Research bolsters foliar nutrients’ role in citrus greening management

10/07/2011 01:32:00 PM
Vicky Boyd

A nutrient cocktail developed by citrus grower Maury Boyd appears to not only rejuvenate citrus trees infected with citrus greening, but also improves their vascular system.

Three years of trials conducted by Bob Rouse, an associate horticulture professor at the Southwest Florida Research and Education Center in Immokalee, have backed up Boyd’s earlier observations that the nutrient blend helped counteract the effects of greening, also known as huanglongbing or HLB.

In addition, preliminary laboratory tests by Ron Brlansky, a plant pathology professor at the Citrus Research and Education Center in Lake Alfred, show the application of the nutrient blend has enabled the plant to produce new phloem—the plant’s nutrientcarrying vascular tissue—in HLB-infected trees compared to untreated ones.

“This raises more questions,” Rouse says of Brlansky’s preliminary findings. “All we know is the first time we ran the dye, and there was a big distinction between the open phloem in the trees that were treated compared to the trees that were not treated. And with Maury’s trees, the samples we pulled from his were open, too.”

Despite the apparent good news, Rouse says he doesn’t view the nutrient cocktail as a silver bullet. Instead, he says, it is a shortterm bridge to keep growers in business until research yields longer-term solutions.

Rouse says the trial results also back up his contention that trees were nutritionally deficient even before citrus greening entered the state.

As fruit prices dropped during the past few decades, growers cut costs, including foliar nutritional programs.

HLB, yet another stressor, simply pushed the trees over the brink.

“We moved from a good nutritional program in the last 20 years to probably a weak foliar nutritional program,” Rouse says. “We were putting everything on the ground because we had been told that a little micronutrient deficiency in the leaves doesn’t affect production. But that doesn’t hold up in the era of greening.”

Early trials

Rouse began his trial in early 2008 at the SWFREC with HLB-ravaged trees that had nearly 100 percent infection. He had nine different treatments, including the full Maury Boyd cocktail, different components of the cocktail individually and an untreated check.

He also had two similar trials in nearby commercial citrus groves, one with about 50 percent to 60 percent infection and the other with about 15 percent to 20 percent infection.

As Boyd has, Rouse applied macronutrients by ground and made foliar applications of micronutrients and systemic acquired resistance-inducing materials during each tree flush.

“My feeling was we might be able to save those two commercial groves that had up to 50 percent infection, but the one here I didn’t expect to pull out of it. But citrus trees have great recuperative power.

“My whole deal was if this comes back, it will be a miracle. The other two I really had hopes for.”

As an offshoot of the study, Rouse says in 2009 he began an extensive study of the roots of the trees treated with the Maury Boyd cocktail compared with the untreated trees.

“We found that we were not losing root density or root mass,” he says. “We didn’t have sloughing or declining roots.”

The study involved collection of 1,000 samples from among nine different treatments and five reps.

Rouse repeated the sampling in 2010 and found trees that received the Maury Boyd cocktail actually had increased root density compared with 2009 results.

A balancing act

The initial trial results prompted Rouse to consider whether buckhorning a tree to bring the canopy more in balance with a smaller root system, then feeding it with the micronutrient mix, would be a viable alternative to tree removal.

In 2010, he established a trial at the SWFREC that compared various nutritional regimes applied to both unpruned and buckhorned trees. The buckhorned trees were mechanically hedged and topped to 4 by 4 feet.

“If it works, then a grower could set up a strategy to rehabilitate through pruning some percentage of his grove every year until it was back into production and it wouldn’t totally eliminate his cash flow,” Rouse says.

A buckhorned tree should regrow and be back in production within about two years, he says. But a replant typically takes four years to come into production and another three years to where production will repay the investment.

Even though the buckhorning removed most of the 2010 crop, Rouse says they still harvested 1/3 box from each tree that received the severe pruning and the nutritional sprays the first year.

That compares to 1/4 box per tree average before buckhorning.

Trees that were left unpruned but received the nutritional sprays yielded 1/2 to 2/3 box each.

Boyd and grove manager Tim Willis, who are closely following Rouse’s trials, buckhorned trees in January in part of a grove with the poorest soil near Felda.

The trees already have significant regrowth and appear to be responding well to the nutritional program, Willis says.

“We’re trying to do everything we can to avoid having resets because that is a costly proposition,” Boyd says.

Both he and Willis say one of the keys is to include a strong psyllid management program with the other cultural practices.

Nutrient recommendations

Based on the results of the original trial, Rouse says he recommends a balanced program that includes both ground- and foliar-applied nutrients.

And at least one of the micronutrients should be in the phosphite form, such as zinc phosphite or manganese phosphite, which enhances fruit yield.

“At this point, we don’t see the benefit from the SARs. It doesn’t mean they’re not doing something, but we don’t see they’re doing anything for greening,” he says.

Salicylic acid, sometimes called an SAR-inducing material, is actually a growth regulator that releases buds from dormancy, Rouse says. It may benefit recovering trees by promoting more leaves.

Typically, the macronutrients—nitrogen, phosphorus and potassium—are soil applied whereas the micronutrients tend to be foliar applied.

Rouse also cites Liebig’s law, which states that plant growth is limited by the scarcest resource. If one micronutrient is in short supply, it will affect the tree even if all of the other micronutrients are adequate.

‘Evidence that something is working’

Last fall, Ron Brlansky, a plant pathology professor at the Citrus Research and Education Center in Lake Alfred, collected leaf samples from trees in citrus grower Maury Boyd’s Orange Hammock grove.

Brlansky also collected leaf samples from trees in Bob Rouse’s trials that received Boyd’s nutrient mix without systemic acquired resistance-inducing materials and from untreated trees.

Brlansky stained them, so the vascular tissue had more contrast, and viewed them under a conventional light microscope.

The phloem—the nutrient-carrying vascular tissue within the plant—from the trees receiving the Boyd cocktail wasn’t nearly as plugged and appeared to have new phloem being produced compared with untreated trees.

Boyd, who has championed his nutritional cocktail as an interim way to stay in business, says he was surprised by Brlansky’s findings.

“To me, this is the first evidence that something is working,” Boyd says.

He was quick to add that the nutrition program only works with a strong psyllid control program to help protect new flushes from reinfection.

This spring, Brlansky again collected samples from some of the plots that received nutrient combinations other than the Boyd cocktail and repeated the process.

Results are pending, Rouse says.

“If we can narrow down what are the important parts of this [cocktail], then we can cut the cost to the grower,” he says.

The next step is to collect additional leaf and stem samples for microscopic observation with light and electron microscopes “to see the bacteria and plugging to find out more and provide a description of what’s happening here,” Rouse says.

Brlansky also plans to determine whether the phloem in treated trees is functioning with tests using a fluorescent dye.

A grower advisory group within the Citrus Research and Development Foundation Inc. recently recommended funding Brlansky’s project.

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Manahar Kadariya    
Nepal  |  December, 16, 2011 at 11:43 AM

I also stressed to support this result. here we are doing a trial on 10 different groves for rejuvenation of more than 50 percent declined trees. But we do not have any resources for lab testing of these trees for greening. But most of the plant showed the typical symptoms of greening. Can i go further with farmers to disseminate technology ?

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