Unmindful of the heat and the humid air circulating from an industrial fan in the glasshouse, Casiana Vera Cruz, plant pathologist at the International Rice Research Institute (IRRI), met with her team to check the progress of the rice plants that exhibit resistance to bacterial blight. Several rows of plastic boxes with plants from different rice-growing countries filled the facility.
“Some varieties are from South Asia while others are from Southeast Asia,” said Dr. Vera Cruz. ”The plants look healthy for now but, in the next few weeks, we will see the plants that are more resistant to bacterial blight, as they have been inoculated.”
A deadly disease
“Among rice diseases, bacterial blight is one of the most costly,” said Dr. Vera Cruz. “It can damage as much as 60–70% of the plant and can even result in crop failure, especially when disease strikes at the seedling stage.”
Once infected at the seedling stage, the leaves turn grayish green and roll up. And, as the disease spreads, the leaves turn yellow to straw-colored and then wilt. The result can be a grim nightmare for farmers as they helplessly watch their seedlings dry up and die.
This is exactly what happened to farmers in Haryana and Punjab states in India in 1980 when for the first time, the rice they were growing succumbed to a bacterial blight outbreak. It is the same disease that has been associated with major epidemics that ruined the fortunes of farmers in China, Korea, Indonesia, the Philippines, Sri Lanka, Myanmar, Laos, Taiwan, Thailand, and Vietnam. The disease also occurs in Australia and Africa.
It is no surprise that farmers are taking this disease seriously. Although there are chemicals developed to control this disease, none of them are completely effective at eliminating outbreaks.
Breeders at work
However, farmers no longer need to worry spend very much on chemicals to combat bacterial blight thanks to the scientists at IRRI and other research organizations who have been scouring the world for rice plants that have natural resistance to bacterial blight.
“Many improved rice varieties now have major genes for resistance to the disease,” said Dr. Bertrand Collard, IRRI plant breeder. “Thus, the chances of farmers losing their crop to bacterial blight are lower.”
As early as the 1970s and ‘80s, rice scientists found varieties TKM6 and DV85 that had inherent resistance to bacterial blight. Recently, researchers have identified more than 30 genes (named Xa1 to Xa38) that impart blight resistance.
“Making rice resistant is not only most economical, but it is also a sustainable way of controlling bacterial blight,” said Dr. Vera Cruz. “A good example is IR20, one of the elite varieties that has been promoted by IRRI since 1975. Even after more than 35 years, IR20, which carries the Xa4 gene, is still resistant to some strains of bacterial blight.”
At IRRI, more than 80% of the elite lines have the Xa4 gene and, since 2000, released cultivars such as PSB Rc82 carry combinations of genes with resistance to the predominant population of the pathogen. Other elite lines have also been developed with different combinations of Xa5, Xa7, Xa13, and Xa21, among other genes. Some elite lines and released cultivars show broad-spectrum resistance, indicating that unknown or novel genes may be present in these lines and cultivars.
Nevertheless, bacterial blight continues to be an important concern due to the capacity of the pathogen to change and overcome the deployed resistance genes. Government agencies know well that the stakes are high. Whenever susceptible rice varieties are grown in environments that favor bacterial blight, the disease can turn green rice fields into wastelands of dry and wilting leaves, and empty grains—wiping out investments and potential profits.
Balancing genetic protection
Recently, Dr. Vera Cruz’s team made a discovery that will further improve the resistance of rice to the disease. They learned that it is not just the presence of resistance genes Xa4 and Xa7 that is important, but that environmental factors such as temperature also play an important role in how the genes protect the plant. They found that Xa4 is more effective as temperatures drop, while Xa7 does its job better at higher temperatures.
“Since the two genes compensate for each other‘s weaknesses, this will also help farmers withstand changes in weather patterns,” she said. “Climate change could radically alter temperatures during the dry and wet seasons.”
Since pathogens co-evolve with the plant, growing a single resistant variety over large areas will “push” the virulent form of the pathogen to become dominant. Therefore, one key questions for breeders and plant pathologists is how to deploy the resistance genes to prevent pathogen epidemics while maintaining yield.
Diverse genetic resources
This is where the importance of genetic diversity comes into play. The good news is that IRRI has a genetic gold mine of different types of rice including wild rice accessions that is stored in its International Rice Genebank. The genebank continues to provide rare versions of genes to enrich and diversify the sources of resistance to manage bacterial blight.
Aside from finding a critical mix of genes, IRRI scientists are aiming to map the genome of the blight pathogen and understand what role genes play in the plant. With this information, they can precisely target certain genes of the pathogen that cause virulence in the plant host. According to Dr. Vera Cruz, this will radically shorten the breeding process for designing blightresistant rice varieties.
No one can tell what challenges the future may bring. With the ever-evolving diseases and changing climate patterns, IRRI scientists are not resting on their past successes. They are constantly searching for better ways of doing things to deliver what farmers need to win the battle against this insidious disease.