click image to zoomCourtesy Agricultural Research ServiceRoot feeding by Diaprepes root weevils can cause added stress to citrus trees already weakened by citrus greening, also known as huanglongbing or HLB.Scientists studying huanglongbing—or HLB—have made an unexpected discovery.
Even though the disease is introduced into citrus trees through a vector—the Asian citrus psyllid—the infection causes root damage even before above-ground symptoms are seen, says Jim Graham, soil microbiologist at the University of Florida's Citrus Research and Education Center in Lake Alfred.
The finding has caused researchers and growers to hone their plan of attack against citrus greening.
“Now that we know this, we focus squarely on root health," he says.
By the time symptoms are showing, the tree shows a minimum of 30 percent root loss and 30 percent fruit loss, Graham says.
As if root damage caused by HLB weren’t enough, Florida growers still must contend with other prevalent root health issues—Phytophthora, parasitic nematodes and Diaprepes root weevil.
In the past, these pests and pathogens were growers’ main concerns, but now they pile on additional root damage, after HLB.
But it may not always be necessary to treat for Phytophthora or Diaprepes, Graham says.
Only if they are present at damaging levels is treatment recommended. Otherwise, concentrate on managing the HLB effect on root health.
How do you know if these are present at damaging levels?
Detect and quantify the pest and pathogen with traps and soil tests, Graham says.
“These assays provide growers the information they need to help make a decision to treat," he says.
Chemical manufacturers can assist in performing the assays.
For example, Syngenta has a program that tests for Phytophthora, and DuPont has a testing program for nematodes.
Diaprepes is not present in every area, either. It’s most devastating in the east coast and southwest flatwoods areas of Florida and also causes damage on the central ridge.
“When the weevil teams up with HLB in the destruction of roots, the result can be devastating,” Graham says.
There are ways you can manage the weevil with insecticides to kill the adults and soil applications of beneficial entomopathogenic nematodes—or EPNs—to control the insect’s larvae.
Phytophthora is in every grove, but may not cause damage until the number of propagules per cubic centimeter of soil exceeds the 10-20 threshold, he says.
In that case, consider a fungicide treatment.
Like Diaprepes, Phytophthora teams with HLB and can add to root damage.
One way to ward off Phytophthora is by choosing more resistant or tolerant rootstocks.
HLB bacterium and the root weevil attack all rootstocks. Rootstock tolerance to HLB or Diaprepes depends mostly on ability of rootstock to adapt to soil and water quality conditions in a given grove site.
There’s another stress factor in Florida that appears to top even Phytophthora and root weevil—calcium bicarbonate, manifested in the form of high bicarbonates in well water and soil.
Most wells in Florida are high in calcium bicarbonate because of water management district policies requiring growers to dig deep wells, Graham says.
In HLB-affected groves, it is now advisable to keep roots out of stress by reducing bicarbonates and increasing availability of water and nutrients as needed, he says.
Orange-Co LP, Arcadia, which owns and manages 14,600 acres of citrus, is at the forefront when it comes to combating high bicarbonates and high pH levels in irrigation water and improving citrus root health.
Jerry Newlin, vice president, citrus operations, says he became aware of the positive effect of lower-level pH range and bicarbonates through the work of Graham and Bryan Belcher, production manager at Davis Citrus Management Inc., Avon Park.
Although deep-well water generally is considered good quality from a salinity perspective, it has a fairly high pH level—in the 7.5-8 range—and it has bicarbonates in the range of 180-230 parts per million. Ideally, the pH should be below 7.0 and bicarbs of less than 100 ppm.
Before citrus greening came on the scene, deep-well water was “just fine” for citrus, Newlin says, although golf courses and vegetable growers had been treating their water for a couple of decades.
“Greening obviously causes impairment of the roots,” he says, and that, in turn, adds stress to the tree.
“We found that bicarbonates create a situation where roots can’t get certain elements, such as calcium, potassium, phosphorous and other minor elements,” he says. “By mitigating the bicarbonates and neutralizing the water, you at least make the water no more part of the problem.”
There still could be soil issues from long-term use, however.
Orange-Co now treats its water using enhancements like acidified fertilizer and sulfuric acid.
Finding the right way to treat the problem has been “a real challenge,” Newlin says.
For example, tanks and other equipment are needed for the water treatment program.
“We went through several different types and brands until we figured out some that could handle these types of acids.”
The company only has been treating its water slightly more than a year, but good results already are showing up.
“The trees are looking better where we’re doing it than where we’re not doing it" in terms of overall health, leaf size, color and response to greening, he says.
No one thinks water treatment is a cure for greening, Newlin says. “But we have learned that if we can reduce the stresses to the tree, it at least can survive better.”
There is an extra cost involved, but Newlin says the results are well worth it.
“Now that our cost of caretaking is quite a bit higher, to spend an extra $50 to $150 an acre in terms of water remediation or spoon-feeding nutrients seems to be a prudent thing to do to make trees healthier relative to greening,” he says.
If you’re just treating the water, the cost might be in the $50-$70 range. If you also spoon feed the nutrients, the cost will be in the $150-plus range, depending on how much fertilizer you use.
“It’s too early to say it’s reversing anything at this point, but it’s certainly taking some of the stress off the tree,” Newlin says.
Newlin is optimistic about the future of these programs.
“The good news is a lot of smart growers and vendors are starting to do this,” he says, “so we’re coming up with some pretty ingenious ways to make it easier and more cost effective.”
The Florida citrus industry has implemented good horticultural management practices for maintaining root health through proper water, nutrition and fighting the psyllid vector Graham says.
“We emphasize practices that promote root system function of HLB-affected trees to maintain root density, yields and fruit quality,” he says.
So far though, research has not determined whether root loss due to HLB is reversible.
The future of the citrus industry in Florida depends on replanting grove sites with the best soil selecting a rootstock adapted for the soil type to ensure that the root system sustains uptake of adequate water and nutrients, Graham says.
“It is not recommended to grow citrus on marginal soil anymore,” he says.