are sadly needed, but the
little we have suggests the health effects of mycotoxins on the poor
are much more serious than recognized previously (e.g., see ref. 10). Bacterial diseases similarly cause large crop losses. Particularly deadly are diseases caused by the genus Xanthomonas, which include blights in rice and cotton and, more recently, banana wilt, a serious new disease of the African Highland
Banana, the major staple crop of Uganda.
Viruses
are no less a problem for many crops. Among the RNA viruses are papaya
ringspot, cassava brown streak, and cucumber mosaic virus which affect
many vegetables. In Africa, the ssDNA geminisviruses such as maize
streak and cassava mosaic virus are particularly deadly; worldwide,
others, such as tomato leaf curl and banana bunchy top, are also
important. And finally, in certain parts of subSaharan Africa, the
parasitic weed Striga can be one of the most serious constraints to the
yields of crops such as maize, sorghum, and cowpea, whereas another
parasitic weed, Orobanche, is an important pest to several crops in
countries like Egypt and India.
This
list of constraints is by no means comprehensive. To my mind, the best
approach to identifying key constraints is to establish a much better
dialogue with between bench scientists and those key breeders in the
developing world who actually talk to farmers and understand local
agriculture. One of the most rewarding experiences I have had in recent
years was to organize a workshop that brought together breeders of
African crops with some of the key scientists working on genes that
control flowering time and plant architecture. When forced to avoid
jargon specific to their trade and to resist talking just about their
most recent great discovery, these two groups were excited to learn
from each other and to identify some imaginative new approaches to crop
improvement, some of which are now being funded by the Rockefeller
Foundation.
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