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Spruce needle rust

Do your spruce trees look like the neighbors have been sneaking in at night and flocking them for Christmas?

Heavy spruce needle rust has been seen in scattered locations in northern Minnesota the past couple of weeks. Fungal spores produced on the current year needles are sometimes so abundant that trees take on a tan or pinkish cast. So, some of these trees almost look as if they have been flocked for Christmas.

Two rust fungi occur in Minnesota that infect the current year's needles, Chrysomyxa ledi and Chrysomyxa ledicola. Labrador tea is the alternate host for both rusts. White, black and blue spruce can all be infected but this year the heaviest infection seems to have been on blue spruce trees. Each needle is infected individually and the fungus does not spread into the twig.

The rusts overwinter on the leaves of Labrador tea. In early summer a spore that develops on the Labrador tea is wind blown to the spruce trees. The spores infect current year needles of the spruce. By about the first part of August, a orange to yellow spore stage develops on the spruce needles. When very heavy this gives the tree a tan to pinkish cast from a distance. Seventy five percent or more of the current years needles may be infected. Heaviest infections levels often seem to be in the tops of trees. These spores then spread the disease back to Labrador tea. Infected spruce needles turn yellow and drop off the tree in the fall.

Chemical control is usually not necessary. Heavy infections occur when there is wet weather at the same time that the spores produced on the Labrador tea are infecting the spruce needles. It is common to have very heavy infections one year and then have trouble finding any infections at all the next year. Since only current year needles are infected, unless the trees are very small or unless heavy infection occurs for a number of years in a row, trees will normally not be killed.

Needles that are infected now cannot be saved. Fungicides can not kill the infection in the needle. To be effective fungicides would have had to been applied this spring and early summer at the time the infections were occurring. Fungicides have to be in place on the needle at the time of infection so that when a spore lands on the needle it is killed before it is able to infect the needle. This makes control of many foliage fungal diseases difficult. Multiple applications are necessary to keep the needle coated with the fungicide during rainy periods. And to make matters worse, when conditions become rainy enough to get infections, it is difficult to get dry enough weather conditions to make the fungicide application. Fortunately, control is not necessary in most years.

Rhizosphaera needlecast

Rhizosphaera needlecast was found on many Colorado blue spruce trees in the same residential area where an ongoing spruce budworm outbreak has occurred for the past six years near Andover in Anoka County. Many residents in this area have planted Colorado blue spruce trees as ornamentals, which now comprise the second largest number of conifers in the area after white spruce. The intensity of the damage to this species was very severe.