Until the last six or seven years, band canker, a disease that strikes 3-year to 6-year-old almond trees, had occurred only sporadically since it was first reported in California more than 40 years ago.
But now, it has become much more prevalent, damaging or killing almond trees from Kern County in the south to Glenn and Butte counties in the north. The disease has also been found to a lesser extent in walnuts, and to greater extent in pistachios. But it isn’t killing pistachios.
Some of the same species of fungi that cause shoot blight of walnut trees and panicle and shoot blight in pistachio orchards are the culprits behind the disease, which leaves one or more broad bands of cankers circling almond tree trunks.
Vigorous-growing almond varieties like Carmel, Nonpareil and Padre are most susceptible to the disease, says Themis Michailides, University of California plant pathologist, based at the Kearney Agricultural Center, at Parlier. Sometimes, the Botryosphaeria fungi infect the crotch of a tree, weakening it and leaving it vulnerable to splitting by high winds. Michailides has also seen one orchard where fungi had infected nut stems so severely that the nuts didn’t fall off the tree when shaken by a nut harvester.
Band canker is caused by seven different species of Botryosphaeria fungi, he explains. These fungi can infect 45 different species of trees in California, including those in riparian areas like willows, cottonwoods, elderberries, and others. They provide the spore sources for the disease. Spores enter the tree through openings in the bark — growth cracks in the trunk, cracks in shoots from bending in the wind, pruning wounds, fruit peduncles, lenticels or rough bark at the base of large shoots. Fungi are also attracted to trees that have been stressed for water or other stresses.
Once they get inside the bark of healthy tree, the spores can remain dormant until conditions are right for them to flourish and infect the tree. Trees infected in the spring develop larger cankers than those infected later in the season.
“We haven’t done much research in walnuts, but at least three different species of Botryosphaeria can infect trees with the disease,” Michailides says. “However, instead of encircling the trunk, cankers form on major branches in the lower and middle areas of the canopy.
“In early May, I saw a walnut orchard with severe cases of both Botryosphaeria and Phomopsis cankers, which sometimes occur together. That’s also common with clusters of pistachio nuts infected with Botryosphaeria. The fungi have been found in pecan trees, too, but, so far, we haven’t seen them cause any problems.”
Once almond trees have been infected, the band canker can’t be controlled, he notes.
In Michailides’ trials, spraying, painting or injecting the cankers with fungicides or biological agents in the cankers has failed to cure the disease. But, in greenhouse experiments, he’s painting pruning wounds with a fungicide to see if that will protect the trees from becoming infected.
He has limited spread of infection by installing a splitter in sprinkler nozzles to keep tree trunks dry when irrigating. “We’ve been able to reduce incidence of the disease in orchards by 50 percent doing this,” he says. “For drip or micro-sprinkler systems, move the emitters and nozzles far enough away from the trunks so the perimeter of the wet circle just touches the base of the trunk.”
Stumps left after an infected tree has been removed can still harbor the fungal spores to continue spreading the disease. Remove those stumps, Michailides recommends.
Currently, California researchers don’t know how, if at all, Botryosphaeria canker can be controlled in walnut orchards. “The only way to protect other trees is to avoid wounding them and to remove any infected trees,” he says.”