Before the 1960s, there were no antibiotics to help orchardists control fireblight, one of the most destructive diseases of pears and apples. Copper materials were the mainstay back then.

But coppers are returning as fireblight control products, particularly for organic orchardists. Oxytetracycline, an effective antibiotic, could lose its place on the list of acceptable organic products, a list compiled by the National Organic Standards Board. The NOSB will decide this spring if its organic use will be extended beyond October 2014.

The coppers of today are improved from the old formulations, says Dr. Ken Johnson, Oregon State University plant pathologist. Companies have made coppers safer on fruit—not as harsh on fruit finishes—by solubilizing the mineral and reducing the percentage of metallic ­coppers in comparison to the metallic amounts of fixed ­copper products.

The new soluble coppers have a low phytotoxicity to the fruit and tree, are tightly absorbed by the waxy plant layers, and can be used at petal fall to give a long, seven- to ten-day residual.

One soluble copper product currently available is ­Phyton 27AG, which is registered by Phyton Corporation. Phyton 27Ag is labeled for a variety of crops, from tomatoes and vegetables to stone fruit, pome fruit, and grapes. Johnson doesn’t know if the material has been accepted as organic. Additionally, more research is needed on pear and apple fruit finish.

A new copper product coming soon is Previsto from Gowan Company. Registration is expected in 2013 and components of Previsto have already been accepted as organic, Johnson said, adding that extensive testing has resulted in no problems on fruit finish.

Lime sulfur thinning

There’s also good news for apple growers using lime sulfur routinely as a thinning agent. “Lime sulfur does help with fireblight control, and is actually a bactericide,” Johnson said. The thinning effect of fewer flowers also helps in the fight against fireblight because there are fewer flowers for the disease to infect.

“That’s why it’s much easier to achieve nonantibiotic control in apples than pears—because of the routine thinning of apples with lime sulfur,” he said.

But there is concern that lime sulfur can affect the efficacy of biological agents, like Bloomtime (Pantoea agglomerans strain E325, Northwest Agri Products), BlightBan A506 (Pseudomonas fluorescens strain A506, Nufarm Americas, Inc.) and Blossom-Protect (Aureobasidium pullulans, Westbridge Agricultural Products). If lime sulfur applications are followed immediately with biologicals, he believes there is reduced colonization of the ­biological agent.

Johnson recommends that orchardists do their thinning with lime sulfur, then wait until about 70 percent bloom to follow with other fireblight control treatments. He also suggests delaying application of biologicals until after the second lime sulfur thinning application.

Research shows that integrating both biological agents and antibiotics for fireblight can result in better control than using each alone.

“We got better control using two applications of biologicals and two antibiotics,” said Johnson. “The biological allows beneficial bacteria to colonize the stigma and protect from the fireblight. Then, the antibiotic leaves a residue in the floral cup to prevent further infection.”

Serenade Max (Bacillus subtilis), now owned by Bayer Crop ­Science, is short lived and lasts only 1.5 to 2 days, but it does provide control, he said.

After four years of studying an integrated approach on Gala and Golden Delicious apples, Johnson found that lime sulfur and fish oil, plus two applications of the biological Blossom-Protect, provided equally good control as the streptomycin and biological Bloomtime treatment. The Blossom-Protect yeast has been a consistent performer, colonizing the flower with yeast as a biofilm that makes it hard for the fireblight bacteria to grow, he said.

In comparing the economics of an organic and conventional fireblight control program, he noted that the organic costs are higher because Blossom-Protect costs about 2.5 times more than ­streptomycin.

“The economics of organic production must factor any grade loss for fruit quality. There’s still a question about fruit finish and quality when using the organic products,” he said.

Johnson presented his talk at the Washington State Horticultural Association’s annual meeting.