Trees ooze, turn black after hailstorms help spread bacteria

Spring hailstorms that damaged trees along the Front Range made it easy for fire blight to cause a large number of trees to ooze and develop dark cankers.

“It’s been a pretty dramatic year,” said Kevin Marks, a master arborist with Davey Tree Expert Company who has worked in Denver for more than 20 years. “This is the worst I’ve seen it.”

Or as Anthony Koski at Colorado State University’s Department of Horticulture and Landscape Architecture put it: “This year was kind of an epic year for fire blight.”

The damage is particularly extensive in Louisville, Arvada, Broomfield and Westminster, Marks said, though he has seen the bacteria all over the Denver area. He said he’s seen the blight affect every crab apple tree in an entire neighborhood.

The bacteria infects trees in the rose family, especially pear, apple and crab apple trees, Koski said. As the weather warms in the spring and rain starts to fall, the dis ease is spread by animals, insects and splashing rain.

Hailstorms this past spring were timed perfectly to help spread the disease, Marks said. The damage caused by the hail creates openings for the bacteria to enter the tree.

“It’s just like people, you get a wound on your skin you can get an infection,” Koski said.

The bacteria then spreads, turning leaves black and brown. Infected twigs ooze a creamy substance. Fruits dry on the branch instead of falling. Dark cankers appear on branches and stems. If left untreated, the bacteria can kill an entire tree.

The bacteria makes the tree look like it was lit on fire, the effect that gives the disease its name, Marks said. Fire blight has been an issue for decades and can be found across the U.S., Koski said.

Winter is the best time to treat trees with fire blight because the bacteria becomes inactive, Marks said. Prune leaves and branches at least 8 inches from the infected site, he recommended. “You might have to aesthetically challenge your tree for a season,” he said.