Canker & Greening: Science Searches For Answers

The last couple of years of drought have played havoc on crops and water tables, but one beneficial side effect of the dry weather was to slow the spread of citrus greening and canker. With the rains returning, one can expect the two diseases to pick up their rate of spread in the coming months. Greening has been confirmed in 25 counties in the southern half of Florida. The hurricanes of 2004-2005 spread canker to the extent that it ended hopes for a successful eradication effort.

According to Dr. Timothy Gottwald, USDA-ARS research leader and plant epidemiologist, that makes current and future research dedicated toward greening and canker all the more important.
“Currently, there is a flurry of research going on both at USDA and IFAS, dealing in multiple approaches concerning these diseases,” he says. “The USDA has put together a large, comprehensive proposal including 95 individual projects aimed at the two problems.”

Diagnostics/Detection

“You have to find your enemy before you can do anything about it,” says Gottwald. “Finding these diseases early is very important.”

According to Gottwald, that’s why a good deal of research on identifying and confirming the presence of canker and greening is underway. “In greening research, there is a lot of molecular characterization work being done,” he adds. “This work is helping us determine what strains of greening we are dealing with and the pathogen’s relation and effect on the plant. And we are studying how the vector, the psyllid, transmits the bacteria and the latency of the pathogen after transmission.”

High spectral imagery also is being evaluated for its ability to show outbreaks of canker and greening. Research is even underway utilizing dogs to sniff out the diseases.

“We are looking at the volatile compounds coming off the fruit when disease is present,” says Gottwald. “We are seeing that K-9s can detect canker, so there must be something coming off the plant. If we can isolate that compound, we might be able to develop a chemical nose to mimic the dog’s ability to detect the problem.”

Resistance

The holy grail of greening and canker research is the development of resistant cultivars. Much work is currently being done to find germplasm that shows some resistance.

“At this point, to our knowledge there is not a gene in the citrus genome that can bring resistance to greening,” says Gottwald. “But we continue to work with germplasm whether through traditional breeding or by transformation in search for resistance to greening.”

Researchers are finding some hope in the search for resistance to canker in the kumquat. “Kumquat is demonstrating some resistance to canker, so researchers at USDA and IFAS are trying to isolate this gene, which is imparting resistance. Once isolated, we’ll then look at how we might transfer that gene into citrus cultivars.”

Here And Now

While science searches for answers to the greening and canker crisis, Gottwald reminds growers of the most important things they need to know about both diseases. “For canker, we need to look at the best control methodology that we have at the moment, which is windbreaks and cooper treatments,” he says. “Probably, the number one thing is figuring a way to control the wind to help reduce the bacteria in groves.

“For greening, it is very important that you know where it is, whether in your groves or your neighbors. Control psyllids the best you can and remove infected trees to keep the inoculum down.”