In a period of less than a hundred years, the number of food crops cultivated today has dropped from an estimated 7000 species to 150. Crop diversity, both between (inter) and within (intra) species has given way to uni-variety cropping and to large scale, genetically homogenous, cropping for industrial purposes. As a result the genetic base has narrowed considerably.
The potential negative consequences of planting large areas to single uniform crop cultivars were recognised as early as the 1930s by agricultural scientists. When farmers sow cultivated varieties with uniform resistance to a pest or disease, crops can become susceptible to attack by pathogens able to overcome their resistance and epidemics can result. The Irish potato famine is one of the most dramatic examples of genetic uniformity leading to devastating loss of the crop. Susceptibility of five major commercial cultivars of banana to the fungal disease black sigatoka resulted in Central America countries losing nearly 47% of their banana yield. Rice blast epidemics in Korea in the 1970s caused 30-40% yield losses.
Up to 30% of the world’s annual harvest continues to be lost to pest and diseases, with developing countries experiencing the greatest devastation. The resulting economic and food resource costs are, to a significant extent, a consequence of the continuing evolution of new races of pests and pathogens that are able to overcome resistance genes introduced by modern breeding creating the phenomenon of boom and bust cycles. Breeding programs are in place to develop new varieties and to replace varieties that have lost their resistance. However, the maintenance cost of the current system is high. The International Center for Wheat and Maize (CIMMYT), based in Mexico, reportedly spent 35% of its budget in 1989 on ‘maintenance research’. The inherent instability and thus risk for farmers and industry lead to a reliance on various generations of pesticides and more recently genetically modified (GM) crops.
Small-scale farmers in developing countries depend on genetic diversity to maintain sustainable production and meet their livelihood needs. Loss of genetic choices, reflected as loss of local crops or cultivars, diminishes farmers’ capacities to cope with changes in pest and disease infestations, and leads to yield instability and loss.
Local cultivars are a primary source for the new resistant germplasm, providing about 39% of the resistant germplasm used in the breeding programmes of major crops such as maize and barley. Most if not all known resistance to arthropod pests and pathogens in crops are derived from accessions collected from farmers who traditionally grew them in genetically diverse systems. Even so, the development of new cultivars grown as monocultures continues to be central to modern agriculture. Most breeding programs use single genes to provide resistance across many types of environments. In single variety strategies, resistance to only few diseases can be incorporated leaving the crop susceptible to other diseases.
Genetic resistance continues to be part of the disease management strategy in traditional, genetically diverse systems. Maximum numbers of genes for disease resistance have been found in landraces in areas where host and pathogen had coevolved for a long period of time. In effect, ex situ seed collections of farmer landraces and varieties with landrace parentage are the source of virtually all genetic resistance in modern varieties.
In many regions of the world, farmers have local preferences for growing mixtures of cultivars, which they understand to provide resistance to local pests and diseases, and to enhance yield stability. However, the extent to which this is done and its effectiveness are not known. What is known is that farmers apply a variety of agronomic techniques, such as crop rotation and timed planting. Farmers also use high-yielding modern cultivars, shown to be resistant to pests and diseases, and pesticides. Integrated pest management (IPM) strategies, which have focused on using agronomic management techniques to modify environment around predominantly modern cultivars, have excluded the potential of using within-crop diversity, for example, through variety mixtures, multilines or the planned deployment of different varieties in the same production environment to minimize pest and disease pressures on-farm.
The main purpose of genetic mixtures (crop variety mixtures) for pest and disease management is to slow down pest and pathogen spread. The basic principle that enables varietal mixtures to reduce the severity of disease was stated by Wolfe in 1985: “Host mixtures may restrict the spread of disease considerably relative to the mean of their components, provided the components differ in their susceptibility.” This is considered to be the mixture effect.
A diverse genetic basis of resistance is beneficial for the farmer because it allows a more stable management of pest and disease pressure, than a monoculture allows. This is because when resistance in a monoculture breaks down the whole population succumbs, while in a genetically diverse field it is much less likely that different types of resistance will all break down in the same place for comparable pest or disease damage. The effectiveness of a given mixture to do so depends not only on the resistance available, but also on the nature and speed of the life cycles of the pathogens as well as their means of spread. Mixtures serve to decrease the spatial density of susceptible plants, provide a barrier effect by resistant plants that fill the space between susceptible components, and induced resistance by non-pathogenic spores such that normally pathogenic spores that land in the same area are prevented from infecting or are limited in their productivity.
Although the general mechanisms that contribute to the ‘mixture effect’ are now fairly well understood, there is inadequate information on the biological mechanisms that function in complex farmer (not simple researcher) managed intra-specific genetic diversity systems. Few studies are available to shed light on how farmers manage diverse genes in plant populations either to manage single constraints, or as complexes of pests and diseases. Surprisingly, few in depth studies are available on cultural methods that aid the use and longevity of genes. Local preferences exist for growing mixtures in part, because they provide resistance to local pests and diseases and enhance yield stability.
As people move around the globe with genetic resources, so does resistant and virulent germplasm. Resistance genes evolve in response to new pathogens and pest, as well as there being remnants of resistance from old diseases in other regions. This phenomenon has resulted in the occurrence of resistance outside the primary centre of diversity, such as the development of resistance to chocolate spot in faba bean (Vicia faba) in the South American Andes although the crops primary centre of diversity is the Fertile Crescent. This phenomenon creates the potential to find resistance diversity in countries of secondary centres of diversity not found in the primary diversity centres.
The outcome of the project will be that resource-poor rural populations will benefit from reduced crop vulnerability to pest and disease attacks through increased use of genetic diversity on-farm. By providing farmers and NARS researchers with the tools and practices needed to manage local crop (intra-specific) genetic diversity, farmers’ options to combat pest and disease on-farm will be expanded, food security will be increased, genetic diversity conserved, and ecosystem health improved. The project will develop tools to determine when and where intra-specific crop diversity can be used to manage pest and disease pressures by integrating existing farmer knowledge, belief and practices with advances in the analysis of crop-pest/disease interactions. Unlike Integrated Pest Management (IPM) strategies, which have focused on using agronomic management techniques to modify environment around predominantly modern cultivars, this project is unique in that it concentrates on the management of the local crop cultivars themselves as the key resource, making use of the intra-specific diversity among cultivars maintained by farmers.
Project Crops, Pests and Diseases
National partners selected crops, pests and diseases to cover a range of systems and circumstance so that the methodologies developed could be replicated and applied to other systems. The project crops, rice (Oryza sativa), maize (Zea mays), barley (Hordeum vulgare), common bean (Phaseolus vulgaris), faba bean (Vicia faba), banana and plantain (Musa spp), cover a range of breeding systems (inbreeding, outcrossing, partical outcrossing, and clonal) and farmer management systems (managed as populations versus managed as single plants). Pest and pathogens cover those that are determined by major and minor genes (one gene or a complex of genes provide resistance), seed-borne, soil-borne and air-borne diseases, and pathogens or pests affecting different plant organs (aerial and roots). All four countries, China, Ecuador, Morocco and Uganda, contain areas of important crop genetic diversity for these crops, including different types of resistance to major pests and pathogens in their local crop cultivars maintained in traditional farming systems. The countries have at least two target crops in common with another partner country, linking diversity of primary centres of diversity to secondary centres of diversity, in-country initiatives exist upon which the project can build, and each country’s demonstrated commitment to conservation of agrobiodiversity. In addition, the life cycles of major pest and disease that affect these crops are well studied. To now more about the crops and their diseases in this project: The Target Crops and Their Main Diseases