CLERMONT, Fla. Oct. 14, 2015 - Gradually in a fistful of ways, Florida growers and citrus researchers are beating a path back from the onslaught of huanglongbing (HLB), often called citrus greening, that began decimating the state’s groves a decade ago.

In 10 years, HLB has spread to all commercial orchards there, and two-thirds of citrus farmers estimate that at least 80 percent of their orange trees are infected, the University of Florida (UF) reports. The bacterial disease, spread by citrus psyllids, clogs the vessels in citrus tree roots and crown, squeezing life from the trees. The invasion has reduced Florida’s grapefruit harvest by more than a third, and its orange crop by nearly half, since those of 2005-06. The greatest impact overall is juice orange trees, and 95 percent Florida orange trees are for juice – most of them Valencias, a variety highly vulnerable to greening. And while yields have tanked, “our overall costs of production have more than doubled,” with the soaring expense of treating trees or replacing them, and fighting psyllids and the bacteria, says Harold Browning of Florida’s Citrus Research and Development Foundation.

“But even though we’re really struggling here, the outlook is improving,” says Browning, who reports that some strains of citrus that can tolerate HLB are emerging.

Uncle Matt’s Organic, a family citrus company west of Orlando, has a 30-acre grove of orange trees planted back in 1944. The tangerine-like Temple oranges there are sweet, but seedy and not especially popular, in part because they have a brief shelf life. But the trees sport a spectacular trait: a hardy resistance to HLB. While an assortment of citrus varieties tolerate greening at various levels – that is, they’re infected but producing fruit – the Temple seems to have held out all HLB invasion even though the psyllids have carried the disease throughout the Lake Louisa State Park area around the Uncle Matt’s grove.

In fact, Ben McLean, who heads up Uncle Matt’s research and development, says he and an entomologist have tested some of the trees by enclosing them in a screened tent with loads of infectious psyllids for six months, and the trees still test negative for the bacteria. The discovery is as sweet as, well, a Temple orange to McLean because organic farmers can’t use chemicals to kill either the psyllids or HLB germs, and a resistant variety is a clear way around the pathogen.

There’s more good news in a 2015 UF tangerine release, developed by breeder Fred Gmitter and so far just called 7-6-27. It may prove fully resistant to citrus greening, and so far the tasty citrus is proving very popular with growers. “I’ve heard there are 70,000 to 100,000 trees being placed on order” from farmers, Gmitter said.

Jude Grosser, a UF citrus genetics expert, says selections of

HLB-tolerant citrus are growing. Though grapefruit are especially susceptible, a few strains of pomelo are highly tolerant. As a result, pomelos, a grapefruit ancestor that’s sweeter and larger than grapefruit, may enjoy increased popularity. Grosser points to a delicious piece of fruit nicknamed Monster for its bowling-ball size. Breeders will need to shrink it before it’s market-ready because Monster is too large for packing house equipment to handle, he says.

Meanwhile, the Sugar Belle mandarin orange (this one a tangerine-pomelo cross called a tangelo), released by UF back in 2009, has proven moderately tolerant and increasingly popular with growers. Grosser says the first release flopped because citrus greening was raging across Florida at the time, and growers feared planting any new trees. Now that the variety has proven itself, he says, greenhouses are rushing to supply enough Sugar Belle seedlings.

Fort Pierce grower Pete Spyke was raising a variety of Sugar Belle more than a decade ago, years before UF released its strain. He has learned that his strains of Temple, Navel and Page oranges can survive greening. “My personal choice is to plant the varieties that are tolerant, and Temple is at the top of the list,” he says.

But fighting HLB takes much more than improving genetics. Thorough nutrition and good orchard husbandry is proving to be at least as important as varietal choice in keeping citrus trees thriving and productive. Spyke reports: “We're just fertilizing with slow release fertilizer, drip irrigating, and occasionally spraying them with a nutritional spray. Just controlling stress, really. No psyllid insecticides or extreme measures. These trees looked terrible two years ago, and most people would have pushed them out. I kept them, though, and just took good care of them.”

Browning says a lot of growers are moving in Spyke’s direction. With HLB, “you have to provide nutrition in a different way than if the tree were healthy,” he says, because greening chokes the vessels of both the roots and scion (trunk and crown). Hence, drip irrigation can be switched on for short periods three or four times daily, spoon feeding the water and fertilizer at a slow pace that the compromised tree can absorb. That practice also helps manage fertilizer costs. “The enhanced nutrition is not a treatment against the pathogen, but is a way for the tree to remain healthy in the presence of the disease,” he says.

Grosser notes that research has found low doses of several essential nutrients – boron, manganese, calcium, iron and zinc – are also helping HLB-sickened trees to thrive.

A host of other tricks to combat greening have evolved. For example, to knock down most of the greening pathogen for a few months, putting snug plastic covering over a tree for a few days lets the sun bake the bacteria out. A speedier version of that is a blast of steam in the tent for a minute or less. Delivered correctly, the heat actually stimulates a growth spurt in the tree.


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