Citrus greening disease occupies a big part of Bob Rouse’s thoughts these days.
The citrus horticulturist at the University of Florida’s Southwest Florida Research and Education Center at Immokalee, thinks he and his colleagues, as well as the state’s growers, will be wrestling with it for a long time to come.
“We’re going to have to learn to live with it,” he says. “I wouldn’t bank too much on a solution from biotechnology within the next 20 years. If we had a GMO today that would work on it, it would take 10 years-plus to get permission to use it in the field. First, we’d have to test it, then get EPA’s permission to put it in the environment.”
And, whether the public would accept genetically modified citrus is another unanswered question.
But with any biotech solution beyond reach, for the time being, Rouse says, growers have to combat greening with already-available tools. He thinks the best option, in many cases, is an intensive micronutrients program, possibly combined with vigorous pruning.
“As long as the leaf is green and functioning, the tree is fine; where the leaves are functional, the tree will keep going. We discovered that where micronutrients were working, root density could increase on a diseased tree.
“Most growers didn’t know what to do when greening first appeared. They stalled a couple of years and got into a recovery mode.”
In 2008, Rouse began testing trees at the experiment station that were almost 100 percent infested with greening. He was intrigued by the observations by Maury Boyd, a southwest Florida citrus grower, who believed a micronutrient cocktail involving as many as 14 compounds could extend the productive life of greening-ravaged trees.
“Was there something unique about Maury or his location? We wanted to find out. Obviously, something was working. That’s what we learned in the last four years. Our experiments all have controls. Some trees are untreated; other trees get some of the components in this mix, but have one thing or two things left out.
“On control trees the phloem is blocked. Where the micronutrients are applied, the phloem is mostly open and functional, for whatever reasons. We just discovered this last season.”
Exactly why the pathway opens up with doses of nutrients remains a bit of a mystery.
“We want to look into the individual nutrients and see which ones are contributing to this.
“Could it be that the heavy metals are doing it? We don’t know. Or, are the additional nutrients growing the phloem faster than the disease can destroy it? We’d like to find the answers,” Rouse says.
All this makes him think the most workable answer for greening right now is to rehabilitate disease-stressed trees rather than replant them.
“We can rehab a tree in one to two years, rather than the seven it takes to get into production if we replant. And we avoid the costs of replanting. Some may worry about the cost of the nutrient mix— but you’re going to have to do it, anyway on replant trees.
“In two years, the tree will have enough fruit to repay the costs. These trees will go back to their original size the next year and in two years you’ll harvest a crop of Valencias. With Hamlins, it may be only one year.”
That idea goes against the philosophy of destroying the pest or disease before it destroys the crop, Rouse says.
“With replanting, the illusion is that you’re going to get rid of greening. But, it’s here — and it’s going to be here. Neither tree removal nor anything else is going to get rid of greening. So, the objective is to live with it, to be productive in the presence of greening.”
The greening bacteria are spread by the Asian citrus psyllid. Efforts to control the psyllid with insecticides can work, but the vector is tiny and mobile, making eradication unlikely, he says.
“With greening, the roots are dying because the top of the tree can’t support them. As the root area becomes less and less, you have continued dieback from the top. The roots can’t get nutrients to the top, so it’s a cycle. If you remove the diseased top area, you get rid of the bacteria, but it’s already throughout the plant.
“When you prune the tree, the more severe the pruning, the more vigorous the regrowth.”
That led Rouse to investigate pairing pruning with the micronutrient cocktail in an effort to rebalance greening-infested trees.
Want to balance top, roots
“We want to balance the top and the roots so they can grow together and support each other,” he says.
“The top supplies the roots and the roots support the top. The reason the roots are dying is because the top end can’t support them. What we did was buckhorning — extreme pruning that removed most of the top to make it regrow.”
Pruning like that is what growers do after severe freezes to encourage regrowth. The difference, Rouse says, is that the root system is not damaged because of the freeze.
“If we can rebalance the tree and grow back the root and top together, we can use the nutrient cocktail mix we’ve seen is effective and really feed and bring the nutrients to the top of the tree.
“But we’re also feeding it with foliar sprays, with liquid fertilizer to encourage new regrowth. We’re using both macro- and micronutrients — key micronutrients, along with N and K, very little P, and some phosphite added. It has some fungicidal activity and is a very good carrier of nutrients into leaves.”
How long will the rehabilitated trees last?
“Right now, no one can say,” Rouse says. “Psyllids are attracted to young growth. The more we learn about greening, the more we learn we need to investigate. This project just keeps getting bigger and bigger.”