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Contents

Needle Diseases of Spruce Trees

• Rhizosphaera Needle Cast
• Lirula Needle Blight
• Spruce Needle Rust
• Two New Fungi Associated with Spruce Needle Loss

Branch Diseases of Spruce

• Cytospora Canker
• Eastern Spruce Dwarf Mistletoe

Root Rot Diseases of Spruce

• Tomentosus Root Rot
• Armillaria Root Rot

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Needle Diseases of Spruce Trees

Spruce needles can be infected with several different fungal pathogens. Many needle diseases result in needle death and needle drop. It is important for the gardener to understand that spruce needles will not grow back after being lost to a fungal infection like the leaves on many deciduous trees will. New needles are only produced on spruce trees once a year on the tips of branches. As a result spruce trees with a severe needle disease often appear thin or discolored. Needle retention and color is important to the tree for food production and tree vitality. Branches that have lost needles for 3-4 years in a row often die. If the disease is properly identified and treated before branch damage occurs, however, the spruce will gradually regain its needles and appear full again.

Fungal pathogens thrive in moist conditions, and a gardener can reduce the severity and the spread of many needle cast diseases by reducing moisture on the spruce needles. Redirect lawn sprinklers so that water does not hit the spruce needles. When watering the spruce tree itself, use drip irrigation or direct the water at the soil, not the canopy of the tree. If application of water to the needles cannot be avoided, applications should be done early in the day so that needles can dry before evening. Shearing spruce trees creates a dense, compact growth that stays wet longer, and should be avoided when possible. All weeds and tall grasses crowding the tree should be removed to provide better air circulation around the tree.

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Rhizosphaera Needle Cast

This disease is caused by the fungus Rhizosphaera kalkhoffii and is probably the most common needle disease in Minnesota.

Colorado blue spruce, Picea pungens, is highly susceptible to this disease. White spruce (including Black Hills spruce), P. glauca, is intermediate in susceptibility and Norway spruce, P. abies, is relatively resistant. Trees that are stressed from drought, poor planting practices, or other factors are more likely to suffer from Rhizosphaera needle cast.

Identification

• Infected needles may look yellow and mottled by mid to late summer. 
• Infected needles turn brown or purplish brown by late winter or early spring.
• Newly growing needles in the spring do not show symptoms. Needles closest to the trunk of the tree (the older needles) are often discolored while the needles at the tips of the branches remain green.
• With a magnifying glass, look for tiny black spots arranged in neat rows on infected needles. These are pycnidia, fungal spore producing structures. These may be confused with pycnidia of Stigmina lautti, which are very similar in appearance.
• Infected needles typically fall off in the summer, 12-15 months after the initial infection. Infected trees have thin canopies.
• Damage typically starts on the lower branches and moves up the tree.
• After 3 to 4 years of severe infection the lowest branches may begin to die.

Biology

The pathogen overwinters on living and recently killed needles. Spores, called conidia, are dispersed by splashing water spring through early autumn. New needles on the lower branches are most commonly infected but if conditions are very favorable for infection (extended periods of moisture on the needles at 77° F) any needles can become infected.

Management

• Plant Norway or Black Hills spruce instead of Colorado blue spruce or Engelmann (P. engelmannii) spruce. Whenever possible plant spruce trees grown from local seed sources as these plants are likely to be best adapted to the local conditions.
• Avoid planting young spruce near old spruce trees that may be harboring fungal pathogens.
• Reduce stress on spruce trees by watering during periods of drought, mulching the soil around the tree, etc.
• Do not allow lawn sprinklers to spray the spruce needles.
• Space spruce trees to allow good air circulation around the trees.
• Do not shear spruce as shearing creates a dense, compact growth that stays wet longer.
• Chlorothalonil can be sprayed twice in the spring to protect new needles. The first spray should be applied when needles are half the length of the mature needles. A second spray should be applied 3-4 weeks later or as prescribed on the fungicide label. Read and follow all instructions on the label when applying a fungicide!
• Before spraying fungicide, confirm that Rhizosphaera is the fungal pathogen causing damage by sending a lab sample to the University of Minnesota plant disease diagnostic clinic. Several other fungi result in symptoms very similar to Rhizosphaera.
  

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Lirula Needle Blight

Lirula needle blight is caused by the fungus Lirula macrospora.

This disease can occur in Colorado blue spruce, Picea pungens, and white spruce, Picea glauca. White spruce, including Black Hills spruce, is the more susceptible species.

Identification

• In late summer second year needles become discolored, yellow to brown.
• In the following year (third year needles) a raised black line forms along the midrib of infected needles, mostly on the lower surfaces.
• In some cases horizontal black bands form across infected needles.
• Needles on lower branches are commonly infected first.
• Infected needles turn grayish brown after spores are released but remain attached to tree for several years.

  
  Photo by P.Hennon USDA FS, Bugwood.org

  Click photo for larger image

Biology

It takes several years for Lirula macrospora to complete its lifecycle. The fungus overwinters in infected needles on the tree. During late spring through midsummer infected needles release spores that spread in splashing and dripping rain to new needles. First year needles appear to be the most susceptible. On second year needles, a raised black line, which is a spore producing structure created by the fungus, forms on the midrib on the underside of the needle. Spores are released from these structures on third year needles.  

Management

• Avoid planting spruce where Lirula needle blight already occurs.
• Inspect new seedlings and trees for signs of infection prior to purchase.
• Apply Chlorothalonil or Bordeaux mixture when new needles are half the length of mature needles and a second time, one month later. Since the disease cycle is 3 years long, fungicide application must occur for 3 consecutive years to be effective.
• Remove the most heavily infected trees in the landscape to protect neighboring spruce trees.
  

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Spruce Needle Rust

This disease is caused by fungi from several species of the genus Chrysomyxa.

Colorado blue spruce, Picea pungens, white spruce, P. glauca, and black spruce, P. mariana, are all susceptible to the disease. Norway spruce, P. abies, is only occasionally infected.

Identification

• Needles at the tips of the branches (current year needles) turn yellow
• Pale orange to white tube-like projections appear on infected needles in July or August and release powdery orange spores
• Severely infected trees may have a tan to pinkish cast
• Infected needles fall off in September
• Some species of rust can cause witches’ brooms, a clump of small weak branches arising from one point on a large branch

Biology

The spruce needle rust fungi live half of their life cycle on spruce needles and the other half on an alternate host in the Ericaceae family. In Minnesota labrador tea (Ledum groenlandicum), leather leaf (Chamaedaphne calyculata), and bearberry (Arctostaphylos uva-ursi) can all serve as alternate hosts for spruce needle rusts. In the spring, spores are released from the alternate host plant and infect young growing spruce needles if cool wet weather is present. In the summer months, these infected needles develop spore producing structures that release orange powdery spores. These spores can only infect the alternate host plant and do not reinfect the spruce. Infected needles often turn yellow and in most cases, fall off at the end of the growing season. The fungus that causes witches’ brooms in addition to needle infection can overwinter within the infected spruce branches. All other spruce needle rust fungi overwinter in the alternate host plant.

Management

• In most cases spruce needle rust is a cosmetic problem and no management is needed.
• Reduce moisture on needles by redirecting lawn sprinklers away from spruce trees and spacing spruce trees to allow good air circulation around the tree.
• If witches’ brooms are present these can be pruned out and destroyed.
• Removal of all alternate host plants within 1000 ft will reduce disease on spruce but is often impractical.
  

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Two New Fungi Associated with Spruce Needle Loss

In recent years two new fungi have been found on spruce trees with needle discoloration.

Trees with symptoms and signs almost identical to Rhizosphaera needle cast were often found to be infected with the fungus Stigmina lautii. In fact it requires magnification of the fungal spore producing structures on the needles to differentiate between Rhizosphaera needle cast and trees infected with Stigmina lautii. This is best done by sending a sample to the University of Minnesota Plant Disease Diagnostic Clinic.

In addition many spruce trees have been recorded with a disorder known as SNEED or spruce needle drop. These trees show yellowing and eventually browning of older needles. By the end of summer, all of the needles on affected branches fall off except the newest needles on the tips of the branches. Although the discolored needles themselves show no signs of fungal infection, the twigs are often covered with tiny black pimple-like spore producing structures of the fungus Setomelanomma holmii. Branches affected by needle drop may be scattered throughout the tree or the entire tree may be affected.

Both fungi have been found on Norway, white and Colorado blue spruce trees. Unfortunately we know very little about the fungi Setomelanomma holmii and Stigmina lautti. These fungi may be new pathogens or they may simply be fungi taking advantage of a tree stressed by drought, heat, poor planting or other environmental factors. Although research on these two fungi is continuing, there are currently no management recommendations for either condition.

The general guidelines listed above about reducing moisture on the spruce needles can be used to make the environment less favorable to any fungal pathogen. In addition, reducing stress on the spruce tree by providing water during periods of drought, mulching the area around the tree, and taking care not to wound the tree are always beneficial to the overall health of the tree.

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Branch Diseases of Spruce

Cytospora Canker

This disease is caused by the fungus Leucostoma kunzei.

Cytospora canker is an opportunistic pathogen common on trees stressed by drought, winter injury or other factors. In Minnesota this disease is most common on Colorado blue spruce trees although all spruces commonly grown in Minnesota can be infected under the right conditions. Trees are more commonly infected once they are 10 years or older. 

Identification

• Needles on one or more branches turn completely brown or purplish brown then fall off.
• Scattered dead branches occur throughout the tree, often starting on the lower branches and then spreading upward.
• Clear to white to bluish-white resin encrusts cankers on infected branches.

Biology

The fungus that causes Cytospora canker can often be found in healthy branches. Disease begins in response to a wound or stress from insect feeding, snow or ice damage, drought stress or other factors. A sunken canker forms on infected branches and is often coated in a thick layer of resin. It may take several years for the canker to girdle the branch. Once girdled, the branch dies and the needles turn brown and fall off. The fungus quickly colonizes the dead branch, but rarely grows into the tree trunk. The tree trunk can be infected through wounds. Dark raised pimple like spore producing structures form on infected branches and release yellow tendrils of spores in wet weather. These spores are carried on wind or splashed by rain to new branches.

Cytospora canker rarely kills spruce trees but it can severely deform them often making them unsuitable for the landscape.

Management

• Plant Norway or white spruce instead of Colorado blue spruce.
• Reduce stress on spruce trees by watering during periods of drought and mulching the soil beneath the tree.
• Diligently prune out and destroy infected branches during dry weather. It may take two consecutive seasons of pruning to significantly reduce the disease.
  

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Eastern Spruce Dwarf Mistletoe

Dwarf mistletoe is a parasitic plant (Arceuthobium pusillum) that most commonly attacks black spruce (Picea mariana) in northern Minnesota, although white spruce (Picea glauca) are also highly susceptible to the parasite. Dwarf mistletoe can also attack white pine, red pine, jack pine, eastern larch, balsam fir, and Colorado blue spruce. These trees are typically only infected with dwarf mistletoe when planted close to groups of spruce infected with the parasite.

Identification

• Witches’ brooms, a clump of small weak branches arising from one point on a larger branch, form in infected trees.
• Needles within the witches’ broom remain green, needles on the rest of the tree yellow and fall off, typically from the top of the tree down.
• Short (1/2 – 1 inch) brown to orange dwarf mistletoe stalks can be seen during the growing season but fall off after seed dispersal in August or September. This occurs only after 4-5 years of infection.

Biology

Dwarf mistletoe is a parasitic plant that lives its entire life within the canopy of the tree. Female flowers shoot seeds, coated in a sticky layer, up to 55 feet away. Seeds can also stick to birds or other wildlife and be carried to trees much farther away. If these seeds land on a spruce branch, they germinate and colonize their new host. Root-like structures develop within the tree branches and rob the tree of nutrients and water. Witches’ brooms develop on infected branches, and as the infection continues, the dwarf mistletoe continues to steal nutrients, and the spruce tree begins to decline. After 4-5 years the first dwarf mistletoe shoots appear on the infected spruce branches. These short (less than 1 inch) branches are yellow orange in color and have only small scale like leaves.

Management

• Witches’ brooms can be pruned out to improve the look of the tree and reduce the spread of the dwarf mistletoe. Often however, the dwarf mistletoe reappears in other parts of the tree within 2-3 years from deep root-like structures or new infections.
• Trees infected with dwarf mistletoe with over 50% dead branches should be removed.
• Do not plant spruce or other susceptible conifers near infected trees.

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Root Rot Diseases of Spruce

Tomentosus Root Rot

This disease is caused by the fungus Inonotus tomentosus.

Black, white and Colorado blue spruce trees (Picea mariana, P. glauca, and P. pungens) are commonly infected with tomentosus root rot. Larch (Larix laricina) and Norway spruce (P. abies) can also become infected.

Identification

• Heartwood of infected roots and trunks is initially reddish brown.
• As infection continues white pocket rot develops. Decayed wood has elongated pockets or pits, and may appear honeycomb-like in cross section.
• Infected trees have reduced growth and thin canopies, produce large amounts of cones and eventually die.
• Infected trees frequently break or lodge during storms.
• Mushrooms that are velvety brown above and porous and buff colored below appear around the base of the tree in late summer.

Biology

Spores from the fungus that causes tomentosus root rot can infect wounds in roots or in the wide base of the tree, known as the root flair. In addition the fungus will spread from infected roots or tree stumps to healthy roots that grow close by, often resulting in a cluster of infected trees. Once inside the tree, I. tomentosus causes discoloration and decay of the heartwood and sapwood of both the tree roots and the tree trunk. Infected roots eventually die. The above ground parts of the tree are not able to receive the needed water and nutrients. Growth slows and the canopy looks thin and yellow. With time the tree dies. Often, however, infected but living trees fall over or break in a storm due to trunk and roots weakened by decay. The fungus can survive in the root system and stump of an infected tree for over 30 years, and will infect new trees planted nearby.

Management

• Mulch the soil around the base of all spruce trees to prevent injury from lawn mowing or other garden equipment.
• Have infected trees assessed by a certified arborist to determine its structural stability.
• Remove unstable trees to prevent damage that might occur if the tree were to fall.
• Remove the stump and as many roots as possible of infected trees.
• Do not plant spruce or other susceptible conifers at a site where trees have been infected with tomentosus root rot. Most deciduous trees are immune to the disease and can be safely planted.
  

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Armillaria Root Rot

This fungal root rot is caused by several different species of Armillaria.

Armillaria species have a very broad host range and can infect many deciduous and evergreen trees and shrubs. In addition to living as a pathogen in live plant tissue, Armillaria can also survive as a saprophyte on dead wood.

Identification

• Infected trees have poor growth, dead branches in the upper canopy, browning needles, may produce an abundant crop of cones, and eventually die.
• Clusters of honey-colored mushrooms may grow at the base of the tree in fall.
• Flat white sheets of fungal mycelia (mycelia fans) grow between the bark and sapwood at the base of infected trees.
• Thick black, shoestring-like fungal strands sometimes grow in a net on infected trees and in the soil around the base of the tree.
• The base of the tree just below the soil surface may be encrusted in resin.
• Wood is decayed, white, soft and stringy and may extend from the base of the tree well up into the trunk. Trees frequently break or fall over in storms.

Biology

Armillaria can survive many years in colonized wood debris like an old stump or root system. New infections occur when healthy roots grow close to diseased roots, or when the black shoestring-like fungal rhizomorphs grow through the soil to encounter healthy roots. Rhizomorphs can grow 10 feet from an infected tree or stump. Once inside the roots, the fungus colonizes the roots and the base of the trunk, resulting in decay. A vigorous tree may be able to isolate the fungus and slow its growth, but stressed trees frequently succumb to the disease quite quickly. Trees die of Armillaria root rot when the infection girdles the base of the trunk, when the trees lodge due to loss of roots, or when the weakened trunks break.

Management

• Reduce stress on trees by mulching the soil around the base of the tree, providing supplement water during drought and not wounding trees.
• Have infected trees assessed by a certified arborist to determine their structural stability.
• Remove unstable trees to prevent damage that might occur if the tree were to fall.
• Remove the stumps and as many roots as possible of infected trees.
• Do not plant spruce trees where oak trees or other hardwoods have recently been removed due to the fact that the Armillaria is likely to be present in these areas.