Blue Spruce Problems in Montana Landscapes

Why do blue spruce decline in Montana yards?

Mostly from stress that builds over decades. Montana State University Extension's article on common conifer issues makes the point that most conifer problems are not caused by an insect or disease at all but by growing conditions or weather. The diseases that do attack spruce here are largely opportunists. MSU describes cytospora canker as a disease that mainly affects stressed trees, says sudden needle drop only affects stressed trees, and says spider mites respond to stressed trees.

The Montana DNRC nursery rates blue spruce as highly drought tolerant and increasingly so as it matures, with only fair tolerance of alkaline soil. In a yard the stresses are usually crowding, sprinklers wetting the foliage, lawn competing for water, and dry open winters. The sections below take the specific problems one at a time.

Why are the lower branches dying?

Lower-branch dieback on spruce has four common explanations, and they overlap. Shade is the simplest: branches at the bottom of a dense tree, or pressed against a building or a neighboring tree, lose needles because they no longer get light. Needle cast diseases start on the lower branches. Cytospora canker kills lower branches first. Spruce spider mite damage is concentrated on older needles in the lower part of the tree.

The details separate them. Look at whether the newest needles at the tips are still green, whether there is resin on the branch, and whether dead branches are scattered or progress upward. An accurate diagnosis matters because the responses are different, and one of these causes has no spray at all.

What is needle cast and how do you recognize it?

Needle cast is a group of fungal diseases that kill older needles and cause them to drop. MSU Extension describes Rhizosphaera needle cast on Colorado blue spruce this way: in spring or early summer the older, innermost needles turn brown to purple while the younger needles at the branch tips stay green. Infected needles fall in late summer or fall, about 12 months after infection, and branches thin over time. With a hand lens, rows of tiny black dots can be seen emerging from the needle pores.

Stigmina needle cast looks very similar, with fuzzy black fruiting bodies and needles dropping one to two years after infection. MSU also describes sudden needle drop, in which older needles yellow or brown and are shed by fall on branches scattered through the tree. All three are spread by wind and splashing water in mild, wet weather, so a spruce hit by lawn sprinklers every morning is an ideal host.

What does cytospora canker look like?

It kills whole branches rather than individual needles. MSU Extension describes cankers on infected spruce branches that are often covered in a whitish to bluish resin. The fungus girdles the branch from within, the needles beyond the canker yellow or brown, and the branch dies. Branches appear to die in an upward-progressing spiral.

The key fact is in MSU's conifer article: fungicides are not recommended for cytospora. The management is to prune out and destroy affected branches, in dry weather, disinfecting tools between cuts, and to support the tree's vigor with adequate water and nutrients. Pruning removes the dead wood and some of the fungus. It does not make a stressed tree resistant.

Are spider mites the problem?

Sometimes. Spruce spider mite feeding gives needles a grayish cast with fine flecking, and MSU Extension says heavier infestations cause early needle drop, with fine silk sometimes visible. The mites usually attack older needles low in the tree.

The timing surprises people. MSU describes spruce spider mite as a cool-season pest that is dormant in the warmer months, and puts the best treatment windows in May and again in September. It recommends miticides, horticultural oil or insecticidal soap over broad-spectrum insecticides, which kill the predators that keep mites in check. MSU also cautions elsewhere that some oils can discolor spruce needles. With any product, the label directions are the law, and restricted-use products need a licensed applicator.

Galls, dead tops and other spruce pests

Brown, cone-like growths on the branch tips are Cooley spruce galls, formed by an aphid-like insect. MSU Extension says the galls rarely harm the tree, and that once they have turned brown the insects have already left, so cutting them off accomplishes nothing but tidiness.

A leader that suddenly wilts and dies in early summer, curling into a shepherd's crook, is white pine weevil, which attacks spruce despite its name. MSU's non-chemical fix is to clip out the infested top before the end of July, while the larvae are still inside, destroy it, and train a side branch as the new leader. MSU also lists budworm and engraver beetles among spruce pests. Those are covered in the guides to western spruce budworm and pine engraver beetles.

How do drought and winter affect spruce?

Winter injury is the common one in the Missoula and Bitterroot valleys. MSU Extension explains that it happens when there is minimal snow cover and low humidity over winter, causing conifers to dry out. Needles turn red, yellow or brown in spring, especially at the tips and on the south and southwest sides of the tree.

MSU's prevention is water: thorough watering during dry periods in the growing season and into the fall, and irrigating evergreens well before the ground freezes. For drought generally, it recommends slow, deep watering, removing competing grass from under the canopy, and three to four inches of mulch out to the canopy edge. Aim the water at the soil, not the needles. The guide to watering trees through drought covers how much and how often.

How much space does a spruce need?

Far more than the nursery pot suggests. MSU Extension's identification guide lists Colorado blue spruce as a large tree, 50 to 150 feet tall, with stiff horizontal branches. Good spacing between trees to improve air circulation appears in MSU's management advice for every one of the spruce needle diseases.

That leaves owners of crowded spruce with a few choices: remove every other tree in a tight row so the rest can dry out, raise the lowest dead limbs to open air under the canopy, or accept bare lower trunks as the price of the screen. What does not work is shearing the sides or topping the tree to fit the space. Where spruce stand close to a house in fire country, spacing and limb height are also a fire mitigation question.

Can a declining spruce be saved?

It depends on how much live crown is left and what is killing it. A spruce with needle cast on the lower third and healthy growth above is a reasonable candidate. MSU's program is sanitation, dry foliage, better spacing, and protective fungicide in spring when new needles are half grown and again three to four weeks later, with its conifer article recommending treatment for two consecutive years. MSU is clear that fungicides only protect new needles and cannot cure infected ones, so bare branches stay bare.

A spruce with cytospora through much of the crown, a dead top, or more dead than live branches is not coming back to the tree you remember. If it is large and close to the house, removal is often the better use of money than years of treatment. Our guide to the signs a tree is dying helps with that judgment.

A tree health and risk assessment will identify what your spruce has and whether it is worth treating. It is one of our tree services in Missoula and Ravalli counties, and more species guides are in the Montana Tree Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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