California’s Citrus Scourge
May 1, 2013 
It was always just a matter of time until the news came that Huanglongbing (HLB), the deadly and — for now, anyway — incurable disease of citrus, was discovered in California. After all, it had ravaged Florida and then moved to the citrus-growing districts of Texas and Mexico. Carried by an innocuous looking pest just the size of an aphid, the Asian Citrus Psyllid (ACP), the disease spread inexorably West.
News of the spread of ACP was disconcerting to citrus experts like IPM specialist and research entomologist Beth Grafton-Cardwell of University of California (UC)-Riverside, because she knew a dirty little secret about HLB. It was likely already in California, imported on budwood by people who simply didn’t know any better, and grafted onto backyard citrus trees. “That’s how we think it got to Florida, and that’s what is probably happening in Los Angeles,” she says. “It’s just ignorance, people not realizing they are bringing in diseased plant material.”
That meant there were, as Grafton-Cardwell calls them, “ticking time bombs” in some of the state’s backyards. Finally, in March 2012, one of the bombs went off when HLB was detected in a residential tree in the city of Hacienda Heights in Los Angeles County. That tree was carefully removed and destroyed, and every citrus tree within an 800-meter radius was sprayed with pesticides.
The California Department of Food and Agriculture (CDFA) continues to sample citrus trees within a half-mile of that Hacienda Heights tree, and one year later all samples continue to turn up negative. But there are likely other trees with HLB, so now the task is to keep both the disease and its vector, ACP, out of the state’s commercial groves.
Unfortunately, ACP has already established in some of the commercial groves of Riverside County. Now the fight is on to keep it out of the coastal Ventura area, long known for its excellent lemon production, and especially the San Joaquin Valley, where 75% of the state’s citrus fruit is grown. These two areas have had some finds, though they have been isolated. Grafton-Cardwell also serves as director of the Lindcove Research and Extension Center, which is located in the heart of the San Joaquin Valley Citrus Belt that runs roughly along the base of the Sierra Nevada foothills. She graciously provided Western Fruit Grower with a publication describing UC research on ACP/HLB and then sat down to answer some questions.
The University of California has a five-pronged approach to ACP/HLB: Ensuring clean plant material; Managing the psyllid; Detecting HLB-infected trees; Finding long-term solutions; and Engaging and educating the public. Regarding that first angle, ensuring clean plant material, have any HLB-infected citrus trees been planted in any commercial groves in California?
“No, not that we know of, growers fund a comprehensive citrus protection program to test budwood,” she says. “All citrus budwood has been tested by CDFA or the Citrus Clonal Protection Program and is disease-free.”
The UC Citrus Clonal Protection Program, directed by Georgios Vidalakis, is housed at UC-Riverside, and is considered the gatekeeper of California citrus. In addition, two other scientists at UC-Riverside, Tracy Kahn and David Karp, are developing a legal source of plant material for popular non-citrus ACP hosts, such as the bael tree, a native food plant of India. Hopefully, providing people with clean plants will reduce the incentive for smuggling plants and plant material into California. In Florida it was an ornamental ACP host popular with residential landscapers, orange jasmine, that is believed responsible for spreading HLB so quickly and so widely. To prevent such a problem happening in California, all such ornamentals are sprayed with an insecticide, says Grafton-Cardwell.
“Retailers can’t treat with pesticides, so the state requires wholesale nurseries to do it. So even if psyllid lands on a plant before the plant is sold, it dies,” she says. “It’s a way of protecting the state of California; all wholesale plant material — before it is shipped to retail nurseries — is treated.”
Researchers are also working on the possibility of developing a repellent that could be sprayed on trees to keep ACP away. I’ve never heard of such an approach. Has it been used to repel other pests?
“It’s unusual, and certainly not commonly used in agriculture. For one thing it’s tough to get the concentration of the repellent high enough. Also, it’s expensive to create, but this is such a serious situation it’s being explored as one of the things in the toolbox that helps,” she says, adding that on the other side of the coin, they are also looking at attractants so an attract-and-kill approach can be employed. “You can run odors by the pest to see what excites it. It’s a little electrode, really little, that is attached to the insect. It is really cool new technology.”
What about the treatment plan for organic groves?
“Oil works pretty well because it kills ACP on contact, but it doesn’t last long so you have to spray constantly,” she says. “To keep numbers as low as possible, it’s an option. Again it’s one of the tools. It will reduce the numbers (of ACP) but it won’t stop the spread of the disease.”
All groves within 800 meters of a psyllid find are treated — all blocks need to be sprayed because you can’t assume they are just in the blocks where you have psyllid finds. “Growers don’t know when the disease will hit. They have to assume it will hit eventually so they must spray to reduce the psyllids so they can’t find infected trees and spread the disease,” she says. “So far only one infected tree has been confirmed in California, but it could be spreading and we can’t see symptoms for many months.”
What about biocontrol? I was surprised to see the UC publication state that biocontrol won’t eradicate ACP.
“Biocontrol always lags behind the target pest’s numbers,” she says. “It can help limit the psyllids, but it can’t eliminate them.”
Mark Hoddle, a UC Cooperative Extension entomologist at UC-Riverside, collected two natural enemies of ACP in Pakistan. The two tiny wasps, Tamarixia radiata and Diaphorencyrtus aligarhensis, lay eggs underneath or inside ACP nymphs; the hatching larvae eat the nymphs, killing them. Hoddle released T. radiata in Southern California in 2012 and is monitoring its establishment and distribution. And while the natural enemies won’t eradicate ACP, scientists hope they will reduce densities of the pest in urban areas, giving other control practices a better chance of working.
There are an awful lot of scientists working on this effort. Any idea how many there are?
“I don’t have a number, but there are dozens and dozens of plant breeders, plant pathologists, entomologists, molecular biologists — you name it — everyone is working on this,” she says. “Of course there are citrus industry and federal funds available because this is such a serious problem, and that helps.”
HLB doesn’t spread through a tree quickly. Why is that?
“It’s just the way the disease is. The growers keep treating for ACP in Florida even though many of the trees are infected because one localized infection is not as bad as several throughout the tree,” she says. “It’s currently incurable, so instead of getting rid of the disease we may be looking at protectants. I’m not sure if we’ll ever find a curative treatment. What we’re probably looking for is a protectant so the disease never gets in there to begin with.”
But that also means it can be hard to detect, correct?
“Yes, with the current methods of testing, if the bacterium isn’t in the part of the tree that is tested, you miss it,” she says, adding that’s why researchers are investigating whether HLB changes the tree’s metabolism. “If you can test for something that affects the whole tree, that can be detected sooner and easier because you can take a sample anywhere on the tree. Hopefully it’s an earlier detection technique.”
Doesn’t the eventual answer lie in genetically modifying trees to resist the disease?
“Not necessarily, the answer might lie in modifying the insect, not the plant,” she says. “Perhaps we can prevent the insect from putting its mouth part in the plant so it can’t feed and it can’t spread the material. The answer may be with the bug, not the plant. Or perhaps we can purposefully infect the trees with a mild virus carrying a gene that resists HLB. So instead of developing new trees, we can protect existing trees.”
How important is it to educate the general public about what a threat ACP/HLB presents?
“It’s absolutely essential. For one thing, there are more citrus trees in the states’ backyards than are grown commercially. About 60% of backyards have citrus trees,” she says. “This disease will change the landscape of California. It’s not just a commercial citrus problem. So it’s very, very important that the general public participates.”
How can the general public help detect ACP if the pest is so tiny?
“The pests are attracted to the new flush of growth, the tiny new leaves,” she says. “We recommend everyone examine flush and report the psyllid, if they find it, to their local Ag Commissioner. We also recommend that PCAs do tap sampling with a white sheet under the branch. You can see the adult psyllids with the naked eye. They are small, but you can see them.”
The main booklet issued by the UC says “when HLB becomes established in California,” not “if.” You have no doubt that will occur? If so, when?
“I have absolutely no doubt. It’s now made it to Texas and Mexico, and we’re next,” she says. “But there is no time line. It took three years to spread throughout Florida, but they didn’t do anything about the psyllid and didn’t know they had the disease at first. We’ve had the psyllid since 2008. So we’ve already slowed the spread for five years. I think the disease will likely start showing up as early as this coming fall.”
So now we are just buying time?
“Yes, now we’re just buying time for the researchers to develop preventatives or cures for the disease,” she says.
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