citrus

published on January 5, 2018 - 12:56 PM
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UC RIVERSIDE — Normally, Mikeal Roose would spend his days with fellow researchers and students working to develop new varieties of citrus.

But things changed for the geneticist and other scientists working on citrus and insect control after the first Asian citrus psyllid was found in August 2008 in Southern California.

After that, Roose and many other researchers in academia and the private sector were sort of drafted, many setting aside their normal research to either try to find a cure or an inoculant against huanglongbing, a bacteria that infects and kills citrus trees, or to at least slow its spread through the state.

Disease is nothing new for farmers, but this one — commonly known as “HLB” — has got commercial citrus growers scared, as there is no cure for infected trees nor a way to inoculate them from the bacteria spread by only one insect, the citrus psyllid.

“It’s frustrating, because we have spent — the U.S. citrus industry — well over $150 million in research, and we’re finding out what doesn’t work, but we haven’t found out anything as far as a cure is concerned,” said Joel Nelsen, president of California Citrus Mutual, an Exeter-based trade group for California citrus growers.

“HLB is such a problem that there’s a lot of money to work on these things, and there are a lot of different labs doing it,” said Roose, adding that the work looks to attack a variety of problems, from finding a cure for HLB to more effectively slowing or stopping the spread of Asian citrus psyllids, which have been found across Southern California and in lesser numbers in the Central Valley — the state’s primary citrus-growing region — and the Bay Area.

Just in terms of detection, researchers are trying to find ways to identify HLB-infected trees earlier — as some trees infected recently would register negative under current testing methods — and new methods of detection are being researched, from electronic devices to training dogs to sniff out diseased trees, Roose said.

In fact, one of the U.S. Department of Agriculture research projects he’s assisting on is to develop of a trap installed in trees that would encase psyllids in antifreeze, so they don’t dry up and end up so desiccated that researchers can’t determine if they’ve been infected with HLB, a problem with most of the insects caught in sticky traps placed on citrus trees.

“The problem is, we currently don’t have a way to attract the insects, nor does the sticky trap,” said Roose, adding that researchers at UC Davis may be on the verge of solving that problem by identifying a psyllid sex pheromone that may become the needed attractant.

But something that can cure infected trees or at least make them more resistant to HLB remains the Holy Grail for the citrus industry.

A major area of that research involves looking at citrus varieties that are resistant to HLB.

“One major approach is we want to understand the genetics. How is it inherited? We also want to locate what causes it — the resistance. There may be more than one gene. We don’t know that yet,” said Roose, adding that many of those cousins to U.S. commercial citrus grow in Australia, though some would hardly be recognized as an orange, lime, etc.

“This would be more like the difference between humans and gorillas.”

The next step would be to breed the attributes that make trees immune or resistant to HLB with trees that produce commercial varieties or use genetic engineering to mix the desired attributes.

One problem with breeding is that it can take 5-7 years, so researchers are looking at ways to speed that process, Roose said.

Another approach being studied is the use of other viruses that affect citrus but are relatively harmless to the trees and adding to them a “payload,” a molecule designed to attack and kill the HLB virus, he said, adding that tests on this method are underway in Florida.

“There’s a new technology, gene editing, in which the gene causing the resistance has a close relative in [commercial] citrus, but the gene is not turned on at the right time in order to cause resistance [to HLB], or the gene has a variation that makes it ineffective,” so it may be possible to develop seeds for existing varieties of commercial citrus with disease-resistant genes essentially turned on, Roose said.

Gene-editing techniques currently being studied to prevent ailments in humans could have applications in fighting HLB in citrus, he added.

Indeed, Nelsen said Citrus Mutual may ask researchers in Kansas to consider applying to do research at a new lab for HLB testing being built near UC Riverside to see if their work in fighting cancer could be applied to impede the progress of HLB bacteria in infected trees.

For his part, Roose said the research into HLB has allowed agricultural scientists to delve into areas of research and techniques they might not have without the funding and focus that has been put into fighting the disease and its spread.

“If they were just looking for ways to improve fruit in general, the money would be harder to get and not nearly as many people would be working on it,” he said.

The work is exciting, “but this isn’t what I wanted to do. It’s somewhat of a stretch for me in some ways, because it’s not a particular field that I wanted to be as focused on historically, as I have to be now,” Roose said.

Still, he said he feels a strong obligation to solve the HLB problem or at least contribute to a solution. He feels as if he’s on the front lines with other researchers fighting off a major threat.

“If I and the scientists don’t address this problem, it won’t get solved, and citrus will disappear in California.”


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