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Major Constraints to Crop Production
- Agriculture in the developing world: Connecting innovations in plant research to downstream applications

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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