Non-Native Spruce Trees Vulnerable To Needle-Stripping Fungus

Dec 19, 2018

Rhizosphaera Needle Cast progresses in Spruce trees by browning the interior needles, then causing them to fall off the tree.
Credit University of Wisconsin

Spruce trees are a common choice for Christmas decorations, but a regional fungal disease can strip away their needles.

Rhizosphaera Needle Cast is a fungus that can infect several varieties of spruce trees, with one of the most vulnerable types being the Colorado Blue Spruce.  Taylor Hennelly is a Forestry Supervisor for Rockford. He says the city’s tree is infected with Rhizosphaera which can be tracked by the gradual browning of needles.

“The innermost needles will fall off the branches, then work its way toward the tips, starting from the base, then moving its way up the tree,” he said. 

This doesn’t pose a threat to Rockford’s Christmas festivities, but in the long term, it can affect the health of the tree. Dr. Frederic Miller, a professor of horticulture at Joliet Junior College, explains.

Rockford's Christmas tree is infected with Rhizosphaera Needle Cast. But the needle loss is inside-out, bottom to top, and slow enough that residents won't see any significant change over the holidays.
Credit Chase Cavanaugh

“Rhizosphaera doesn’t necessarily kill trees outright, but it can kill individual branches, needles, along with some other pathogens that we see with spruce, and of course, then that leads to a weakened tree, and you have bark beetles coming in, just a whole myriad of things," he said.

Miller says over time, these health problems, along with needle loss, may necessitate cutting off entire branches. 

“You may have a bare trunk up five or six feet, depending on how big the tree is and then the rest of the tree kind of looks like a lollipop tree, as we call it," he said. 

And that’s just what happens when a tree is infected.  Brian Hudelson is Director of the Plant Disease Diagnostics Clinic at The University of Wisconsin. He says Rhizosphaera Needle Cast, like many fungus-based diseases, thrives in wet environments.

“(You) have to have a thin layer of water on the surface of the needle for the fungal spores to be able to germinate and successfully infect," he said. "So anything that traps moisture, that’s going to slow down drying, and that’s going to allow for these leaf wetness periods to be longer."

Hudelson says water can accumulate in branches when the spruces aren’t spaced far enough apart or if their canopy is too thick. But those aren’t the only sources.

“We also run into problems if people have some sort of irrigation system that ends up spraying water onto the needles of the trees and keeps them wet for an extended period,” he explained.

Miller adds that once the fungus accumulates, it spreads quickly.

“The foliar fungi that cause foliar diseases spread by wind or rain so it’s very easily moved around from tree to tree or even across large areas," he said. "There’s some cases where the spores may be carried along the feet of birds.”

Ordinary fallen branches from Rockford's Christmas Tree. Branches affected with Rhizosphaera Needle Cast would have a dark brown color.
Credit Chase Cavanaugh

But the main reason the disease is so common among spruce is the Midwestern climate. Miller says trees like the Colorado Blue Spruce aren’t used to growing in such a consistently wet climate.

“If you go back and look at historical records, you don’t hear the explorers or the early settlers talking about spruce and pine trees here in Illinois. That’s just very unusual. We were grasslands and savannahs and things like that with more native species.”

Miller says for all the problems the 2012 drought caused, there was a significant drop in foliage diseases because of the lack of moisture. 

It’s possible to slow down the spread of Rhizosphaera Needle Cast with antifungal sprays and by planting trees further apart.  But Hudelson says in the long term, the fungus will force owners to trim off branches.

“People hate me when I say that about Blue Spruce because really what they want kind of in their yard is an outdoor Christmas tree year round, and by pruning out branches, you end up kind of ruining that form," he said.

Rockford’s Christmas tree won’t be downtown long enough for residents to notice the effects of Rhizosphaera Needle Cast. But Hudelson says the fungus is one of the most common diseases he finds at UW’s plant diagnostics clinic.  Because of this, he suggests people consider planting more native trees if they want to preserve their beauty in the long term.