Homegrown: Blue Spruce facing fungus threat

The following is by Iowa State University Linn County Extension Master Gardener Intern, Jane Schildroth:

Many people are noticing that Blue Spruce trees are looking stressed this summer.  According to the experts with the Plant & Insect Diagnostic Clinic at Iowa State University Extension, the fungus Rhizosphaera causes an infectious needle disease and Blue Spruce are susceptible.  White Spruce are classified as “intermediate” in their susceptibility.  It is possible this fungus is the cause of what you are seeing on the lower branches of your trees.  It will work its way up gradually. Needles in their second year will turn a purple or brown color and then fall from the tree.  Branches may die after several years of needle loss.  Sometimes the symptoms just appear on one side of the tree, giving it a patchy look.

To be absolutely certain of the diagnosis, you should contact your extension office.  They – or you – will need a magnifying glass to see small black spots in rows on the discolored needles.  These spots are the fruiting structures of the fungus.  Even green needles may show these fruiting structures.  It could also be that the cause of stress in your trees is environmental or site-related rather than the fungus described above.

If Rhizosphaera is present, it will overwinter in infected needles on the tree and the ones that have fallen.  Splashing and dripping water in the spring will spread the fungus so that emerging needles become infected.

A fungicide should be sprayed on the diseased tree – next May and again 4-6 weeks later.  Be sure to follow the specific rate and timing on the instructions if you want to achieve the best results.  The label should specify “for Rhizosphaera needle cast” and will include Daconil 2787, Daconil Ultrex, Terranil 90, Thalonil 4L, Thalonil 90, Manicure Flowable, and Twosome Flowable.

Remember to space trees to allow for good air circulation, mulch and water when needed and avoid shearing the trees when the foliage is wet.