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Finding resistance, combating rice diseases remain focus

While there are numerous fungicides available to treat rice diseases, they shouldn't be seen as the ultimate goal in protecting a crop. The true aim of Don Groth's research is to find and foster disease resistance.

“This is my 23rd year in the field,” Groth said at the Rice Research Station field day in Crowley, La. “We've made some very good progress and are getting better on others.”

As Groth, a plant pathologist and research coordinator at the station, spoke, he stood before vast research plots used — among other things — for screening disease resistance.

“We're testing that resistance to see if it's effective in yield trials. We've also been doing a number of fungicide trials as a kind of stop-gap effort until we have the resistance needed.

“Resistance is really how we want to control diseases. That's why we spend a lot of time on it. You'll see a number of locations around (the station) marked ‘disease nursery’ or ‘sheath blight resistance study.’ Those are all aimed at getting resistance to diseases like sheath blight, blast and bacterial panicle blight — the three big diseases for us.”

After finding the needed resistance, when incorporating it into a variety “we need to maintain the yield and quality expected,” said Groth, while pointing to several plots. “That's what's back here: a yield loss trial. We have seven varieties ranging from moderately resistant to very susceptible. All of these are evaluated in both uninoculated and inoculated plots.

For example, consider sheath blight.

“If we have something that is moderately resistant to moderately susceptible, we don't have to worry. And we're getting some varieties in that range.”

Blast is much more erratic. Groth wants to find a moderately resistant to resistant variety. With that type of resistance “you don't really have to worry about it. Only if there's an extended drainage period or there's a lot of it in the area would you have to worry about blast.”

Because this year has been very dry, Groth has been hearing few reports of disease. Most farmers are likely getting along without using a fungicide.

“It's critical, though, even in these conditions, to go into the field and open up the canopy in a number of locations. Just see what's going on. We've had reports of quite a bit of blast — especially where there's been drainage for water weevil or straighthead control. If it turns moist, you need to know that disease is out there so you can plan a fungicide application.”

Chuck Rush, an LSU AgCenter plant pathologist stationed in Baton Rouge, shared the tour stop with Groth. Rush's focus was on sheath blight and bacterial panicle blight.

“For sheath blight, we're still making crosses and are extensively using foreign varieties with partial resistance. There's no complete resistance. We've crossed those with our commercial varieties. The lines we're testing now have several sources of resistance.

“This area,” said Rush pointing behind him, “contains our yield test where we take lines with the characteristics necessary: the height, milling, yield and quality characteristics and sheath blight resistance. We put those with a commercial variety as a check to test them.”

Researchers are seeing success. Lines have been developed that yield with commercial varieties and have “virtually all” the characteristics necessary. The best ones go into the uniform yield nursery every year. There, they're tested by breeders and are available for crossing.

“The second thing we're focused on is bacterial panicle blight. In 1996, we discovered that panicle blight, which has been around forever, is caused by a bacterium. We began studying it and identified sources of partial resistance. Again, there's no complete resistance known in the world.”

Partially resistant varieties have been crossed with commercial varieties and selections are being made from those.

Rush and colleagues just finished a series of studies on the causes of panicle blight. They found two species of bacteria cause the disease.

“The main one is seed-borne and that's how the disease is best spread around. Since the early 1990s, it has spread from Japan to become a serious problem virtually all over Asia, in Central America and South America and other rice-growing areas. We were just in line for this problem like everyone else.”

In 1998, many fields south of Interstate 10 lost up to 40 percent of their yield. It seems to be a problem in years that are unusually hot — especially when high temperatures extend into the night.

“When you're watching the 10 o'clock news and it's still in the 90s…you may find damage from this disease.”

That doesn't mean the disease isn't there in cooler years.

“In our tests, we've found that even when you're not showing symptoms — when the panicles are standing up straight — there can be enough aborted and damaged florets to cause losses of several hundred pounds per acre. And you'll hardly know you've been damaged.

“We don't want to forget about this disease. The last serious problem with it was in 2000. But it's out there all the time. The next time the temps are right, it'll come back in a very serious form.”

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