Blue Spruce: Common Health Issues in the Landscape
Blue spruce (Picea pungens), also known as Colorado spruce, is commonly planted in landscapes for its attractive blue color and conical form. Although the blue color is popular and common with cultivars, the green form is more common in the wild. Native to high elevation forests of the intermountain U.S., blue spruce is hardy in zones 2 to 7. When young, plants have a conical form with stiff, horizontal branches all the way to the ground. The form becomes more open with age. Landscape plants typically grow slowly to 30-60’ tall and 10-20’ wide, but can get larger. The stiff, prickly needles range in color from gray-green to blue green and are spread around the stem. Needles are four-sided with six stomatal lines on each side. Cones are cylindrical but narrow slightly at each end. Young cones are green with a purple bloom (coating) and become brown with age. Cone scales are wavy, oval, and jaggedly toothed.
Cultural Considerations & Environmental Stresses
Blue spruce prefer an organic, moist soil in full sun but are adaptable to different conditions. It is more drought tolerant than other Picea species. While blue spruce can tolerate some shade, disease incidence and severity increases with the amount of shade trees receive.
Blue spruce does better in cooler, more northern climates. In USDA hardiness zone 7, blue spruce struggles with high nighttime temperatures. Although tolerant of short-term drought, water stress is one of the primary predisposing factors that facilitates disease development. Deep planting and the subsequent development of girdling roots can be a major issue for some landscape trees. Container-grown trees should have their roots thoroughly distressed prior to planting. Ensure the flare is exposed and is level or slightly above the soil line and provide supplmental water regularly during the first few years following transplanting.
Rhizosphaera needle cast of spruce, caused by Rhizosphaera kalkhoffii, can be a destructive disease to blue spruce in the region. In addition, white and Norway spruce can also be severely affected, while on occasion, other landscape spruces (e.g. Oriental and Serbian) also suffer from the disease. Rhizosphaera primarily initiates new infections on susceptible trees during wet and mild periods in the spring and early summer. However, the fungus can attack susceptible needles at any point in the growing season and a second wave of infection also occurs in autumn. Rhizosphaera invades the needles through the stomata (pores used for gas exchange), but symptoms and signs of infection typically may not appear until one year after infection. This latent period between infection and symptom development is typical of many needle disease pathogen. When trees are stressed, by drought for example, symptoms may develop more quickly. As a result, needles can be infected but appear green and healthy during much of the growing season. Symptoms of infection include browning and premature shedding of needles, especially on interior branches lower in the canopy where moisture and shade are more abundant and humidity levels are higher. Infected needles often become purple to brown before being shed from the canopy. Small, black-colored spore-bearing structures emerge through the stomata and may be readily visible with a hand lens. They typically develop on diseased needles in the spring and early summer after mild and wet periods but can also be abundant during the autumn months as well. Needle cast diseases do not result in mortality by themselves. However, when needles are shed prematurely, the tree has reduced resources for growth, maintenance and defense against opportunistic pests and pathogens. As such, Rhizosphaera needle cast is an important contributor to decline and death of blue spruce in landscape settings.
Stigmina needle cast, caused by Stigmina lautii, is not believed to be as widespread or damaging as Rhizosphaera needle cast, but increasingly, the disease is associated with declining spruce in landscape settings. Just like Rhizosphaera, Stigmina is found on several species of spruce, but blue spruce seems especially susceptible to infection. The fungus behaves very similarly to Rhizosphaera and is easily confused. One distinguishing characteristic is that Stigmina can be found sporulating on green needles, whereas Rhizosphaera will produce spores only on needles that have become purple/brown first. These fruiting bodies appear as a small black-colored dust on the surface of the needles. To date, it appears that Stigmina is not as aggressive as Rhizosphaera but much remains to be understood. Symptoms appear as early needle drop on interior branches in the lower canopy and older needles becoming brown, especially later in the summer. Rhizosphaera and Stigmina can be found co-occurring on the same tree, confusing our understanding of the actions of Stigmina alone, along with several other insect pests and fungal pathogens. However, based on samples submitted to the UMass Plant Diagnostic Lab, Stigmina is clearly becoming more prominent pathogen in our region on declining blue spruce and warrants further study.
Cytospora canker, caused by Cytospora kunzei, is most often found on blue spruce, but has a broad host range among conifers. While Cytospora invades both young stems and large branches, it is most often found on branches >1–2” in diameter. Infection results in the formation of small, eruptive cankers. As infections expand, the stem cankers coalesce, killing the cambium tissue. While the underlying sapwood is also colonized, it may not show symptoms of infection until advanced stages. Infections by Cytospora leads to stunted growth of stems and needles, yellowing of needles, early needle drop and branch dieback. A prominent symptom of infection on spruce is the presence of hardened resin that has oozed from cankered branches. The cankers often have rough callus tissue on the margins of the infection site. Signs of infection include very small, black-colored fruiting structures that rupture through the bark. They are typically most conspicuous on the margins of symptomatic cankers but may not present. Like many cankering fungi, Cytospora is an opportunistic pathogen, taking advantage of stressed trees. Infections usually begin on older branches towards the base of the tree and gradually spread upward in the canopy. Shade and free moisture favor the pathogen and during extended periods of wet weather in late spring and early summer, Cytospora producesspores that are spread by wind and splashing rainwater. Bark wounds created by strong winds, snow/ice loading, insect feeding and mechanical wounds provide infection sites for dispersed spores. Stresses that helps to facilitate infection include drought, needle cast infections, spider mite infestations, root compaction and girdling/circling roots.
Phomopsis canker, caused by a variety of Phomopsis species, is a common disease on a wide array of woody plants. Increasingly, Phomopsis can be found on declining blue spruce in landscape settings. The fungus typically becomes established on current year's shoots, or it invades stressed or weakened plant parts through some type of wound. In comparison to Cytospora canker, Phomopsis canker is most often found on small-diameter stems and shoots <1” in diameter but will sometimes attack larger branches. Needle cast caused by Rhizosphaera and Stigmina is often associated with infections as it feeding by the spruce spider mite. Phomopsis can also enter buds and slowly progress down the stems, resulting in sporadic crown dieback progressing from the tips inward. Numerous abiotic stresses can favor infection such as transplant shock, rough handling of nursery stock, drought stress, freeze/heat injury and insect feeding. Like many other cankering fungi, Phomopsis produces an abundance of very small cankers, from which pads of fungal tissue erupt to release large volumes of microscopic spores. Spores are washed or blown to nearby stems, creating new infection centers. Overall, Phomopsisis an opportunistic pathogen that is widespread in forest and landscape settings, making eradication unfeasible.
Spruce needle rust (also known as Weir’s cushion rust), caused by Chrysomyxa weirii, is a minor but sometimes highly conspicuous disease of blue spruce. While blue spruce is the primary host, many other spruce species (e.g. Oriental spruce) are also susceptible. The primary symptoms exhibited are needle blisters that appear brown at first, becoming to orange to red in color as they mature, surrounded by yellowing tissue on the needle surface. The blisters swell during the spring (mid-April to mid-May, sometimes earlier) and rupture to release spores that infect needles on adjacent branches or nearby trees. Spores are dispersed by wind and rain to infect newly emerging needles on adjacent branches or to nearby trees. Wet weather will prolong the spore production period. Newly infected needles may not show symptoms until the following winter or spring. Rarely are trees severely affected by spruce needle rust but the disease is often very conspicuous. However, a large buildup of the pathogen can weaken the tree and detract from its aesthetic appearance.
Insect and mite pests of spruce that may be encountered include but are not limited to: bagworm, balsam twig aphid, bark beetles, Cooley spruce gall adelgid, eastern spruce gall adelgid, gypsy moth, spruce bud scale, spruce spider mite, and white pine weevil.
Defoliators such as bagworms (Thyridopteryx ephemeraeformis) and gypsy moths (Lymantria dispar) can be pests of conifers, including blue spruce, if populations are high. Bagworms are not historically an outbreak species in Massachusetts, however local, patchy increased populations can have a significant impact on individual plantings/specimens in the landscape. While spruce are also not a favored host for gypsy moth caterpillars, when outbreak populations occur, this insect has been observed feeding on blue spruce needles when favored hosts (ex. oak) are defoliated. Other defoliating insects, such as certain sawflies, may be occasional pests of Norway spruce.
Piercing-sucking insect pests of spruce include the balsam twig aphid, Cooley spruce gall adelgid, eastern spruce gall adelgid, and spruce bud scale. The balsam twig aphid (Mindarus abietinus) has a complex life cycle which includes three distinct forms of aphids (in addition to eggs) per generation. It has been recorded on Colorado blue spruce, white spruce, as well as fir and juniper. The balsam twig aphid produces copious amounts of honeydew, so much so that in high populations needles may stick together. Balsam twig aphid feeding also twists and curls needles, which remain distorted for as long as they adhere to the tree. Needle stunting and distortion may be most apparent on terminal twigs of new growth.
The Cooley spruce gall adelgid, Adelges cooleyi, is a native insect that also has a complex life cycle. It has at least five different morphological forms, and requires 2 years and two hosts to complete its normal life cycle. Galls (pineapple shaped/cone-like and at the tips of twigs) are produced on Colorado blue spruce, Engelmann, Sitka, and Oriental spruce and cause needle injury (yellow spots and distortion) to Douglas-fir. Eastern spruce gall adelgid (Adelges abietis) is a primary pest of Norway spruce, however on occasion it will damage Colorado blue, white, and red spruce. This insect is non-native, and was introduced into the United States from Europe before 1900. Galls are small, sometimes pineapple shaped/variable, but produced on the basal portion of the shoots, such that the twig extends beyond the gall. Twig dieback may occur.
Spruce bud scale (Physokermes hemicryphus) is a pest of various types of spruce, most notably Alberta and Norway spruce. Mature scales are globular, reddish brown, and found in clusters of 3-8 at the base of new twig growth. They so closely resemble buds of the host plant that they are very easily and often overlooked. Lower limbs are more likely to be infested and when populations are high, lower branches may be killed. Weakened trees (trees with predisposing stressors) may support higher numbers of these scales than healthy trees.
Various species of bark beetles (Ex. Ips spp.) that attack spruce are attracted to trees that are stressed due to other factors such as drought, site characteristics, etc. Different species may attack different plant parts, for example branches, upper trunk, or lower trunk.
The white pine weevil (Pissodes strobi) is often the most frequently discussed weevil pest of eastern white pine, however this insect can also be destructive of blue spruce. White pine weevil adults chew holes to feed on leaders near terminal buds causing pitch flow. Eggs are laid in these holes and hatch. Larvae bore into the leader, causing stunting, flagging, and dieback.
The spruce spider mite (Oligonychus ununguis) is an insect relative that sucks the chlorophyll from spruce needles, leaving tiny yellowish flecking or stippling. If infestations are high enough, entire needles can turn yellow and eventually brown. Small trees may be killed and larger trees can suffer dieback. Action is not typically recommended, however, unless damage impacts 10% or more of the needles and beneficial mites or predatory beetles are not found. Broad spectrum chemical management options that also kill beneficial mites and insects may lead to subsequent increased spruce spider mite populations.
Common Cultivars (>200 cultivars are known)
· ‘Fat Albert’ – bright blue needles, 15’ tall
· ‘Glauca Pendula’ – blue-needled, sprawling, spreading form
· ‘Hoopsii’ – strong blue color, pyramidal
· ‘Moerheimii’ – dense, 30’ tall, irregular when young, good blue color
· ‘Montgomery’ – silver blue needs, global shape
· ‘Thompsenii’ – very thick, pyramidal tree, silver-blue foliage, one of the best cultivars
· ‘The Blues’ – weeping and irregular habit with bright blue needles
- Canaan fir (Abies balsamea var. phanerolepis): Very dense growth; susceptible to deer injury
- White fir (Abies concolor): Susceptible to deer injury
- Korean fir (Abies koreana): Prefers cool and moist conditions; susceptible to deer injury; harder to find
- Alaska yellow-cedar (Chamaecyparis nootkatensis): Weeping forms mostly available; subject to winter burn in exposed settings
- Norway spruce (Picea abies): Highly adaptable, tolerates some shade; various pest/pathogen issues
- White spruce (Picea glauca): Susceptible to many of the same pests/pathogens as blue spruce; salt tolerance
- Serbian spruce (Picea omorika): Not tolerant of strong winter winds; slow growth rate
- Oriental spruce (Picea orientalis): Slow growth rate; susceptible to winter burn
- Swiss stone pine (Pinus cembra): Slow growth rate; harder to find
- Korean pine (Pinus koraiensis): Slow growth rate; similar health issues to eastern white pine
- Douglas-fir (Pseudotsuga menziesii): Does not tolerate extreme heat or drought; must be sited in full sun
- Carolina hemlock (Tsuga caroliniana): Excellent shade tolerance; susceptible to hemlock woolly adelgid and elongate hemlock scale