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• 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.
UNIVERSITY OF FLORIDA Citrus Horticulturist Bob Rouse’s research shows that a micronutrient program, combined with vigorous pruning, might keep greening-infested trees producing longer.
“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.