More likely, he said, it will be a combination of solutions. Those could include breeding trees immune or resistant to the huanglonbing bacteria – commonly known as “HLB” – as well as finding ways to reduce populations of Asian citrus psyllids, the gnat-sized insects that spread the disease from one infected citrus tree to another.
“One important component going on now is to try to slow the spread of the insects,” through agricultural quarantines and spraying, said Roose, a professor of genetics at the university’s Botany and Plant Sciences Department, where at least 15 research projects are underway to deal with California’s HLB and psyllid problems.
Unfortunately, since the psyllids were found there in 2008, they’ve bred and migrated throughout Southern California, prompting efforts by the citrus industry and state agricultural officials to focus heavily preventing their migration north into California’s primary commercial citrus-growing region.
Despite those efforts, psyllids are being found with increasing frequency in the Central Valley and Central Coast, including numerous finds in Tulare County – California’s top commercial citrus-growing region – that has prompted a county-wide quarantine restricting the movement of commercial and home-grown oranges, lemons and other citrus, as well as citrus trees.
No more of the insects have been found outside the city or in commercial lemon groves south of Salinas.
So far, only a few HLB-infected trees have been found in the state, within residential neighborhoods in and near San Gabriel, but growers and others in the citrus industry worry that if more diseased trees are found, the psyllids could spread the bacteria to commercial groves at a rate that could threaten the state’s $2 billion citrus industry, as there’s no cure for HLB.
Once infected, trees produce mottled, bitter fruit and die. It’s a threat so great that the industry-funded California Citrus Research Board has called HLB an “industry killer.” HLB can ruin citrus production here and wipe out jobs, as it has in Florida, where it is found in most groves, and where commercial citrus losses have totaled more than $7.8 billion since 2007.
To put the damage in perspective, Florida produced about 291.8 million boxes of citrus in the 2003-04 growing season. But the U.S. Department of Agriculture’s production estimate for the 2015-16 season is just 69 million boxes – 3.11 million tons.
With so much at stake, California citrus growers have put up $15 million a year for the past seven by self taxing the fruit they sell to pay for HLB-related research and efforts to educate both commercial and home growers about the threat of the bacteria.
On top of that, the federal government has allocated an average of $35 million over each of those seven years to fund research in the nation’s citrus-producing areas. At least $10 million goes to California researchers each of those years, said Joel Nelsen, president and CEO of California Citrus Mutual, a trade association representing about 2,200 citrus growers.
So the battle against HLB and Asian citrus psyllids is being fought on many fronts, with researchers in Riverside and elsewhere in California and the world on different aspects of the disease threat.
For example, last month, the Cal Poly Pomona College of Agriculture finished construction on a 5,040-square-foot expansion of its research and insect-production greenhouse to breed Tamarixia radiata, a tiny wasp —not big enough to sting humans — native to Pakistan that has been imported to the U.S. because it attacks psyllid nymphs.
Roose said the wasps already released are reducing psyllid numbers in residential areas of Southern California. But, Roose said, but the beneficial insects, spraying and improved insect traps aren’t going to eradicate psyllids because they are far too plentiful and established in SoCal.
Instead, the goals of researchers is to reduce their numbers and to slow their progression into commercial citrus areas.
“They’re trying to buy time so other solutions can be developed,” he said.
In California, much of those efforts are focused on finding better ways to detect infected citrus trees, Nelsen said.
Roose noted that not all branches and leaves of infected trees test positive for the bacteria, at least in the early stages, so the disease can be missed.
Riverside scientists are working to identify protein molecules that would exist in an entire tree if it were infected and other factors that could be detected through chemical tests or through smell, using mechanical smellers or specially trained dogs.
“Of course, that doesn’t solve the problem,” said Roose, explaining that improved early detection of HLB can help contain the spread of HLB, but “after that, the next level is to develop mechanisms to cure the trees or make trees resistant.”
Georgios Vidalakis, a plant pathologist and director of the Citrus Clonal Protection Program in Riverside, said the plan is to remove cells from up to 50 varieties of citrus from Florida to remove any HLB bacteria and then use material that’s left to grow new, disease-free plants.
“We want to create mother plants for bud wood and see how they grow in California,” Vidalakis said.
He said, though, that a Mandarin or other citrus tree tree from Florida might produce fruit with an unpleasant taste when grown in soil here.
The goal is to have resistant trees ready to sell to growers as soon as possible, though between the testing, determining which varieties actually are HLB resistant and commercially viable in California groves and then growing the the trees will take years, Vidalakis said.
And at UC Riverside, the research is being hampered by the fact that scientists here can’t work directly with HLB bacteria, as they can in Florida. That’s because “Pretty much every grove in Florida is infected,” but here in California there are concerns the bacteria could somehow spread outside a lab and infect trees here.
That’s why the citrus industry is funding the construction of a biosafety level-three lab and greenhouse on the Riverside campus. Once finished, some time next year, researchers will be able to work directly with HLB and infected material there.
The new lab also will be important because if a cure is developed elsewherem, Roose said, that doesn’t mean the cure will work anywhere else. It will have to be tested here.