In the tree world, each plant or group of plants tends to have their own set of issues. In fact, many plant groups have developed a reputation or an association of sorts in that whenever the plant’s name is referred to, you think of the problem as well as the plant. For example, when crabapple is mentioned, it’s hard to avoid thinking of apple scab and cedar apple rust as well as attractive blooms and a nicely shaped tree of small to medium size.
Knowing what to expect can make inspection for insects and diseases affecting spruce and fir easier. This is especially true if you’ve seen the pest before, even if your observation was in a book or leaflet. Walking up to a tree with an image in your mind of the insects and disease symptoms generally associated with that species begins the recognition process. The next step is to look closely at the trunk, needles, buds, fruit, root flare and crown for well-known symptoms. A thorough assessment is best, one that involves taking the time to walk twice around the periphery of the tree, as well as the remaining landscape and even neighboring landscapes.
Regular inspection should be an integral part of the total service package you offer customers. An approach that leads with periodic inspections and assessment — charging appropriate fees for time, effort and applied knowledge — and treating pest infestations as they arise is much preferred to the old “spray and pray” methodology, where the arborist sits by the phone waiting for a customer to call them to come out and apply something to make their tree better.
Several canker diseases can infect spruce and fir trees; one of the most common is cytospora canker. Lower branch death is usually the first symptom to appear. Over time, one or more branches become affected, eventually killing the tree. Close inspection is essential, as the actual cankers are often seen at the base of the branch attachment. In general, they are not sunken or of a different color than the rest of the bark, as is the case with other canker pathogens. Like other canker diseases, however, resin flow is dramatically restricted as a result of infection and deterioration of conductive vessels. Once cytospora canker is suspected, cutting into the affected area will reveal brown cambium and sapwood that has ceased to be functional. Needles appear yellow, turning to a uniform brown as the nutrient and water flow cease.
Unfortunately, effective fungicide treatments for cytospora canker are not available. The best approach is to provide best management practices for woody landscape plants, which includes separating trees from turf; irrigating to keep soils moist but not soggy; mulching with 2 to 3 inches of a coarse woody material to cover the roots (not the root flare); control of other pests; and avoidance of compaction, bark and stem injuries.
While cytospora canker is primarily a problem of the conductive tissues, Rhizospaera needlecast is (you guessed it) a needle-related malady. The infection generally begins on the lower branches and progresses to the upper needles. Although the new growth becomes infected in late spring or early summer, the symptoms usually do not appear until late summer or fall. Affected needles turn from green to purplish-brown and usually drop from branches. Tiny fungal fruiting bodies will be visible using a 10x hand lens, appearing as black dots. During optimal conditions of cool, rainy springs, spores are released and splashed on healthy foliage to spread the disease. Effective control of this disease can be achieved through multiple applications of chlorothalonil, liquid copper fungicides or manganese/zinc combination products.
Sirococcus shoot blight
In many ways, Sirococcus shoot blight is similar in appearance to Rhizosphaera needlecast, at least at first glance. Symptoms of Sirococcus also resemble frost or winter injury. However, cold temperature injury tends to be uniform, affecting the entire tree or one side of the tree, whereas Sirococcus activity is usually scattered. Sirococcus causes damage to branch tips, especially on the current years’ terminal growth. The disease infects the needles of spruce and fir at the base, causing chlorosis in the short term and reddish brown discoloration in subsequent weeks. In some cases, the growth takes on a curled or bent shape, which can be confused with herbicide or insect damage.
Fortunately, Sirococcus infections are usually confined to the lower branches. Sirococcus is also spread through rain splash. If practical, removal of infected tissues is recommended to reduce spore habitat. Severe infections can be lessened through early spring applications of thiophanate-methyl, propiconazole or liquid copper fungicides.
Bagworm infestations have been a bit like the stock market in recent years. In the ’80s, they were a major pest of most conifers and many broadleaf trees. In the ’90s, populations crashed and bagworms lost status as a primary pest to control. For the last 10 to 12 years, bagworm numbers have risen and fallen somewhat erratically and for shorter durations. Fortunately, bagworms are easy to identify. Early in the season, tiny caterpillars hatch from eggs and emerge from the bags in which they overwintered. They immediately begin to feed and construct cocoon-like structures around themselves. These bags look pretty much the same throughout the season, but expanding in size as the season progresses.
Hand removal of these bags prior to egg hatch in the spring can effectively reduce numbers on small trees. Once eggs have hatched and small caterpillars are present, applications of acephate, Bacillus thuringiensis, bifenthrin, carbaryl, deltamethrin, permethrin, spinosad or tebufenozide should help bring these infestations into check.
Pine needle scale
Though normally thought of as primarily a pest of pine trees, pine needle scale can also be damaging to spruce and fir. Timing is critical when controlling needle scale. Scales overwinter as mature adults under a white waxy covering that aids in winter survival. In midspring, crawlers emerge from under the overwintered scale in search of a new attachment site. During this brief period – usually 10 days or so – they are vulnerable to insecticidal sprays. Soon after, the insect begins secreting a new waxy covering. With each passing day, they become more and more resistant to control efforts.
Many products can be used to control scale insects including acephate and permethrin. Avoid using insecticidal soaps and horticultural oils on spruce and fir, as these products are known to remove the desirable needle color associated with the species.
Spruce spider mites
While spider mites are not insects, their feeding habits and potential for damage to trees put them in the same general category. Two major differences between adult mites and insects are that spider mites have eight legs and two body segments, whereas insects have six legs and three body segments.
Mites can be detected by placing a clipboard holding a white sheet of paper under a branch and rapping it a couple of times with your gloved hand or a medium size stick. If present, mites will drop from the needles and be visible on the paper. (Lots of other debris and a few larger creepy crawlers may also fall on the paper, but mites will usually be the only tiny yellow to reddish dots moving around.) This is a good technique to use while explaining to clients that pest control is necessary; it’s a good visual, especially if a magnifying glass is available.
If one or two mites are found, continue to inspect the tree. If the general pattern remains the same, then control measures are not needed. However, if many mites are observed, then applications of acephate, abamectin, etoxazole and hexythiazox should help reduce their numbers.
Cooley spruce galls
These galls can be eye-catching on a blue spruce, but rarely cause serious injury to the tree. Cooley spruce galls are swollen, cone-like structures covered with perforated openings. Each cone is inhabited by numerous aphid-like insects called adelgids. Cooley spruce galls first appear on the new growth of spruce trees in the spring and enlarge throughout the summer. Cooley spruce gall adelgids require two hosts (spruce and Douglas fir) to complete their life cycle.
Insecticide trials conducted at Colorado State University suggest that foliar treatments of carbaryl (Sevin) and permethrin are most effective for control of Cooley spruce galls, but must be applied before galls begin to develop. Once galls have formed, insecticides are usually ineffective because the adelgids are protected within their galls.
Spruce needle miners
Spruce needle miners are the caterpillars of a small (.5 inch) brown moth with grayish white bands on the front wings. Overwintered adults emerge in late May to mid-June, mate and lay eggs on the spruce needles. After hatching, the tiny greenish-brown larvae enter the base of the needle and hollow it out. As caterpillars feed, they move from needle to needle producing clusters of matted needles held together with dense webbing.
The best time to treat for spruce needle miners is in early spring. Insecticides should be applied in time to kill hatching larvae before they can tunnel into needles. Application of acephate, Bacillus thuringiensis, bifenthrin, carbaryl, deltamethrin, permethrin, spinosad or tebufenozide should provide two to three weeks of control.
Editor’s note: This article was originally published in December 2011 and has been updated.