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.
#30
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