CABI Bioscience, Bakeham Lane, Egham, Surrey, TW20 9TY, UK

The Bruntland Commission (WCED, 1987) classified agricultural economies into three basic types: industrialized, 'green revolution' and resource-poor, and recognized that it was the third category that was benefiting least from developments in agricultural science. Over a quarter of the human race depend for their livelihoods on resource-poor farming systems which occur over a diverse agro-ecological range. They are mainly rainfed, often undulating, with fragile or problem soils of limited potential, and with complex farming systems that are inherently risk-prone and dominated by small family farms. The challenge is to both improve the productivity and stability of these farming systems, in which the application of scientific advances in agricultural technology has had the smallest impact. Since the 1980s new approaches have become more widely adopted in order to understand and tackle the constraints of these systems, using holistic methods which involve farmers, social and biological scientists in interacting partnerships.

Plant disease and the resource-poor farmer
In traditional 'undisturbed' agricultural systems, plant diseases tend to be low on the list of problems facing farmers - they are one of the many biotic constraints kept in balance in these long-evolved systems. Farming practices have evolved to incorporate strategies which directly or indirectly control crop diseases. Avoidance strategies such as long-term fallow rotations, supported in some cases by seed selection, provide the main limitation to build-up of plant pathogens. Disease development is also restricted by heterogeneous cropping patterns and by the general tolerance of landraces to local pathogens. Yet plant diseases occur widely and farmers recognize those which have more overt symptoms, such as leaf blights, smuts, root and tuber rots at or after harvest, and may take direct action against them although they often do not understand the causal biology. Many other diseases such as root pathogens, which exact a toll on plant growth, go unrecognized.

The productivity restriction caused by plant diseases in these traditional systems is often difficult to partition from other factors. Problems become more evident as these systems are changed or adopted for use in new environments, usually through the consequences of development, and the increased vulnerability to and economic impact of plant diseases is one of the greatest. Rotations are shortened and cropping patterns changed as farmers strive to absorb new practices to increase productivity in endeavouring to meet the new demands of a changing society and changing economy. New crop species/varieties are grown which expose unforeseen susceptibilities. New, often ecologically more marginal lands are cultivated as populations expand, increasing predisposition. New pathogens (or pathotypes) spread to vulnerable crops/areas where resistance to them is low.

Helping farmers to help themselves
Assisting resource-poor farmers to overcome crop disease problems must be in the context of their existing farming systems, as resources are often limited to the farmer and the local agro-ecology. Initially there must be adequate problem identification and analysis with farmers; inadequate perceptions and diagnoses remain major obstacles in many areas. Equally, an understanding of farmers' objectives, their actions to achieve these and the constraints encountered is an important prerequisite before realistic options (rather than pre-formulated packages) for crop disease management can be presented. Biological options for resolution of problems may become apparent from existing knowledge or require the acquisition of new knowledge. Much of the existing knowledge about the causal biology of diseases can be used to improve disease control and presented to farmers in simple ways, but there are large gaps in knowledge which restrict the development of ecologically sustainable options suitable for resource-poor farmers. How options may be incorporated into farming practices through adaptive and participatory research is the critical step needed for impact at the developmental level, and requires that both farmers and pathologists understand the principles and processes involved.