Scientists, growers refine nutrient blends for HLB-infected trees

04/11/2013 03:10:00 PM
Renee Stern

While the citrus industry seeks better solutions to huanglongbing—also called citrus greening—growers can use nutritional programs to help keep greening-infected trees productive as long as possible.

Tearing out infected trees as soon as the disease is detected isn't always the most practical solution, says Fritz Roka, associate professor of agricultural economics at the University of Florida's Southwest Florida Research and Education Center in Immokalee.

"To make roguing work, you have to be very vigilant on scouting," he says. That means starting early and never letting up.

Replanting after tearing out infected trees leaves growers waiting two to three years for young trees to come into production, Roka says.

That was the calculation Maury Boyd, president of McKinnon Corp. in Oakland, made when he first realized his Orange Hammock grove was severely infected with greening.

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In 2006 the decline that he'd initially attributed to hurricane damage was diagnosed as HLB; 70 percent of the grove's young trees and 43 percent of his mature trees were infected.

"I was told then the trees would probably be gone in two to three years," Boyd says. "I didn't buy into the prediction of utter doom."

The tipping point had come and gone before he'd even been aware.

"We couldn't remove the infected trees," he says. "That would've been half the trees in our groves."

Nor would it be a solid long-term solution, with the rest of his trees probably infected but not showing symptoms. That, combined with infected groves on neighboring properties, meant any new trees he planted were certain to pick up the disease in a short time.

A ‘Mediterranean diet’ for trees

Growers in Western states, now on the alert after watching Florida deal with greening, may be able to avoid reaching that tipping point, Boyd says.

He focused on tree nutrition, supplementing a soil fertility program with foliar applications of what's come to be known as the Boyd cocktail full of minerals and other nutrients.

Controlling Asian citrus psyllid, which vectors the disease, also remains essential.

His trees now produce the same fruit yields as before infection, he says.

"HLB made the trees look like they were starving to death," he says, likening his nutritional program to feeding his trees a Mediterranean diet rather than a McDonald's diet.

"If a tree is satisfied with micronutrients, it doesn't express [greening] symptoms" or expresses them to a lesser degree, says Bob Rouse, citrus horticulturist also at the university's Immokalee center.

Keeping trees well fed

Fighting off HLB's effects on leaves and keeping them green helps the tree continue producing enough food to beat back other symptoms, Rouse says.

One recent study suggests addressing phosphorus deficiency can help reduce greening symptoms.

Over the years Boyd has tweaked and tinkered with the cocktail's formula, which includes phosphite, magnesium sulfate, manganese sulfate and zinc sulfate. His latest change adds trace amounts of nickel.

Most growers have followed Boyd's lead and adopted some form of a nutritional program as a greening defense, Rouse says.

Several companies offer their own premixed formulations.

"Some growers are more successful than others" with the nutritional programs, he says.

"The Boyd program is the hardest and most cumbersome for most people" because it requires gathering and mixing all the components, says Joe Davis Jr., president of Davis Citrus Management in Avon Park. "But he's made them work beautifully."

Davis' company uses a Boyd variation in some of its groves and two commercial versions in others, depending on specific problems with the location and citrus variety. A formula that offers added help against canker and greasy spot, for example, is the obvious choice for a grove also battling those diseases.

Foliar micronutrients remain crucial

Rouse has been studying Boyd's original formula since 2008, testing which mix of components works best and which might be dropped with little or no impact.

So far a mix that omits only the systemic acquired resistance, or SAR, products shows the best performance, Rouse says. The micronutrients in the foliar applications appear to be the crucial element in combination with a liquid nitrogen-potassium-phosphorus fertilizer to cover all nutrient bases.

Three to four foliar sprays over the growing season supplement a soil fertility program, he says. Greening inhibits nutrient uptake through the roots.

Refocusing on root health

While Davis plans to continue attention to foliar nutrition, this coming year will see greater efforts to improve root health.

"Nutritional enhancement is good, but don't neglect root health and therapy," he says.

That includes not only liquid fertilizer applications but also treatments to control beetles and phytophthora, as well as ensuring proper levels of irrigation.

"Any stress that tree is under, greening makes the problem exponentially worse," he says.

The advent of greening underscores a basic truth, Roka says.

"Good horticultural practices will probably be the salvation of getting through this."

Like any other business owners, growers must maintain their assets—and citrus trees are among a grove's most crucial assets, he says. "They don't respond well to jumping in and jumping out" of basic horticultural maintenance.

But when prices drop, growers tend to reduce costs.

Micronutrients whose correlation to tree health and yields aren't clear-cut may seem obvious candidates for cuts, Rouse says.

Roka has calculated cost comparisons of six foliar nutrition programs, including Boyd's original formula. Total costs top out at $433 per acre for the Boyd cocktail and range downward to $190 per acre for a foliar nutrition program from Chemical Dynamics Inc.

Rouse and Roka are trying to determine which parts of the nutritional programs are most effective and whether they return growers' investment.

"It's hard to tease these apart," Roka says. Zinc by itself, for example, may do less than its contribution in concert with other micronutrients. Cumulative effects may be more important than a single year's applications.

"We're still spending a lot more money on foliar programs than we were before greening," Davis says.

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Maury Boyd    
Florida  |  April, 15, 2013 at 05:15 AM

The article is well written. We apply the materials while the new emerging and expanding flush is developing, before maturity is reached and the cuticle forms. Of primary importance is for the materials to absorb into the new growth. Additionally, where we have poor soils, sandy and low CEC , we apply to the soil compost ( SWFla ) and recently have increased our soil application of calcium nitrate over that of ammonium nitrate. Regarding roots we find no P but have Fusarium ( secondary ?) found on blighted trees ( citrus blight a + 100 year old disease & cause is unknown ). This was confirmed for me when last year we reshapped our furrows ( recut every other middle, deeply, to increase water drainage ) and we found we had a heavy root system through out. Soil testing by all has been limited. Maury Boyd

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