A risk-minimizing argument for traditional crop varietal diversity use to reduce pest and disease damage in agricultural ecosystems

Much of the worlds' annual harvest loss to pests and diseases occurs as a consequence of crops grown in monocultures, or crop varieties with uniform resistance. This uniform resistance is met by 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. Until recently, IPM methods have concentrated on using agronomic techniques to modify the environment around predominantly modern cultures to reduce the need for pesticides, making limited use of the opportunities offered by the effective deployment of the intra-specific diversity of local crop varieties themselves within IPM practices. One of the few assets available to small-scale farmers in developing countries to reduce pest and disease damage is their local crop varietal diversity, together with the knowledge to manage and deploy this diversity appropriately. Through a research partnership, over the past eight years, among Biodiversity International and over 30 national and local government and non-government organizations in China, Uganda, Ecuador, and Morocco, high levels of diversity were found within the traditional varieties of a set of globally agreed upon staple crops for the specific pest and disease systems in the four countries: (i) maize (Zea mays): northern leaf blight (Setosphaeria turcica) and stem borer; (ii) common bean (Phaseolus vulgaris): angular leaf spot (Phaeoisariopsis griseola), anthracnose (Colletotrichum lindemuthianum), rust (Uromyces appendiculatus), and bean fly (Ophiomyia phaseoli; O. spencerella); (iii) faba bean (Vicia faba): aphids (Aphis fabae), chocolate spot (Botrytis fabae), seed pod weevil (Bruchus rufimanus; B. dentipes); (iv) banana and plantain (Musa spp.): black leaf streak (black sigatoka; Mycosphaerella fijiensis), Fusarium wilt (Fusarium oxysporum f. sp. cubense), nematodes, and weevils (Cosmopolites sordidus); (v) barley (Hordeum vulgaris): net blotch (Pyrenophora teres) and powdery mildew (Blumeria graminis); and (vi) rice (Oryza sativa): rice blast (Pyricularia grisea) and rice plant hopper (Nilaparvata lugens). Resistance of traditional and modern varieties to the above pests and diseases was assessed from both participatory diagnostics of farmer knowledge and from cross-site on-farm and on-station trials. By performing cross-site onfarm experiments, it was possible to identify traditional varieties with more effective resistance to pest and diseases when grown outside their home sites in all four countries. Increased diversity of crop varieties, measured by number of varieties (richness) and their evenness of distribution across the farmer's fields corresponded to a decrease in the average damage levels across sites and to a reduction of variance of disease damage for common bean and plantain in Uganda, and for maize and rice in China. In sites with greater disease incidence in Uganda, households with greater levels of diversity in their production systems had less damage to their standing crop in the field compared to sites with lower disease incidence. The results support what might be expected of a risk-minimizing strategy for use of diversity to reduce pest and disease damage.