Rice Farming

Bakanae disease swept through California rice regions in 2002, causing significant yield losses in a few fields and invigorating efforts to find suitable seed treatments and recommendations to keep the relative newcomer from becoming more serious.

Foolish seedling disease, as Bakanae is also known, is transmitted primarily by seed. It was first found in 1999 in Butte and Colusa counties and has since spread to every rice producing county in California.

“Four years from the original find, about 80 percent of the fields [in California] now have some level of Bakanae in them, all from the seed,” says Jack Williams, University of California farm advisor in Sutter, Yuba, Placer and Sacramento counties.

“I was very surprised and a bit stunned by the damage I saw this year in a very few fields. That doesn’t mean it’s an unmitigated disaster, but it’s a high level of concern and if we don’t clean it up in the seed it will cost us some yield losses.”

Grower Walt Trethevan, who farms 500 acres of rice in Sutter County, says he has experienced Bakanae to some degree since 1999, and symptoms seem to increase slightly every year. But 2002 was the first season Trethevan experienced significant yield loss as a result of the disease.

In 2002, Trethevan first started seeing symptoms in his most affected field about 30 days after planting with taller, elongated and slightly chlorotic plants.

As the season progressed, he says, those plants didn’t stool as well as other and about a third of the heads in the field were empty of grain at harvest.

Ultimately, he says, that 160-acre field produced 62 100-pound sacks dry compared to 85 to 95 sacks on neighboring fields.

History of Bakanae 101
Foolish seedling disease was likely introduced to California through illegally imported infected seed.

Most evidence points to Japan as the likely seed source because the country is one of the few known rice producers internationally to have the disease in its seed stock, and California growers cater heavily to consumers with Japanese-style varieties.

Until 2002, yield losses had been minimal in California rice fields, and researchers and rice industry groups bought time studying possible chemical and cultural practices that could help stem the spread and potential field damage of the disease. But last season it hit some growers with a vengeance, causing an estimated 30-percent yield loss in some fields and creating a sense of urgency among the industry to find suitable solutions in advance of a potential outbreak.

Caught by surprise
California Extension rice specialist Jim Hill says when the disease was initially isolated in California, yield losses were not affected. With the high seeding rates used in California fields, healthy plants compensated to overcome the loss of infected seedlings. That was not the case last season.

“Bakanae caught everyone by surprise in 2002,” says Hill, who’s based in Davis,Calif. “The disease in some fields persisted through heading, thus causing many completely blanked heads and reducing yields in some fields from over 8,000 pounds to less than 6,000 pounds per acre.”

Hill says researchers have little understanding why some fields with infected seedlings went on to produce significant losses at harvest while others were able to compensate under the same seed and growing conditions and survive with normal yields.

But because seed is the source, he says the growers’ first line of defense now is to buy from a supplier who can assure them the seed is relatively clean of Bakanae spores. From there, growers should consider treating the seed should a pending seed treatment become available in time for this season.

Bleach seed treatment
The industry is hoping the state will register common household bleach as a seed treatment by seed handlers and growers during the soak and drain period to reduce disease spores on seed prior to planting.

Should the treatment become available, the California Cooperative Rice Research Foundation will be treating certified breeders seed at the Rice Experiment Station in Biggs, Calif., before that seed is released, says plant pathologist Jeff Oster. That treatment alone will not eliminate the presence of Bakanae in the lot, but can reduce spores by 80 to 90 percent.

Oster for the past few years has been studying a number of fungicide and bleach treatments in an effort to find suitable seed treatments to protect growers against Bakanae disease. While several fungicides showed promise at the greenhouse level, Oster says those results did not pan out under field conditions.

However field studies have shown that a solution of 3,000 parts per million (ppm) chlorine in soak water can significantly reduce Bakanae levels in seed. As a result, rice experts are recommending commercial seed companies and growers of seed rice in particular consider a preventative bleach treatment in the seed soaking process.

Clorox Corp. is currently pursuing a label amendment that would add rice to the crops, along with asparagus, tomatoes and peppers, which are registered in California to use Clorox as a seed treatment.
Roberta Firoved, manager of industry affairs for the California Rice Commission, which is supporting the label amendment, says registration is required for bleach to be used as an antimicrobial agent to treat seed.
Registration is anticipated in time for the coming season, and if granted, would provide major relief for growers such as Trevethan.

“I absolutely am going to do this bleach treatment if it’s available,” Trevethan says. “Early on I said that if we don’t have something, I’m not going to plant this year. With the cost of production, I’m not going to take a chance of losing a third of my production.”

Trevethan is also hoping he will be permitted to burn the infected fields based on his levels of stem rot and sheath spot to remove overwintering Bakanae inoculum as well.

“While seed is the primary source of inoculum, spores in straw residue may also contribute to disease levels, particularly in heavily infected fields,” Williams says. “Until we know otherwise, growers should burn these fields to destroy spores, consistent with current burning regulations.”

Soak seed for first two hours
As for the bleach treatment, if approved, researchers suggest growers soak seed for the first two hours of the typical 24-hour soak period in a solution of 5 percent Clorox Ultra, which contains 6 percent sodium hypochlorite, the active ingredient in bleach. That equates to about 1 gallon Clorox Ultra to 20 gallons of water. Soak seed for two hours in bleach solution, drain and replace the tank with fresh water for the remainder of the soak period, then drain and fly on seed as usual.

Oster says it is important to apply the bleach solution only during the first two hours of the soak period, because bleach can damage germinated seed. In addition, he notes, bleach treatments in excess of two hours can lead to a significant loss in seedling vigor.

One of the difficulties in applying bleach seed treatments is disposing of the solution after soaking.

Oster suggests growers spread the discarded solution out on fallow ground to allow the bleach to dissipate.
Firoved adds that information on disposal will be provided on the pending label and growers are cautioned to read and follow label instructions carefully.

Cultural control
In addition to seed treatments, Oster says there are a few cultural practices growers can do to minimize the incidence and spread of Bakanae in the field, with an overall goal of keeping the incidence below 1 percent to prevent inoculum from spreading.

Oster says growers should avoid draining fields early, because the aerobic fungus will reproduce rapidly in the presence of oxygen. In addition, he says, soak and drain periods longer than the typical 24 hours each can increase Bakanae incidence in fields planted from that seed. Growers also should rid fields of straw residue because the fungus persists better in soil with a protective structure such as straw debris.

But the real solution will come in seed treatments, and researchers continue to explore options as they recommend the 5-percent bleach solution as a stop-gap measure. Oster says there are three or four compounds in the pipeline that have performed under controlled settings as a possible bleach replacement, and field trials will continue until they find optimum control measures for growers in the future.

Don’t be fooled by this disease Use cultural practices, seed treatment to reduce Bakanae in fields
By Marni Katz
Bakanae disease swept through California rice regions in 2002, causing significant yield losses in a few fields and invigorating efforts to find suitable seed treatments and recommendations to keep the relative newcomer from becoming more serious. Foolish seedling disease, as Bakanae is also known, is transmitted primarily by seed. It was first found in 1999 in Butte and Colusa counties and has since spread to every rice producing county in California. “Four years from the original find, about 80 percent of the fields [in California] now have some level of Bakanae in them, all from the seed,” says Jack Williams, University of California farm advisor in Sutter, Yuba, Placer and Sacramento counties. “I was very surprised and a bit stunned by the damage I saw this year in a very few fields. That doesn’t mean it’s an unmitigated disaster, but it’s a high level of concern and if we don’t clean it up in the seed it will cost us some yield losses.” Grower Walt Trethevan, who farms 500 acres of rice in Sutter County, says he has experienced Bakanae to some degree since 1999, and symptoms seem to increase slightly every year. But 2002 was the first season Trethevan experienced significant yield loss as a result of the disease. In 2002, Trethevan first started seeing symptoms in his most affected field about 30 days after planting with taller, elongated and slightly chlorotic plants. As the season progressed, he says, those plants didn’t stool as well as other and about a third of the heads in the field were empty of grain at harvest. Ultimately, he says, that 160-acre field produced 62 100-pound sacks dry compared to 85 to 95 sacks on neighboring fields. History of Bakanae 101 Foolish seedling disease was likely introduced to California through illegally imported infected seed. Most evidence points to Japan as the likely seed source because the country is one of the few known rice producers internationally to have the disease in its seed stock, and California growers cater heavily to consumers with Japanese-style varieties. Until 2002, yield losses had been minimal in California rice fields, and researchers and rice industry groups bought time studying possible chemical and cultural practices that could help stem the spread and potential field damage of the disease. But last season it hit some growers with a vengeance, causing an estimated 30-percent yield loss in some fields and creating a sense of urgency among the industry to find suitable solutions in advance of a potential outbreak. Caught by surprise California Extension rice specialist Jim Hill says when the disease was initially isolated in California, yield losses were not affected. With the high seeding rates used in California fields, healthy plants compensated to overcome the loss of infected seedlings. That was not the case last season. “Bakanae caught everyone by surprise in 2002,” says Hill, who’s based in Davis,Calif. “The disease in some fields persisted through heading, thus causing many completely blanked heads and reducing yields in some fields from over 8,000 pounds to less than 6,000 pounds per acre.” Hill says researchers have little understanding why some fields with infected seedlings went on to produce significant losses at harvest while others were able to compensate under the same seed and growing conditions and survive with normal yields. But because seed is the source, he says the growers’ first line of defense now is to buy from a supplier who can assure them the seed is relatively clean of Bakanae spores. From there, growers should consider treating the seed should a pending seed treatment become available in time for this season. Bleach seed treatment The industry is hoping the state will register common household bleach as a seed treatment by seed handlers and growers during the soak and drain period to reduce disease spores on seed prior to planting. Should the treatment become available, the California Cooperative Rice Research Foundation will be treating certified breeders seed at the Rice Experiment Station in Biggs, Calif., before that seed is released, says plant pathologist Jeff Oster. That treatment alone will not eliminate the presence of Bakanae in the lot, but can reduce spores by 80 to 90 percent. Oster for the past few years has been studying a number of fungicide and bleach treatments in an effort to find suitable seed treatments to protect growers against Bakanae disease. While several fungicides showed promise at the greenhouse level, Oster says those results did not pan out under field conditions. However field studies have shown that a solution of 3,000 parts per million (ppm) chlorine in soak water can significantly reduce Bakanae levels in seed. As a result, rice experts are recommending commercial seed companies and growers of seed rice in particular consider a preventative bleach treatment in the seed soaking process. Clorox Corp. is currently pursuing a label amendment that would add rice to the crops, along with asparagus, tomatoes and peppers, which are registered in California to use Clorox as a seed treatment. Roberta Firoved, manager of industry affairs for the California Rice Commission, which is supporting the label amendment, says registration is required for bleach to be used as an antimicrobial agent to treat seed. Registration is anticipated in time for the coming season, and if granted, would provide major relief for growers such as Trevethan. “I absolutely am going to do this bleach treatment if it’s available,” Trevethan says. “Early on I said that if we don’t have something, I’m not going to plant this year. With the cost of production, I’m not going to take a chance of losing a third of my production.” Trevethan is also hoping he will be permitted to burn the infected fields based on his levels of stem rot and sheath spot to remove overwintering Bakanae inoculum as well. “While seed is the primary source of inoculum, spores in straw residue may also contribute to disease levels, particularly in heavily infected fields,” Williams says. “Until we know otherwise, growers should burn these fields to destroy spores, consistent with current burning regulations.” Soak seed for first two hours As for the bleach treatment, if approved, researchers suggest growers soak seed for the first two hours of the typical 24-hour soak period in a solution of 5 percent Clorox Ultra, which contains 6 percent sodium hypochlorite, the active ingredient in bleach. That equates to about 1 gallon Clorox Ultra to 20 gallons of water. Soak seed for two hours in bleach solution, drain and replace the tank with fresh water for the remainder of the soak period, then drain and fly on seed as usual. Oster says it is important to apply the bleach solution only during the first two hours of the soak period, because bleach can damage germinated seed. In addition, he notes, bleach treatments in excess of two hours can lead to a significant loss in seedling vigor. One of the difficulties in applying bleach seed treatments is disposing of the solution after soaking. Oster suggests growers spread the discarded solution out on fallow ground to allow the bleach to dissipate. Firoved adds that information on disposal will be provided on the pending label and growers are cautioned to read and follow label instructions carefully. Cultural control In addition to seed treatments, Oster says there are a few cultural practices growers can do to minimize the incidence and spread of Bakanae in the field, with an overall goal of keeping the incidence below 1 percent to prevent inoculum from spreading. Oster says growers should avoid draining fields early, because the aerobic fungus will reproduce rapidly in the presence of oxygen. In addition, he says, soak and drain periods longer than the typical 24 hours each can increase Bakanae incidence in fields planted from that seed. Growers also should rid fields of straw residue because the fungus persists better in soil with a protective structure such as straw debris. But the real solution will come in seed treatments, and researchers continue to explore options as they recommend the 5-percent bleach solution as a stop-gap measure. Oster says there are three or four compounds in the pipeline that have performed under controlled settings as a possible bleach replacement, and field trials will continue until they find optimum control measures for growers in the future. Internet Hotlink For information on Bakanae in California, visit the University of California, Davis, rice page at http://agronomy.ucdavis.edu/uccerice/ For questions or comments, contact Rice Farming editor Vicky Boyd at (209) 571-0414 or vlboyd@att.net.
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