A white spruce tree infected by spruce needle rust fungus, a cosmetic disease that does not kill the tree. Photos by Ned Rozell. (Courtesy Photo / Ned Rozell)

A white spruce tree infected by spruce needle rust fungus, a cosmetic disease that does not kill the tree. Photos by Ned Rozell. (Courtesy Photo / Ned Rozell)

The cause of orange trees in the Alaska Range may surprise you

It was as if someone had pepper-sprayed the Denali Highway.

By Ned Rozell

While wandering middle Alaska this summer, I noticed orange spruce trees along the entire length of the Denali Highway, from Paxson to Cantwell. In what looked like a dendrological case of frostbite, tips of every branch were afflicted with something.

The real show happened when the wind blew: An entire valley glowed apricot. After the wind died, a Tang-like orange powder floated on rivers and puddles. It was as if someone had pepper-sprayed the Denali Highway.

I suspected an insect outbreak — maybe the orange dust was millions of little eggs laid on spruce branches — but insect expert Derek Sikes of the University of Alaska Museum of the North said bugs were not to blame.

It was a tree disease known as spruce needle rust, which infects only the current year’s needles of white, black and Sitka spruce trees.

[River Piracy strikes the Yukon]

The orange powder is composed of millions of tiny spores, which the rust fungus uses to reproduce. Paul Hennon, an expert on forest diseases, wrote about spruce needle rust fungus in a 2001 bulletin for the Alaska branch of the USDA Forest Service.

Those orange spores rely on the wind to spread them to another organism, Labrador tea. The spores need the other plant to complete the spruce needle rust fungus’s life cycle.

Spores of the spruce needle rust fungus on needles of white spruce trees growing off Alaska’s Denali Highway. (Courtesy Photo / Ned Rozell)

Spores of the spruce needle rust fungus on needles of white spruce trees growing off Alaska’s Denali Highway. (Courtesy Photo / Ned Rozell)

Labrador tea emits a smell of turpentine when you walk through it. The knee-high plant is the “alternative host” for the spruce tree rust fungus. Spruce trees and Labrador tea plants are two life forms the fungus cannot do without.

Here’s the part of the life cycle I witnessed: Liberated by wind from spruce needles, some lucky spores land on Labrador tea leaves.

There, those particles generate other spores, which also settle on top of Labrador tea leaves. These new specks spread to the leaves of neighboring Labrador tea plants. The rust fungus then settles down, overwintering on the leaves.

This map shows the location of Denali Highway, where orange spruce trees can be seen. (Courtesy Image / Ned Rozell)

This map shows the location of Denali Highway, where orange spruce trees can be seen. (Courtesy Image / Ned Rozell)

Next spring, another type of spore will morph from the ones that survived the winter on the surface of Labrador tea leaves. The new spores will perhaps catch a favorable wind to infect the emerging, succulent needles of spruce trees.

This interesting life form that enlists two others for its survival had optimal conditions — wet and cool — in summer 2020.

While the rust fungus inhibits the work of new spruce needles — which produces the sugars that feed the tree — the fungus does not kill them. Hennon wrote that the damage to trees is mostly cosmetic, and Alaska foresters have noticed that outbreaks rarely last more than a few years.

• Since the late 1970s, the University of Alaska Fairbanks’ Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute.

More in News

In this July 13, 2007, file photo, workers with the Pebble Mine project test drill in the Bristol Bay region of Alaska, near the village of Iliamma. (AP Photo / Al Grillo)
Pebble developer files appeal with Army Corps

The Army Corps of Engineers rejected Pebble Limited Partnership’s application in November.

This August 2019 photos shows a redline at Treadwell Arena designed by Tsimshian artist Abel Ryan. The arena is adding new weekly events to its schedule, City and Borough of Juneau announced. (Michael Penn / Juneau Empire File)
Treadwell Arena adds new weekly events

Hockey and open skate are on the schedule.

This 2020 electron microscope image provided by the National Institute of Allergy and Infectious Diseases - Rocky Mountain Laboratories shows SARS-CoV-2 virus particles which causes COVID-19, isolated from a patient in the U.S., emerging from the surface of cells cultured in a lab. On Monday, Oct. 5, 2020, the top U.S. public health agency said that coronavirus can spread greater distances through the air than 6 feet, particularly in poorly ventilated and enclosed spaces. But agency officials continued to say such spread is uncommon, and current social distancing guidelines still make sense. (NIAID-RML via AP)
COVID at a glance for Friday, Jan. 22

The most recent state and local numbers.

A Coast Guard Station Juneau 45-foot Response Boat-Medium patrols Auke Bay during an exercise in 2018. A response boat similar to the one in the photo was struck by a laser near Ketchikan on Saturday, Jan. 17, prompting an investigation into the crime. (Lt. Brian Dykens / U.S. Coast Guard)
Coast Guard wants information after laser pointed at boat

“Laser strikes jeopardize the safety of our boat crews…”

This 2020 electron microscope image provided by the National Institute of Allergy and Infectious Diseases - Rocky Mountain Laboratories shows SARS-CoV-2 virus particles which causes COVID-19, isolated from a patient in the U.S., emerging from the surface of cells cultured in a lab. On Monday, Oct. 5, 2020, the top U.S. public health agency said that coronavirus can spread greater distances through the air than 6 feet, particularly in poorly ventilated and enclosed spaces. But agency officials continued to say such spread is uncommon, and current social distancing guidelines still make sense. (NIAID-RML via AP)
COVID at a glance for Thursday, Jan. 21

The most recent state and local numbers.

Gov. Mike Dunleavy addresses the public during a virtual town hall on Sept. 15, 2020 in Alaska. ( Courtesy Photo / Austin McDaniel, Office of the Governor)
Dunleavy pitches dividend change amid legislative splits

No clear direction has emerged from lawmakers.

Joar Leifseth Ulsom, right, wearing a bib with ExxonMobil lettering on it, congratulates Peter Kaiser on his win in the Iditarod Trail Sled Dog Race in Nome, Alaska. The world’s most famous sled dog race has lost another major sponsor as the Iditarod prepares for a scaled-back version of this year’s race because of the pandemic, officials said Thursday, Jan. 21, 2021. ExxonMobil confirmed to The Associated Press that the oil giant will drop its sponsorship of the race. (Marc Lester / Anchorage Daily News)
ExxonMobil becomes latest sponsor to sever Iditarod ties

The world’s most famous sled dog race has lost another major sponsor.

Has it always been a police car? (Michael Penn / Juneau Empire)
Police calls for Friday, Jan. 22, 2021

This report contains public information from law enforcement and public safety agencies.

Most Read