WEIR'S CUSHION RUST ( Chrysomyxa weirii )
Several species of fungi in the genus Chrysomyxa cause rust diseases (leading to defoliation) of spruce needles. Of those, the one of most concern to Christmas tree growers is Chrysomyxa weirii, causing a disease known as Weir's cushion rust. The pathogen is native to the forests of the western U.S. where it causes ocassionally significant, but rarely lethal, defoliation. However, in the last 10 years or so, the fungus has been reported from sites in Vermont, New York, and Pennsylvania. This disease is of greatest concern on Colorado blue spruce where it causes discoloration and pre-mature shedding of year-old needles.
Fig. 1. Blister-like fruiting bodies sporulating on blue spruce needles.
The fungus overwinters on yellowed portions of needles infected the previous year. As spring weather warms, blisters begin to develop in those infected areas and waxy, golden-yellow pustules of spores emerge. They, in turn, produce other spores which can be blown by wind to new needles emerging from the buds. Infection occurs shortly thereafter, and diseased tissue eventually develops yellow spots that will serve as overwintering sites for the next year's generation of spores. Good control of the disease can be had with several applications of a registered fungicide in the spring, during the sporulation period.
Fig. 2. Photo of aecia on blue spruce needles. Note that buds are swollen, but have not yet broken.
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