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.