How to spot trouble on a red oak

Kermes scale: The Kermes scale insect is well-established in the Boulder area and affects pin oak and red oak trees. The adult female is about a quarter-inch in diameter, tan and speckled with brown. The insects are usually found on twigs near the end of the branch but can be confused as part of the limb. Heavy infestations can cause twig dieback and leaf flagging. The bugs can be treated with certain pesticides, with varying results, as well as with organic oils. The city recommends contacting a local tree company for treatment.

Brenneria: This rare form of bacteria, known to attack certain species of oak trees, can lead to wilting leaves or dead branches. The infection can lead to visible ooze on oak acorns. Consult a professional tree company for help if you suspect an infection.

To report potentially diseased trees in the public rights of way, contact the city's forestry office at 303-441-4406.

Plant experts are scratching their heads about a rare bacteria detected in Boulder for the first time, leading to fears that the city's population of giant red oak trees could be at serious risk.

City foresters have already identified three towering oaks near the Band Shell in Central Park that are in such a state of decline that they must be removed in early November. Potentially dozens of other red oaks in the area could also be at risk, and will be treated with pesticides in the coming weeks.

A combination of unusual factors is responsible for the sudden decline, and it's too soon to know whether red oaks citywide are at risk, said Ned Tisserat, a plant pathologist at Colorado State University who is working with the city.

The city has identified an infestation of Kermes scale -- an odd, bud-shaped insect that feeds on plant sap -- as well as the presence of the bacteria Brenneria, which has never before been found on a red oak tree.

"I'm not sure we totally understand what's happening on those trees," Tisserat said. "The Kermes scale is a pest, but why it's causing so much damage is still a bit of a mystery."

Tisserat said the bugs are a relatively common problem in Boulder, but have never been associated with the death of a mature red oak, such as those in Central Park that were planted in the 1920s.

The role of the bacteria is even more confusing, he said. Brenneria is associated with wilting and "bleeding" of sap on some types of oaks, but it has never showed up in Colorado or on red oaks.

"This one is something new," Tisserat said. "We're trying to figure out if the bacterium is part of the process of decline of these very large trees."

Brenneria was first detected on pin oak trees in California during the 1960s, Tisserat said. The bacteria caused acorns to ooze from the infection, leading the condition to be known as "drippy nuts disease."

Kathleen Alexander, the city of Boulder s chief forester, looks at the wilting leaves of a red oak tree in Central Park that is infected with Kermes scale
Kathleen Alexander, the city of Boulder s chief forester, looks at the wilting leaves of a red oak tree in Central Park that is infected with Kermes scale insects and a rare bacteria. Three oak trees in the park will be removed due to damage from the insects and bacteria. ( MARTY CAIVANO )

But the bacteria haven't been reported in the United States for decades, and it isn't clear why the infection is attacking Boulder's red oaks.

"We're still looking into it and scratching our heads," Tisserat said.

He said he hopes to figure out the riddle before other trees in Central Park die.

"You definitely don't want to lose those," he said of the green giants. "That would be tragic."

Whitney Cranshaw, an entomologist at CSU, has also been consulting with the city on the problem.

He said symptoms of the Kermes scale infestation include flagging leaves and dead branches.

He said the insects are not particularly uncommon, but they have not been associated with the bacteria before.

"It looks like they're working in tandem," Cranshaw said. "The symptoms of the disease are right around where the insects are feeding. It's a fairly unusual condition."

The immediate treatment for the city's red oaks includes injecting pesticides into the base of the 20 other red oaks in Central Park and across the city's municipal center to attack the Kermes scale. The plan also calls for spraying branches with organic plant oils.

After the city removes the three dying oaks in early November, the CSU experts will be on hand to take additional samples from the trees to try to figure out exactly what happened.

Red oaks are the second type of trees in Boulder to face a health crisis since 2008, when black walnut trees in the city saw a significant decline. Hundreds of walnut trees were infested with walnut twig beetles, leading to the "thousand cankers disease" that forced the city and many homeowners to remove dying trees.

That disease has wiped out the majority of the city's walnut population on both public and private property, said Kathleen Alexander, the city's chief forester.

She said a new epidemic among the city's red oaks would be a "worst-case scenario" because they are generally old and not easily replaced.

In addition to the pesticides and additional research into the red oaks, Alexander said the city would also work to loosen the soil around the base of the trees in Central Park. Years of foot traffic have compacted the dirt around the roots, she said, which can affect their health.

The city is now investigating options for that work, which could include injecting compressed air into the ground to break up the soil.

This year, the city's annual Tree Health Survey identified a total of 111 trees of all varieties on city property that need to be trimmed or removed for safety reasons. At least 48 trees will be removed from city land this fall. On average, the city removes about 250 trees a year.

The city is responsible for the care of about 36,000 trees, which are valued at $46 million.

Contact Camera staff writer Heath Urie at 303-473-1328, or urieh@dailycamera.com.