By Chris Crawford
Although citrus greening has reared its ugly head as the next crisis disease to hit Florida, growers and researchers say attention still must remain on the fight against citrus canker.
Once the hurricanes of 2004 and 2005 spread canker across the state, eradication no longer was an option. In January 2006, as the government officially put to sleep the Citrus Canker Eradication Program, the focus turned to disease management.
The Citrus Health Response Plan, developed by the U.S. Department of Agriculture Animal and Plant Health Inspection Service, Riverdale, Md., and the Florida Department of Agriculture and Consumer Services, Tallahassee, has become the go-to resource for best management practices for registration, inspection and certification of citrus. The plan directs growers to contact The University of Florida Institute of Food and Agricultural Sciences’ Research and Education Centers, or County Cooperative Extension Service offices for canker management strategies. Information also can be found online at http://edis.ifas.ufl.edu/CG040.
“We want growers to look at the best management practices,” says Tyrone Kemp, public affairs specialist for APHIS. “We encourage them to practice decontamination and grove surveys. The USDA encourages this so that groves can have the largest market for their produce, including Europe, Japan, Korea, etc.”
BMPs for canker control start with planting windbreaks, protecting fruit and leaves with copper sprays and controlling leafminer, according to the “2007 Florida Citrus Pest Management Guide: Citrus Canker.”
The latest canker management developments include:
- Research being conducted on transgenic plants that exhibit canker resistance;
- Windbreak research continuing to evolve as test trees grow and are evaluated in the field and on-site at commercial operations;
- Work with the antibiotic streptomycin being evaluated as an alternative method to copper spraying for canker control;
- And expansion of leafminer control resulting from increased use of Admire to kill multiple pests.
Starting Aug. 1, 2006, after the burden of the eradication program had lifted, growers, packers and shippers had to deal with the Federal Citrus Canker Quarantine of all fresh citrus shipments leaving Florida. Shipments are banned to all U.S. citrus-producing areas such as American Samoa, Arizona, California, Guam, Hawaii, Louisiana, Northern Mariana Islands, Puerto Rico, Texas or the U.S. Virgin Islands.
In addition, Florida citrus shipped to states not producing citrus is required to originate in groves inspected and deemed canker-free no more than 30 days before harvest commences.
But on June 21, APHIS proposed amending the quarantine regulations to eliminate those pre-harvest grove inspections. Instead, absence of disease would be verified by mandatory sampling of each lot of citrus at packinghouses.
APHIS proposed this action because a completed risk management analysis concluded that mandatory packinghouse inspections of commercially packed fruit would provide an effective safeguard in preventing canker spread, according to an APHIS news release.
The proposed rule would continue to prohibit shipments of Florida citrus to citrus-producing states and territories.
The quarantine is unlikely to affect the chances of citrus-producing areas acquiring canker from Florida citrus because the chance of it happening in the first place was low, says Raymond Royce, executive director of the Highlands County Citrus Growers Association Inc., Sebring
As greening has commanded more attention from growers, it also has stolen much of the focus of research. But transgenic plants still are being developed and tested for resistance to canker.
“Transgenic work has shifted more over to greening since it’s a way bigger problem,” says Jude Grosser, professor of plant cell genetics at the Citrus Research and Education Center, Lake Alfred.
Current transgenic research on canker resistance at the center includes post-doctoral research associate Ahmad Omar’s work on Hamlin oranges.
“He transferred a Xanthomonas resistance gene from rice into a Hamlin sweet orange,” Grosser says. “We don’t know (yet) if the genetically engineered plants are resistant, as they are currently being challenged in the quarantine facility in Gainesville. We are also working with several other antibacterial genes in our genetic engineering work.”
The center in Lake Alfred also plans on setting up a similar quarantine facility in the next couple months to test transgenic plants’ resistances to greening and canker, he says. The center’s work with transgenic citrus now is in its third year.
Also being tested is the transfer of canker resistance from kumquats, by conventional breeding, into seedless triploid hybrids. Doing so is useful for breeding canker-resistant limes and possibly other types of citrus such as lemons or mandarins, Grosser says.
“A few of these hybrids flowered this year, and we should get a first look at some fruit—if we’re lucky,” he says. “We have also identified a canker-tolerant pummelo that resembles a grapefruit in size, shape and color, so this approach is also being pursued for grapefruit breeding—i.e., producing seedless triploid hybrids with red flesh, good flavor, and canker tolerance. Hundreds of such triploids have been produced already.
“Variety, improvement and resistant plants are the ultimate answers.”
Just like the trees, windbreak research continues to grow. Pete Timmer, professor of plant pathology at the Lake Alfred center, visited a Vero Beach site in May that was using bamboo windbreaks.
“Bamboo is doing well, but it’s still expensive,” he says.
Timmer says it’s hard to establish tests on windbreaks because you have to plant huge plots. “Windbreaks have an effect on wind speed for about 10 times their height,” he says. “Thus a 30-foot tall windbreak will have an effect for 300 feet. So you probably need several acres for each plot and maybe as much as 50 to 100 acres of uniform grove to set up a single test, depending on the number of plant species to be evaluated. Not impossible, but not your backyard test, either.”
IFAS is studying different trees in various locations around the state to see how they perform. The Citrus Research and Education Center has compiled an extensive windbreak resource online at www.crec.ifas.ufl.edu/extension/windbreaks/.
The USDA’s Florida Natural Resources Conservation Service in Gainesville continues to offer a cost-share program for farms looking to implement windbreaks to control windborne diseases. The Florida office is working with the national NRCS office to provide up to 75 percent cost share for windbreaks, as part of the Environmental Quality Incentive Program.
The cost share will help pay for tree purchases, installation and upkeep, says Bob Stobaugh, public affairs specialist for the Florida NRCS office. Visit www.fl.nrcs.usda.gov for more information.
Because copper sprays remain an effective canker control method, the Citrus Research and Education Center continues to test newer copper options.
Timmer says there hasn’t been a huge difference between copper spray brands.
But DuPont Kocide 3000 is a newer popular fungicide/bactericide with a high level of active copper.
“Kocide 3000 has a better efficacy per pound per copper,” Timmer says. “It has more effect with smaller amounts.”
The newest brand the center is testing is Badge SC manufactured by Isagoro USA Inc., Morrisville, N.C.
Although it’s not as effective as spraying copper, researchers continue to study the antibiotic streptomycin as an earth-friendly alternative.
Copper spraying can lead to phytotoxicity with copper damage of fruit or copper build up in the soil, Timmer says.
Streptomycin has been around a long time but has never been registered in Florida.
Jim Graham, soil microbiologist at the Citrus Research and Education Center, is conducting field work on the antibiotic, including developing programs for application and treatment to specific citrus varieties, Timmer says.
Leafminers don’t spread canker, but a serious invasion of leafminer tunnels by the bacterium greatly increases inoculum levels, making the disease hard to control, according to the “2007 Florida Citrus Pest Management Guide: Citrus Canker.”
Graham has been working on leafminer control in Argentina, Timmer says.
Properly-timed applications of petroleum oil, Agri-mek (abamectin), Micromite (diflubenzuron), SpinTor (spinosad) or Assail (acetamiprid) will reduce damage by leafminer, according to the guide.
“By itself, it (leafminer control) does a lot of good,” he says. “Used with copper sprays, it doesn’t help as much. If you’re on a good copper program already, leaf miner control doesn’t add much to the level of canker control. Once you reach a high level of control, it is hard to improve it.”
As growers look to fight canker and citrus greening, one chemical kills two bugs with one stone.
“What’s going to help is that a lot of growers are using Admire to fight psyllids and also leafminer control.”