Using Genetic Diversity to Achieve Sustainable Rice Disease Management.

Abstract

Host plant resistance is an important tool for rice disease control and has played a key role in sustaining rice productivity, especially in tropical Asia. Deploying resistant varieties as a means of disease control is attractive because it requires no additional cost to farmers and is environmentally safe (62). Furthermore, resistant varieties can be easily disseminated as seeds, leading to wide adoption (12). These are important considerations, because for resource-poor rice farmers in developing countries, the options for managing diseases are few. For example, during the 1970s and 1980s, when epidemics of rice tungro were frequent in the Philippines and Indonesia, farmers expressed more confidence in using resistant varieties than in other control measures. Disease control using chemicals is more common in the temperate or subtropical production environments where farmers apply fungicides for controlling blast (caused by Pyricularia grisea) and sheath blight (caused by Rhizoctonia solani). Despite regional differences in control measures, planting resistant varieties is considered most effective by rice farmers. Hence, breeding for disease resistance has been a major objective in rice improvement programs conducted at international agricultural research centers, such as the International Rice Research Institute (IRRI), and at the national agricultural research systems (NARS) of developing countries. There are limitations, however, in using resistant varieties alone to manage rice diseases. Most varieties are resistant only to a few major diseases that are the subjects of intensive breeding efforts. The rice production environments, particularly in the tropics, are habitats of many rice pathogens causing varying degrees of damage. Even the “minor” diseases collectively could pose a significant threat to production (63). Thus, pathologists and breeders have to deal with yield loss caused by diseases of epidemic and endemic nature. Epidemic loss is dramatic but less frequent, whereas endemic loss is less obvious but pervasive in each cropping season. Recent surveys indicated that an estimated annual yield loss from 1 to 10% was due to a combination of different diseases (80). Thus, resistance against a few targeted diseases offers only a partial solution to rice disease problems. To those diseases caused by nonspecialized pathogens, such as sheath blight and false smut (caused by Ustilaginoidea virens), no useful source of resistance has been identified to improve the resistance of rice varieties. To achieve sustainability of rice production in Asia, we need a rice production system built upon effective resistant varieties with broad resilience to a range of diseases and insect pests. Broad-spectrum resistance at the genotypic level and sustainability at the cropping systems level are therefore complementary approaches in managing rice diseases. Although considerable progress has been made over the past decades, much more can be done to integrate these two approaches to achieve results in farmers’ fields. Modern agricultural development has transformed the diverse, traditional rice production system into a monoculture system that relies only on a few fertilizer-responsive and high-yielding varieties. Farmers’ preference to high yield has led to wide adoption of modern rice varieties cultivated in millions of hectares of rice land. Although most modern varieties have built-in resistance against multiple diseases, genetic uniformity inevitably predisposes the system to disease epidemics, and under certain circumstances can lead to serious yield loss caused by diseases and insect pests (43). Varieties carrying a few resistance genes in a uniform genetic background are vulnerable to rapid adaptation of pathogens and pose uncertainty to farmers. For instance, emergence of new pathogen races caused several blast epidemics in Korea in the 1970s, leading to yield losses of 30 to 40% (38). In the 1980s, other disease outbreaks on a regional scale included epidemics of bacterial blight in northern India and Southeast Asia, tungro in Southeast Asia, and bacterial blight and blast in Japan (38,61,89). Another impact of the monoculture system is the gradual decline in the diversity of varieties grown by farmers. As modern high-yielding varieties expand to millions of hectares, they also replace the traditional varieties. Although useful genes from these traditional varieties are being used in breeding for modern varieties, many unique attributes and gene combinations resulting from years of selection are difficult to reconstitute. To achieve the productivity needed, it is not possible to revert to planting diverse traditional varieties that are poor yielding. However, it is within our capacity to work toward disease management methods that sustain productivity yet maintain adequate diversity and resilience in the production systems. In the past two decades, IRRI has moved toward increasing genetic diversity of modern rice varieties through resistance breeding (12,39,43) and deployment of different resistance genes based on an unCorresponding author: Twng Wah Mew, Entomology and Plant Pathology Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines; E-mail: T.Mew@cgiar.org