Bands from Weir’s cushion rust visible on infected spruce needles

Yellow spots or bands are visible on infected Serbian and Colorado blue spruce needles. Weir’s cushion rust can cause discoloration and pre-mature needle loss.

May 3, 2019 - Author: , ; , MSU Diagnostic Services; and , MSU Department of Plant, Soil and Microbial Sciences

Yellow bands visible on spruce needles
Yellow bands visible on 1-to-2-year-old needles early in spring and prior to bud break. Photo by Jill O’Donnell, MSU Extension.

Weir’s cushion rust (Chrysomyxa weirii) is found in eastern and western regions of the United States. It can infect most spruce species but is most commonly found on Colorado blue spruce. Unlike other rusts that we find on spruce in Michigan, Weir’s cushion rust does not have an alternative host. This means the fungus can complete its entire life cycle on spruce. The fungus overwinters in needles infected during the previous growing season.

Last summer and fall, needles on last year’s shoots developed yellow spots or bands as a result of this disease. These spots and bands may intensify to give the tree a greenish-gold appearance during spring when tiny, blister-like pustules develop in the yellow areas. At bud break the blisters burst, releasing yellow-orange spores. Splashing and wind-driven rain spread these spores to the emerging needles on the same or adjacent trees.

Orange pustules/spots
Orange pustules/spots visible in late summer. Photo by Keumchul Shin.

The infection period can continue for two to three weeks and the infection rate increases with high moisture. Newly infected needles harbor the rust fungus until bud break the next year, when again blisters containing spores develop, completing the fungal life cycle. Infected needles continue to yellow, turn brown and fall off as spring/summer progress. Spores are not produced on dead needles.

To prevent infections of the new needles, apply fungicides containing chlorothalonil when 10% of the buds have broken. Make two additional applications in seven-to-10-day intervals. Continue fungicide applications until needles are mature or the symptomatic needles have dropped to the ground (approximately three to five sprays total). We have seen damage to the new foliage of Colorado blue spruce from chlorothalonil applications. Check the label to make sure the product you select is labelled for use on blue spruce.

For more information, see “Weir’s Cushion or Spruce Needle Rust: Chrysomyxa weirii“ by Cornell University.

Orange pustules with emerging spores
Orange pustules with emerging spores that will be dispersed by water. Photo by Keumchul Shin.
closeup view of the orange spores
A closeup view of the orange spores that emerge out of the orange pustules. Photos by Keumchul Shin.

Tags: agriculture, christmas trees, msu extension, nursery & christmas trees, pest management, weirs cushion rust


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