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Check Spruce Trees for Rhizosphaera Needle Cast

Spruce buds opening. M. Grabowski, UMN Extension
M. Grabowski, UMN Extension

Spring is here and spruce trees are opening their buds and developing young new needles. Now is a critical time to inspect spruce trees for Rhizosphaera needle cast, a common disease of spruce trees in Minnesota.  Rhizosphaera needle cast is caused by the fungus Rhizosphaera kalkhoffii. This fungus infects the young growing needles of Colorado blue spruce, white spruce and Norway spruce. New infections do not have obvious symptoms and many gardeners do not recognize the disease until it is too late to prevent new infections this year.

What to look for
Discolored needles with black spore producing structures. USDAFS
Examine the fully grown needles at the branch tips. These needles were new needles last year and if infected, will be developing symptoms now. Look for discoloration of any kind. Needles should be dark green to blueish green. Needles that look pale yellow green, brown or purplish brown are unhealthy. Use a hand lens to examine the needles. Tiny raised black dots on the needles are suspicious and may be fungal spore producing structures. Spore producing structures may be found on green needles or discolored needles. If needle discoloration or potential spore producing structures are observed, contact an arborist or send a sample to the UMN Plant Diagnostic Clinic to determine if Rhizosphaera is the cause of the problem.

Help for sick trees
Spruce Needles at half their mature length. M. Grabowski
If a spruce tree is infected with Rhizosphaera Needle Cast it can be helped, but gardeners will need to act quickly to protect this year's needles. The fungal pathogen overwinters on previously infected needles. Spores are spread from last year's infected needles to new developing needles by splashing rain or irrigation. A fungicide with the active ingredient Chlorothalonil can be applied to protect needles but timing is critical. The first spray should be applied when new needles are half the length of mature needles. A second application should be made 3 to 4 weeks later. This will protect the needles from infection. Fungicide sprays should be repeated next year, but beyond that trees will just need to be inspected to determine if sprays are needed. Many trees recover from Rhizosphaera needle cast after 2 years of treatment.

If left untreated, infected needles will die and fall off after 1 year. If a branch looses its needles 3-4 years in a row, the branch dies. Untreated trees often have dead branches on the lower part of the tree and infected branches higher up.

Preventing future problems
Needle loss from Rhizosphaera needle cast, M. Grabowski UMN
Gardeners can help keep spruce trees healthy with several simple practices. Rhizosphaera needle cast is most problematic on stressed trees, so reducing stress will reduce disease problems. Remove all weeds and turf grass from around the base of the tree. Add a level 2-4 inch layer of woodchip mulch around the base of the tree to keep moisture in the root zone, reduce competition with weeds, and prevent accidental damage from lawn mowers and weed whips. Water the tree during periods of drought with a soaker hose or garden hose directed at the base of the tree. Redirect lawn sprinklers to keep needles dry. Spores of Rhizosphaera need splashing water and moisture to start new infections.

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