The use of copper to control citrus canker has exploded since 1995.
The use of copper to control citrus canker has exploded since 1995.

For decades, Florida growers have found copper-based products to be an effective means of controlling a plethora of citrus-related diseases.

But the use of copper-based products has exploded since 1995, when growers turned to the metallic element to fight the modern menace of citrus canker.

Now, producers are growing increasingly anxious about the effect copper buildup may have on their groves. And they’re increasingly afraid that the day may come when pathogens display copper resistance.

That’s why plant pathologists like Pam Roberts and Jim Graham are working to uncover alternatives to the popular go-to product whose days may be numbered.

Program approach

Roberts, professor of plant pathology with the University of Florida Institute of Food and Agricultural Sciences’ Southwest Florida Research and Education Center at Immokalee, is taking a program approach to fighting canker that combines copper with other disease-fighting tools.

So far, she hasn’t found anything more effective than copper to control bacterial diseases, such as citrus canker. But there are products that she says work just as well.

Some of the tools that look promising include Serenade Max, manufactured by Davis, Calif.-based AgraQuest Inc.; Regalia from Marrone Bio Innovations Inc. of Davis; and Actinovate SP from Natural Industries Inc. of Houston. All currently are registered for use on Florida citrus.

She also has tested Actigard 50WG from Syngenta Crop Protection LLC of Greensboro, N.C., which is not yet registered for use on Florida citrus.

“There isn’t anything that I would say to spray instead of copper,” she says. “But I see potential in using some of these products.”

This has not been a good year for Roberts’ trials because of unusually high citrus canker pressure in many parts of the state that no product or combination of products can manage.

“It’s just been out of control,” she says.

Roberts says she’s hopeful that the outbreak this season is the result of a heavy rainstorm in March before growers had a chance to spray for canker and is not an indication of things to come.

“Everyone is pretty dismayed by the severity of canker this year,” she says.

More alternatives

Jim Graham, professor of plant pathology at IFAS’ Citrus Research and Education Center in Lake Alfred, also has been working on copper alternatives for canker.

One compound on which he has conducted extensive research is streptomycin, the active ingredient in Firewall from AgroSource Inc. of Mountainside, N.J. It is labeled to fight fire blight on pears and apples.

Graham has been testing the product since 2006 on fresh-market grapefruit, which is most susceptible to citrus canker.

“Every season that we’ve tested it, we’ve been able to demonstrate disease-control activity that replaces or complements the activity of copper,” he says.

Streptomycin is not phytotoxic, nor does it produce side effects or fruit blemishes.

It can be sprayed in place of copper or at a full rate with half the recommended rate of copper.

Using streptomycin as a rotational partner to help combat bacterial resistance to copper can actually promote resistance to streptomycin itself.

Roberts has concerns about Firewall, since pathogens can quickly develop resistance to streptomycin.

“I hope it works,” she says. “People are really going to have to be careful and use it according to the label.”

Resistance concern

Graham is studying the resistance concern, but he emphasizes that streptomycin is only applied twice—in August—in a very targeted way. And it should be tankmixed with copper products.

Growers will want to limit their use of streptomycin anyway, since it costs about three times as much as copper, Graham says. It can be applied by an airblast sprayer.

Before growers can use streptomycin to control canker, the Environmental Protection Agency must register it for that purpose.

A registration application showing that streptomycin meets residue tolerances and an economic justification were submitted in September 2011.

Growers concerned

Cody Estes, president of Estes Citrus Inc. in Vero Beach, is one of several growers who wrote letters to EPA encouraging the agency to approve streptomycin to fight citrus canker.

Graham is conducting trials on some of the company’s grapefruit blocks.

The company manages almost 2,000 acres in three counties and has been using copper in some locations since the 1920s and in other groves since the 1960s.

“It’s extremely important to have an alternative to copper,” Estes says.

He has planted more than 30 miles of windbreaks to help combat canker, and he’s looking to keep costs down on sprays.

“Cost matters a lot,” Estes says, adding that he now spends more on spraying than he used to spend on his entire production program.

Novel uses

Graham also has found a way to control canker through what he says is a highly novel use of soil-applied neonicotinoid insecticides, which are used to manage Asian citrus psyllid on non-bearing trees. Neonicotinoids are potent inducers of systemic acquired resistance in plants.

Graham has shown that these soil-applied insecticides not only control insects, but also control canker by inducing systemic acquired resistance in the leaves.

It’s a long-lasting application. One dose in April provides SAR activity until the fall.

Two such neonicotinoids registered for use on citrus in Florida are imidacloprid (Admire Pro) and thiamethoxam (Platinum 75 SG).

Actigard 50WG, which is used as a spray on tomatoes and peppers to help combat bacterial diseases, also induces systemic acquired resistance as a soil-applied treatment, he says.

“Soil drenches are a novel method of application for Actigard,” Graham says. “When used as a foliar spray, it lasts two weeks. But when soil applied, it provides activity for 30 to 60 days.”

Actigard is not yet registered for use on citrus, but a request has been made to register it for use on trees less than 3 years old or less than 6 feet tall.