Diseases Cause Millions In Crop Damage; Scientist Looking For New Ways To Fight Them

Donald Groth, Schultz, Bruce  |  4/19/2005 10:29:05 PM

LSU AgCenter research associate Marty Frey works at the AgCenter’s Rice Research Station to deliberately infect rice plants with sheath blight disease. The effort is designed to test different varieties’ resistance or susceptibility to the fungal pest. Dr. Don Groth, an LSU AgCenter plant pathologist who is overseeing the tests, said sheath blight can be treated with fungicides, but developing resistant varieties is less expensive.

News Release Distributed 06/11/04 

CROWLEY – Dr. Don Groth held up a Petri dish containing a few spots of a dark mold, and then posed a question.

"You wouldn’t think that would cause $10 million to $20 million worth of damage a year, would you?" asked Groth, a plant pathologist at the LSU AgCenter’s Rice Research Station near Crowley.

He was talking about sheath blight, the focus of much of his work. Groth is in the right place for the research, since Louisiana’s warm, humid climate creates the perfect environment for sheath blight.

It affects rice plants – causing large lesions with cream-colored centers and dark, reddish-brown borders on the leaves near the water line that move up the plant as the season progresses. Sheath blight normally is more severe in thick stands, where there are more than 20 plants per square foot, and excess nitrogen levels are suspected of worsening the disease.

Currently, the best method for fighting sheath blight, once it is found thriving in fields, is applying one of several fungicides on the market, Groth said, adding that those include Quadris, GEM, Quilt, Stratego, Propimax, Tilt and Moncut.

"Compared to 15 years ago, we have a lot better arsenal," the LSU AgCenter scientist said.

But Groth is working to develop even more in that arsenal – rice varieties with increased resistance to the fungus.

"Resistance is free. Fungicide costs $15 to $35 an acre," Groth said.

Groth does his research, which is funded by the Louisiana Rice Research Board, by planting up to 8,000 rows of different experimental lines of rice each year and then inoculating the plants by hand with sheath blight grown in a laboratory. After that, he inspects the plants to determine the levels of resistance.

The scientist said a variety could be released today with sheath blight resistance, but there are drawbacks.

"Farmers would only grow it one year," he said, explaining that plants with more resistance to the disease tend to have lower yields and poor milling – because those plants direct more of their energy toward combating sheath blight instead of producing grain.

"Basically, that energy is going somewhere else to protect the plant," he said.

The ideal is to develop a plant with a happy medium between good yield and improved resistance.

"We’re getting closer and closer," Groth said.

Farmers are advised that a small infestation of sheath blight doesn’t justify spraying fungicide, according to the expert. But application is warranted if farmers see the disease in 5 percent to 10 percent of tillers infected or 30 percent positive stops, he said.

The same pathogen causes aerial blight of soybeans. And rotation of soybeans with rice has increased the prevalence of the disease.

Groth also works to develop rice varieties resistant to another fungal disease, blast, that affects all parts of the rice plant.

"You can find it in every environment," Groth said. "It’s a worldwide problem."

Blast spreads very easily by wind-blown spores, so Groth said he doesn’t have to inoculate his rows with the fungus in his research plots.

Blast-resistant varieties have to be released regularly to combat the fungus’ constant efforts to adapt, the LSU AgCenter expert said, adding that he isn’t looking for plants totally immune or highly resistant to the disease. Immunity in a plant could help evolve the strongest blast fungus, he explained, and that would render rice plants defenseless.

"Sooner or later, blast adapts to the resistance," he said.

Blast is identified by elongated leaf lesions with brown borders and grayish centers. It can develop into rotten neck syndrome, causing grain sterility or the heads to fall off.

It can be treated with Gem or Quadris at heading. Both contain strobilurin fungicides modeled after a naturally occurring substance found in a wood-rotting mushroom.

Most of the time blast is found where the flood has been lost or rice is growing under upland conditions. Blast is worse in upland conditions for two possible reasons, Groth said.

First, rice grown in upland conditions that aren’t flooded undergoes a wider temperature range, allowing dew to form on the plants sooner and subjecting them to more moisture.

Second, the lack of several micronutrients in upland soil may make the plants more susceptible.

Some cultural practices also can open the door for blast. For example, Groth said drill seeding has become more common for rice, and farmers are being encouraged to delay permanent flooding to control insects. Also, the recommended action to fight a condition called straighthead and other midseason soil problems is to drain the field, which increases incidence of blast, Groth said.

In addition, late planting and excessive nitrogen levels should be avoided, he said.

Another disease, panicle blight, flourishes in hot, dry weather, Groth said, when rice reaches the heading stage, usually in mid-July.

The disease renders the grain sterile. If grain develops at all, it usually is shriveled or brown.

It seems to start in the middle of a field and be less severe closer to the edges, he said.

Panicle blight originates from bacteria that reside in the seed. After the seed sprouts, the bacteria migrate up the plant, from leaf to leaf, finally residing in the panicle, Groth said.

Surveys have found it in 70 percent of rice fields and in every rice-producing state, and once the infestation has a foothold, nothing can be done. It can lower yields by as much as 80-90 percent, although a 5 percent to 10 percent yield loss is more common.

It also discolors grain, penalizing farmers at the mill for decreased quality.

Foliar treatments are being tested, as well as seed treatments. In addition, a seed test is being developed to detect the bacteria before rice is planted.

A mutation of the Lemont rice variety is being used to develop a line resistant to the disease, Groth said, adding, however, that it will have to be crossed with more recent varieties to achieve acceptable yields.

"Right now we’re trying to find out if resistance is inherited," he said.

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Contact: Don Groth at (337) 788-7531 or dgroth@agcenter.lsu.edu
Writer: Bruce Schultz at (337) 788-8821 or bschultz@agcenter.lsu.edu

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