Spruce Needle Rusts

Chrysomyxa ledicola (Peck) Lagerh.
Ceropsora weirii (H.S. Jacks.) Aime & McTaggart (=Chrysomyxa weirii) 

Host(s) in Alaska:

Sitka spruce (Picea sitchensis)
white spruce (P. glauca)
black spruce (P. mariana)
Labrador tea (Rhododendron groenlandicum, formerly Ledum); alternate host

Habitat(s): C. ledicola occurs on current-year spruce needles & Labrador tea leaves;
                   C. weirii occurs on one-year-old spruce needles (no alternate host)
 

Current Status & Distribution in Alaska (2023 Update)

After two years with unremarkable spruce needle rust damage in Alaska, significant damage from Chrysomyxa ledicola was detected on the Kenai Peninsula and in the Matanuska and Copper River Valleys, characterized by bright orange sporulation on the needles. About 8,200 acres of damage were mapped in Southcentral during the aerial detection survey: near the confluence of the Copper and Chinita Rivers (5,700 acres), along the Matanuska-Susitna River between Palmer and Matanuska Glacier State Recreation Site (2,300 acres), and on the southern Kenai Peninsula (200 acres). Aerial surveys occurred before the peak of the outbreak and likely underestimate the extent. In early- to mid-August, there were numerous reports of heavy spruce needle rust throughout the western Kenai Peninsula. Overall, 73 ground detection survey records were made statewide, with endemic levels of disease outside of Southcentral Alaska. Eighteen research grade observations were contributed through iNaturalist from as far southeast as Kake, as far southwest as King Salmon, and as far north as Chatanika near Fairbanks.

Ceropsora weirii, which occurs in the spring, was far more common in 2023 than ever previously noted. The disease was recorded on Sitka spruce at 12 locations in Southeast Alaska near Juneau and on Mitkof, Zarembo, Prince of Wales, and Annette Islands. Though common, it caused trace to low levels of damage to the lower tree crowns. 

Spruce needle rust (Chrysomyxa ledicola) has historically been observed throughout much of Alaska’s spruce forests (Detection Map). This disease rarely results in tree mortality, but whole needle cohorts can be missing on trees and in locations with recent severe disease. Elevated disease levels were noted annually from 2017 to 2020, with a lull in disease activity until this year. Ceropsora weirii, another less common and less damaging spruce needle rust in Alaska, is occasionally observed on one-year-old needles in spring. It has been documented in coastal forests from the Kenai Peninsula to Prince of Wales Island (Detection Map).

Historic Activity

There has been a history of large spruce needle rust outbreaks in Alaska. A major outbreak was aerially mapped across 116,000 acres in Western Alaska in 2019. The disease was also prevalent in 2020 but was not quantified by aerial survey. In 2012, large outbreaks were reported in Lake Clark National Park, Katmai National Park, and the Kenai Peninsula, and notable needle rust also occurred in Southeast Alaska. In 2011, large quantities of rust spores washed ashore near the village of Kivalina in northwestern Alaska. This was somewhat mysterious since spruce is locally uncommon but found upriver, and no ground-checks of spruce could be made at the time. Significant spruce needle rust outbreaks also occurred in 2007 (Southeast Alaska) and 2008 (Interior Alaska). Click here for a summary of historic spruce needle activity and outbreak information from annual Forest Health Conditions in Alaska reports from 1969-2001.

Symptoms, Biology & Impacts

Heavily infected spruce trees have a distinctive orange tinge when the rust is fruiting on the needles in summer. However, sometimes trees are not obviously infected despite abundant orange spores on lake surfaces. Outbreaks are triggered by cool, wet weather in May, when fungal spores from Labrador tea (the alternate host) infect newly emerging spruce needles. Trees damaged by spruce needle rust are seldom killed, since damage is restricted to current-year needles, and the conditions for severe infection usually do not occur in the same location in consecutive years. Infected trees may be stressed or experience growth loss, but these impacts have not been quantified.