The same organism that’s responsible for Pierce’s disease in grapes appears to be behind bacterial leaf scorch in Southern highbush blueberries.
But many questions remain about the spread of the blueberry disease, control measures for the insects that spread it and how stricken growers can remain profitable, says Phil Brannen, an Extension fruit pathologist with the University of Georgia in Athens.
“It’s relatively new in the sense that it’s only been the last two or three years that we even realized that was the cause of the problem,” Brannen says. “Before, growers just thought [the blueberry bushes] died of Phytophthora. Often, you would go in and kind of accept the fact that in three to four years, they could be dead.”
Adair Chambers-Peterson, a grower/shipper in Homerville, Ga., says growers are concerned about the disease, especially if they have susceptible varieties.
“If you’ve got an area of infestation, you can lose large acreage,” she says. “Everything can look good, and bam—it’s there.”
Bacterial leaf scorch in blueberries has been identified in Florida, Georgia, Alabama, Louisiana and Mississippi. Southern highbush varieties have been the hardest hit compared with rabbiteye varieties.
“We don’t know a lot about rabbiteyes with this disease,” Brannen says. “Since they are more native, they may carry the bacteria and probably not have major symptoms. With that said, we have seen some rabbiteye that do have it and seen some symptoms. Maybe it’s the same thing in rabbiteye as in muscadine.”
Wild muscadine grapes are resistant to Pierce’s disease, whereas some of the muscadine varieties that have been bred for commercial uses are very sensitive to the disease, Brannen says.
In other rabbiteyes, Brannen says he wonders whether bushes that appear less thrifty and twiggier may have a chronic case of leaf scorch.
Even among Southern highbush varieties, susceptibility varies, with V1, Star and O’Neal being the most susceptible, Brannen says, citing data from a 2008 survey of Georgia blueberry operations.
Windsor only had minimal symptoms, and Emerald, Millennia and Southern Belle showed no symptoms. V5 and Jewel also were found to have no symptoms, although Brannen and fellow plant pathologists Harald Scherm and C.J. Chang cautioned that the number of sites surveyed was limited. Additional work needs to be conducted to confirm resistance in the apparently symptom-free cultivars, they say.
Because bacterial leaf scorch was not a problem in the past, cultivars were not screened for it before they were released. Of concern is Rebel, a fairly new release that had about the same disease incidence in older plantings as Star, based on the survey results.
“What we will have to do now is figure out ways we can look at these varieties during the breeding process and screen for this,” Brannen says.
The bacterium behind the disease
Photos courtesy of the University of Georgia
Bacterial leaf scorch mortality (top) may be confused with root rot-caused plant death. Symptoms (bottom) include a distinct marginal leaf burn surrounded by a dark demarcation line.
Xylella fastidiosa, the bacterium that’s responsible for both the grape and blueberry diseases, can be grouped into two different biotypes or strains. One causes Pierce’s disease in grapes, and one causes leaf scorches in several other crops, including almonds and alfalfa. Both strains are harmless to humans.
Although researchers continue to try to pinpoint to which group the blueberry organism belongs, Brannen says test results are pointing toward the scorch group.
Regardless of the strain, Xylella fastidiosa forms colonies in the xylem, or water-carrying plant tissue. The colonies produce gums and waste materials that eventually clog these conduits, killing branches and eventually the entire plant.
Initial symptoms appear as marginal leaf scorch, which can be confused with drought stress, root rots or fertilizer burn.
In the spring, infected plants have very thin twigs, which drop their leaves and take on a characteristic yellow appearance—hence the nickname yellow twig.
The most unique symptom is that a plant may drop nearly all of its leaves, yet appear otherwise healthy. Eventually it will die.
The following year, a neighboring plant or plants may show symptoms.
The disease is spread when glassy-winged sharpshooters and other leafhoppers pick up the Xylella bacterium while feeding on infected plants and then feed on healthy plants. Symptoms typically appear in one year, with plant death following a year or two later, Brannen says.
Whether the disease can be spread through the use of infected cuttings remains unknown, he says. Regardless, Brannen recommends using clean plant material for propagation.
To pull or not to pull
In California, most researchers agree that grape growers should remove vines as soon as they see symptoms. The goal is to reduce Pierce’s disease sources for the sharpshooters.
But Brannen says he’s not convinced that rouging of infected blueberry bushes—particularly if several bushes show symptoms—is the way to go.
“I haven’t pushed it as much because it doesn’t seem to be as clearcut in the blueberries,” he says. “When you see symptoms in August, by the end of the year the [grape] vine is pretty much gone. It looks like it’s a little bit slower to form an epidemic [in blueberries], even in south Georgia.”
Chambers-Peterson says she’s seen some success with pruning infected branches.
“Eventually it’s going to die, but it will slow that process down,” she says.
Jury remains out on insect control
Dan Horton, a University of Georgia Extension entomologist in Athens, says Extension currently has no recommendations for controlling sharpshooters in blueberries.
Researchers in Georgia have experimented with the glassy-winged sharpshooter control program that California winegrape growers have used successfully.
That program includes foliar and soil-applied systemic insecticide run through the drip irrigation system at leaf flush in the spring.
But the trials on blueberries didn’t yield encouraging results, he says.
“In 2009, a very wet year, leafhopper suppression was poor in all treatments,” Horton says.
He says insecticidal suppression may work better in California because of their arid climate and lack of natural moisture. The grapevines readily take in the chemigation as they begin to leaf out, and it translocates throughout the plant.
But Georgia’s climate, with its feast or famine precipitation, doesn’t provide nearly as conducive the conditions.
“We can have a lack of moisture or be blessed with too much of it, so it’s not really as straightforward. 2009 data didn’t show we’re getting suppression,” Horton says.
Nevertheless, he says researchers will continue to look for effective control programs.
“The history of vector suppression as a means of controlling plant diseases is definitely something that is not a slam-dunk,” Horton says, citing the complexity of vector, disease and host relationships.
Ultimately, he says he believes the solution will involve breeding resistant varieties.
Contact Vicky Boyd at vlboyd@att.net or (209) 571-0414.
