Fungal disease wreaks havoc on area’s elm trees

Victoria O’Brien

When Veda Gerrard visited Whaley Cemetery in Shell in May, she noticed the elm trees ringing the property appeared sickly. Later, she found similar symptoms in two elm trees on her own property and her concerns mounted— she put it down to the warm winter, sudden cold snap, and wet spring, but all of them and so suddenly? Gerrard began making calls, wondering if there was an outbreak or new invasive pest in the Basin and if there was any chance she might be able to save her trees.

Both Gerrard’s elms trees and those at Whaley Cemetery were exhibiting symptoms of an infectious, wind disseminated fungal disease called cytospora canker, which is one of the more prevalent tree-related diseases in the Big Horn Basin and has caused a large-scale die-back among elms and other deciduous trees throughout the Big Horn Basin.

The disease, which is caused by the fungus Leucostoma kunzei, often begins in tree branches and quickly spreads through the canopy. The disease starts when a tree becomes stressed by insect feeding, snow or ice damage, drought or other external factors. Over time, the fungus works outward, forming a sunken canker coated in a thick layer of resin, and spreads further, girdling an infected branch, then killing it. The fungus typically stays in a tree’s canopy system, but can invade the trunk through wounds, such as those caused by mechanical injury.

Other hallmarks of the cytospora canker vary by host, but include dark, pimple-like structures on infected branches that release tendrils of long, yellow-orange spores in wet weather, which are then carried on the wind and rain to other branches and nearby trees. Frequently, bark will split near the canker as the tree attempts to defend itself from infection. Diseased wood inside the tree will become watery and odorous and the bark below the infection is oftentimes stained a dark, reddish-brown.

Jason Byerley, an International Society of Aboriculture (ISA) certified arborist based in Powell, attended to Gerrard’s trees and conferred with Big Horn County Weed and Pest on the dieback matter separately. 

While Byerley acknowledged that the primary stressor driving the increase of infections in trees could be anything, he also concurred with Gerrard’s suspicions. 

“I think about the mild […] and dry winter we just had as an additional stress for the trees with disease already present,” he explained. “It only exacerbated the problem by potentially adding optimal conditions for the disease to continue to spread and not sit dormant. The lack of precipitation didn’t help the trees any, and winter can be a difficult time for homeowners to add supplemental water.

“Another factor that could cause stress would be pest damage, primarily borers that attack the stressed trees by detecting their pheromones. These borers then cause vascular disruptions and add additional stress.  The main two components responsible for the tree decline I’ve observed in the Basin are borers and disease. Both made worse by drought conditions, hot summers without added water, very mild or even very cold winters with no precipitation.”

Late frosts, like those that hit the Basin in April and May, can also be tough on trees, Byerley said. 

“[The frost] damages newly developing leaves that haven’t hardened off and opening leaf buds. Even before leaves seem to be emerging and the bud scales have only slightly opened, they are very susceptible to damage from frost.”

Cytospora canker can affect a number of deciduous trees and shrubs as well as coniferous trees, particularly Colorado Blue Spruce. However, infections are rarely fatal in spruce trees when compared to the disease’s impact on deciduous and fruit-bearing trees. Byerley has observed an uptick in the fungus in apricot, cherry, peach and plum trees around the Basin.

“I am seeing a lot of new infections present in these species,” he said, “and a lot of decline and mortality from long-term infection.”

While possibly fatal, cytospora canker is also susceptible to treatment and remediation. Byerley explained that treatment is more typically prescribed for each site or individual tree based on the severity of disease activity and amount of vascular disruption. 

“Borers are easily controlled by a variety of methods of insecticide application,” Byerley  noted, adding that willows, while susceptible to cytospora canker, are typically less impacted by borers than other trees. He continued, “[Treatment] can be challenging if the infection is severe. There is really only one good option for systemic trunk injection for larger trees and nothing in a foliar or basal spray that controls it well.

“Smaller trees can be treated with a basal spray. [However,] fertilization with available or high nitrogen blends can exacerbate the disease and cause rapid spread. It’s usually treatable if caught early by basal spray or systemic trunk injection, and dormant season sanitation pruning.”

When pruning or attempting to treat a sick tree alone, Byerley adds that it is vital to use best practices in sanitizing equipment. Ethyl alcohol, lysol, or other disinfectants should be used when pruning, especially in-between unhealthy and healthy branches and trees; bleach should be utilized in a 1:9 ratio with water.

In a brief on cytospora canker, Colorado State University directed homeowners attempting to remediate infection alone to first determine whether the wound is new or established. If fresh, a sharp knife should be used to carefully cut and carve away all injured or diseased bark until only live, healthy tissue remains, after which the flesh should be disposed of and the blade sanitized before moving forward in the treatment. If the wound is older, one should remove loose bark pieces, but take care to avoid cutting, removing, or damaging the callus that may be forming at the canker’s edge. After scraping the wound clean, allow the healthy tissue to dry out.

For others, arborists like Byerley are available to diagnose and treat common ailments afflicting trees. In Gerrard’s case, Byerley was unable to salvage the two elms on her property showing signs of disease and ultimately removed both, believing the infection to be too advanced. Gerrard visited Whaley Cemetery again recently and found several of the elms there had seemingly rebounded, which heartened her. 

“It looks like they’re starting to come back,” she said, “which is so nice to see.”