Rice breeders searching high, low for disease resistant genes

Incorporating genetic disease resistance into proven California varieties is a priority for researchers at the Rice Experiment Station (RES) in Biggs, Calif., according to Plant Pathologist Jeff Oster. Oster has been screening varieties for better resistance to blast, sheath spot and stem rot diseases.

It’s a very complex process that transcends the borders of the U.S. rice industry. Researchers are looking at wild species of rice from abroad for better disease resistance traits than are available domestically.

“We’re looking at Indica, which is a different sub-species of rice, as well as other donor varieties that cannot make it here in California as rice varieties due to poor seedling vigor, poor cold tolerance, or other characteristics,” Oster says. “In those cases, we’re backcrossing to extract the genes we want out of the donors and put them into something that works in California rice.”

The goal is to end up with a variety that performs like M-206, but also has disease resistance.

Resistance to blast is one goal of this search. “Blast can have many generations of spores that go on to produce lesions which damage your rice,” Oster says. “It can spread throughout your acreage in a very graphic way if the environmental conditions are conducive. On the other hand, it may happen sporadically, so you may not always have a blast problem from year to year.”

Isolating and incorporating genes for blast resistance into a variety can be tricky. “There are genes with resistance to blast that essentially eliminate symptoms of the disease on the plant,” Oster says. “The problem is that resistance usually breaks down within two to three years after release. So even though we have M-208 that has a resistant gene, we need to have a backup in case blast flares up again and causes some major problems in our industry.

“Right now we’ve finished the backcrossing program which means we have M-206 plus one blast resistance gene in each different line that we’ve developed,” Oster says. “The next step is ‘pyramiding’ or putting those genes together into the same plant.”

Another area of disease resistance is stem rot. “It’s different than blast in that it cannot spread rapidly over wide acreages and cause multiple cycles of disease,” Oster says. “It’s chronic. It just lingers there all the time in the areas where it’s adapted and can cause up to 25 percent yield loss, although it usually does not result in that degree of yield loss.”

There is no major resistance in the worldwide gene pool to combat stem rot, according to Oster. The primary goal is to reduce sclerotia production. Medium grains are particularly susceptible to problems.

“We have long grains and short grains with resistance that yield anywhere from 1,300 – 2,300 pounds per acre greater under inoculated and other disease conditions,” Oster says. “But we haven’t been able to put those genes in the medium grain program.”

Oster started a backcross program for stem rot resistance in medium grains in 2005 and that program has now progressed to backcross 4. “That means we have 96 percent M-206 genes plus the stem rot resistance,” he says.

Breeding disease resistance into existing California varieties takes time and patience. “You have to get all these things – blast, stem rot, sheath spot resistance – into advance backgrounds before you combine them,” he says. “If you do it too early you get varieties that can’t make it on their own, and you never get anywhere. So we’re trying to get all the disease resistances into the M-206 background and then combine them.”

Breeding for sheath spot resistance is particularly tricky because breeders do not have the molecular markers to help them speed the process as they do in other disease resistant breeding efforts.

“We’re early in the backcross process,” Oster says. “We’d like to have the molecular markers as we do for blast because you can detect the disease without having to put the pathogen on, or worry about all the environmental interactions and the uncertainties of year-to-year replication. We could eliminate that if we could find markers or little snippets of DNA that could tell us the genes for resistance are in those plants.”

That could soon change due to DNA work being conducted in some of the Southern rice growing states. “They are doing work with sheath blight which is a similar disease to sheath spot,” Oster says. “They have found some markers to detect resistance for this disease so our hope is that we can use these higher precision, more accurate markers to help us advance our programs in these areas.”

Sheath spot is not as damaging as stem rot under most California conditions. “You’re probably looking at a 5 percent to 10 percent yield loss when you have dew on the plants for an extended period of time,” he says. “It’s sporadic. You don’t see it every year, but if you can build resistance in the plant, you don’t have to monitor and you don’t have to worry about spraying.”

Bakanae is not a focus at this point due to the success of bleach treatments in controlling the disease. “It’s entirely seed-borne,” Oster says. “If you can eliminate the pathogen on the seed it doesn’t persist very well over the winter. It’s not worth our effort in the breeding program right now to breed for bakanae resistance. The seed treatment controls the disease quite well.”

TAGS: Rice
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