Weir’s cushion rust visible on infected spruce needles
Orange, blister-like fruiting bodies are visible on infected Serbian and Colorado blue spruce needles. Weir’s cushion rust can cause discoloration and pre-mature needle loss.
Weir’s cushion rust (Chrysomyxa weirii) is found in eastern and western regions of the United States. It can infect most spruce species, but it is most commonly found on Serbian and 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 the disease. These spots and bands may intensify to give the tree a greenish-gold appearance during the spring, and tiny, blister-like pustules develop in the yellow areas. At bud break the blisters burst, releasing yellow-orange spores. Wind and splashing rain spread these spores to the emerging needles on the same or adjacent trees. 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 and summer progress. Spores are not produced on dead needles.
To prevent infections of the new needles, fungicides containing chlorothalonil should be made when 10 percent of the buds have broken, and two additional applications should be made in seven- to 10-day intervals. Fungicide application should continue 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. Michigan State University Extension advises growers to 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” from Cornell University.