Spread by tiny beetles the size of poppy seeds, thousand cankers starts beneath the bark and eats away at the tree until its limbs wither.
The beetles, which officials say can travel about two miles each year, carry fungus that they leave behind when they burrow into tree bark. Cankers eventually form in the place of the burrows, and, as more beetles tunnel in, the tree becomes unable to take in nutrients. By the time its leaves start thinning and turning yellow, it's too late - any tree that has been infected dies a slow death that can last up to 10 years.
On Monday, the Pennsylvania Department of Agriculture announced an infected walnut twig beetle had been found in southwest Chester County, followed by traces of the disease in nearby trees.
The discovery prompted state officials to enact a wood quarantine in Chester, Delaware, Montgomery, and Philadelphia Counties, a ban that prohibits firewood and walnut material from leaving the counties, excepting processed lumber and finished wood products without bark. Bucks County has been under a similar quarantine since 2011.
The quarantine also prohibits walnut shipments from states where the disease has been found - and that list is growing. Since the disease was identified in Colorado in 2003, it has spread to California, Arizona, Virginia, Idaho, Nevada, New Mexico, Oregon, Tennessee, Utah, and Washington.
How many trees have been affected by the disease is unclear. In Colorado, thousand cankers has wiped out black walnut trees in several cities, the National Forest Service says.
The service's researchers have been looking at ways to slow the disease's spread, and treatments have been developed to rid logs of it. State forestry officials also periodically set beetle traps in disease-prone areas in an attempt to monitor outbreaks.
"We try to slow the spread down as best we can," said Dana Rhodes, whose job as Pennsylvania's plant regulatory official includes overseeing efforts to detect and combat plant pests. "But at this point, we're just waiting for science to catch up."
Black walnut trees make up less than half of 1 percent of Pennsylvania's hardwoods, but they produce high-quality wood used for furniture and other woodworking.
The law establishes stiff penalties for violating the quarantine: civil penalties as high as $20,000 and as much as 90 days in jail. But officials were hard-pressed to identify any past offenders.
Much of the state's lumbering is done in northern counties, which means that for now, the effect of the quarantine is small. Still, it is difficult to enforce.
"We're basically at the mercy of homeowners and residents abiding by it," said Miller, the arborist. "The idea is, if you think your tree has it, to remove it from your property and burn it. But not many people really know what to look for."
Rick Hearne owns Hearne Hardwoods in Oxford, Chester County, a family business that supplies more than 100 types of domestic and imported wood. His company follows all the rules, he said, but since most people don't know they are not supposed to transport some firewood across state lines, it is inevitable that the disease will continue to travel.
"The state will probably end the bans when they recognize that it's everywhere anyway," he said.