Bio-control agents can help in fight against beet diseases

Published online: Mar 31, 2014
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SIDNEY, Mont.—Scientists at the USDA-ARS Northern Plains Agricultural Research Laboratory (NPARL) in Sidney have patented an enzyme that can help in the fight against deadly sugarbeet diseases.

Plant pathologist Robert Lartey and microbiologist TheCan Caesar-Ton at the USDA-ARS NPARL, have been conducting studies on ways to combat sugarbeet diseases using bio-controls, natural enemies of such diseases as Cercospora leaf spot.

The enzyme laccase was identified by Lartey and Caesar-Ton and shown to attack the toxin that causes Cercospora.

“Ultimately, applying some form of the enzyme laccase to sugarbeets could prevent Cercospora leaf spot from occurring,” Lartey said.

The fungus Laetisaria arvalis produces an enzyme called laccase that destroyed cercosporin, which is a toxin produced by Cercospora beticola, that kills sugarbeet plant leaf cells in lab experiments at Sidney.

The successful discovery of the enzyme laccase has been patented by USDA-ARS, Lartey said, and is being licensed for commercial development through ARS Technology Transfer Office in Beltsville, Md.

There has already been some interest in developing these natural fungicides for commercial sale to growers, he added.

However, it can be a long process to take the fungus from the test tube to the store shelf, since companies need to not only find ways to reliably mass produce the fungus, but also to address many other commercial considerations such as shelf life, Lartey said.

“It’s a living organism, but it has to be formulated so that it can be shipped long distances and sit on a shelf under a variety of conditions until it’s needed by farmers,” he added.

These studies on bio-controls started when Lartey, who previously worked with the Laetisaria fungus at Auburn University in Alabama and found it helped with cotton diseases, thought the fungus might be useful in helping with sugarbeet diseases.

“Our lab studies showed that L. arvalis produced an enzyme called laccase that destroyed the cercosporin toxin produced by C. beticola that kills the sugarbeet plant leaf cells,” Lartey said.

After the lab experiments, the scientists took the next step – studying Laetisaria in the greenhouse.

They coated the sugarbeet seed (like a seed treatment) with Laetisaria, then planted it. But because a seed treatment cannot prevent cercosporin from moving up the plant and attacking the leaves, a foliar application of Laetisaria was also applied, Lartey explained.

They also had controls where no Laetisaria was applied.

“In the greenhouse, we got the same positive results; the Laetisaria was detoxifying the cercosporin, essentially starving the disease-causing fungus,” Lartey said.

They found that the combination of the seed treatment and the foliar application again worked very well.

But because the “good” fungi would need to work out in a real-world situation—a farmer’s field—the scientists’ next step has been to take the experiment outside to experimental field plots, where Mother Nature can also weigh in.

Cercospora is always in the soil because it survives over the winter primarily on infected beet debris in soil, Lartey said. Even when growers rotate out sugarbeet two or three seasons and come back, they could still have an incidence of Cercospora because it is still in the soil.

However, for the fungal disease to take off and devastate a grower's field, environmental conditions also play a part, Lartey said.

“Cercospora leaf spot needs high precipitation, high humidity and warm temperatures to infest a field,” he said. “But despite those restrictions, it remains one of the deadliest diseases of sugarbeets.”

Unfortunately, conditions were not consistent for the disease for the past few years so results in the field were not conclusive, which means the researchers will need to continue their field treatments for a few more years of data, Lartey said.

Temperatures between 76 degrees to 95 degrees and high relative humidity between 90-95 percent are crucial for incidence of the disease, he said.

Once a primary infection has taken place, the disease continues to be spread by windblown spores moving from infected plants to uninfected plants over long and short distances, or by spores carried by water slashes from rain or irrigation, he explained.

The fungus on the infected plants destroys the leaves that are responsible for making the sugar in the plant resulting in reduced tonnage and lower extractable sucrose, he added.

In the field trials, which take several years, the same treatments are being applied as in the lab and greenhouse, and the preliminary observations indicate the results are just as good.

“Laetisaria arvalis is showing real promise as an effective agent against Cercospora,” Lartey said. “However, more research is needed.”

They are also looking at other natural fungi to fight Cercospora leaf spot and other diseases such as Rhizoctonia.

“A lot of these bio-control agents like Laetisaria are not just attacking one disease or one crop, so we are looking at these agents from all angles,” Lartey said.

In addition, because malting barley is one of the main rotational crops in the region with sugarbeets, the scientists are also looking at the potential of good fungi like Laetisaria to fight barley diseases as well.

In addition to Cercospora leaf spot of sugarbeet, caused by Cercospora beticola, net blotch of barley caused by Pyrenophora teres is also a very important disease in the area, Lartey said.

The scientists are also studying Laetisaria as a potential natural enemy of net blotch, he said.

They have also recently identified another potential bio-control agent for both Cercospera and net blotch, another fungus call Peniophora nuda.

These bio-control agents could cut down on the amount of fungicides needed in sugarbeets and barley, significantly reducing costs for producers, Lartey said.

“Alternatives to chemicals can help because pathogens frequently develop resistance to the chemical fungicides. Resistance is an issue,” he said.

In many cases, producers can reduce the number of times they need to spray to control the disease if they also use bio-control agents, Lartey said.

“Since the bio-control agents are living organisms like the pathogens, it’s also possible that they can evolve along with the disease-causing agent, thereby avoiding resistance problems common with chemical sprays,” he added.

Sugarbeet producers in eastern Montana and western North Dakota, as well as in the Red River Valley struggle with yield losses totaling as much as 30 percent of the crop’s value due to fungal diseases.

“The hope is that these natural enemies can work together with regular fungicides to more fully control sugarbeet diseases,” Lartey said.