APPLICATION OF RICE GENOMICS TO DEVELOP SUSTAINABLE CROPPING SYSTEMS FOR THE GULF COAST
Title: PUTTING SHEATH BLIGHT RESISTANCE GENES TO WORK IN THE RICE FIELD
Submitted to: Experiment Station Bulletins
Publication Type: Experiment Station
Publication Acceptance Date: June 20, 2005
Publication Date: July 5, 2005
Citation: Pinson, S.R. 2005. Putting sheath blight resistance genes to work in the rice field. Texas Rice, Highlighting Research in 2005. pp. VIII-IX.
PROBLEM: Sheath blight disease has been the most economically significant rice disease throughout Texas, Louisiana, and Arkansas since the early 1970’s. While several rice diseases can devastate yields, including blast, sheath blight, and straighthead, sheath blight occurs the most consistently with several hundred thousand acres of rice requiring fungicide treatment each year to control this disease. Rhizoctonia solani, the fungal ornanism that causes rice sheath blight, also attacks several plant species, including soybeans and several of the weed species commonly found in and near rice fields, such as barnyardgrass, crabgrass and broadleaf signalgrass. Weed hosts and rotation crops may serve as sources of inoculum. Sheath blight is a soilborne disease, with fungal mycelia and sclerotia persisting in the straw and stubble of rice, soybeans, and other grasses. Sheath blight infects the rice plant at the water line then produces mycelia that grow up the leaf sheath. Sheath blight lesions are large with cream-colored centers and broad, dark reddish-brown borders. Alternating wavelike tan and brown bands extend up the sheath and leaves. When the fungus reaches the top of the canopy, a circular area often referred to as a "bird's nest" appears (fig. 1). These spots often coalesce, forming large areas of the field that appear grayish (fig. 2). The fungus spreads in the field by growing from tiller to tiller on an infected plant, and from leaf to leaf or across the water surface to adjacent plants. Yield is severely limited in plants exhibiting symptoms on all or part of the flag leaf, and grain milling quality is significantly reduced in plants exhibiting less severe leaf symptoms and yield losses.
Chemical control is costly and incomplete. While several fungicides can slow the progression of R. solani infection, multiple applications repeated every 10 to 14 days are generally required, increasing the cost of chemical control. Scouting and chemical control must be accomplished during early plant development, starting with 1st internode elongation. SOLUTION: Varietal resistance and genetic control. In the case of blast disease, the USDA-ARS Rice Research Unit in Beaumont, TX has demonstrated tremendous success in using molecular markers, or “gene-tags”, to assist breeders in identifying and selecting breeding progeny containing desired blast resistance genes. Because single rice genes are known to confer clear and complete resistance to several races of blast pathogen, developing molecular gene-tags was relatively easy compared to the situation with sheath blight. Unlike blast, there is no complete resistance to sheath blight in rice, and the reduced levels of susceptibility demonstrated by Jasmine 85 and Saber are known to be due to a combination of several genes, each with small individual effect. In 1995, a team of researchers led by Dr. Shannon Pinson (USDA, Beaumont, TX) was first to identify specific chromosomal regions containing genes associated with increased resistance to sheath blight disease in rice. More recent findings by Dr. Pinson’s team have clarified and confirmed the genetic location and effects of six of the resistance genes first reported in 1995, and uncovered the presence of nine additional disease resistance genes (Fig. 3). The genes identified by Dr. Pinson’s team are now providing a solid scientific base for a 4-year, $5 million, 8-State, Rice Coordinated Agricultural Project (RiceCAP) that was initiated January 2005 with funds from USDA-CSREES-NRI. RiceCAP goals are to 1) identify chromosomal regions containing genes for milling quality, and 2) develop molecular markers tagging the SBR genes identified by Dr. Pinson so that breeders can use markers to select SBR breeding progeny as they currently can for blast resistance. Other Texas scientists participating in the RiceCAP effort are Dr. Anna McClung (USDA-ARS Breeder), Dr. Bob Fjellstrom (USDA-ARS Molecular Geneticist), Dr. Rodante Tabien (TAES Breeder), and Dr. Arun Sharma (USDA/TAES Postdoctoral Scientist).