Integrated Pest Management in Christmas Tree Production

Deborah G. McCullough
Department of Entomology and Department of Forestry, Michigan State University

Melvin R. Koelling, Professor
Department of Forestry, Michigan State University 


Photograph: Good management practices can result in production of high-quality Christmas trees like this Fraser Fir grown in Michigan.

What would Christmas be without a Christmas tree? Most people don't think about Christmas trees until November or December, when they go out to select that perfect tree for the family. However, Christmas tree production is a multi-million dollar industry that requires year-round management efforts. Christmas trees occupy a unique niche, incorporating aspects of both agricultural production and forestry. Insect and disease pests can have major effects on tree growth, appearance and value. Managing these pests, along with weeds and even rodents, is a critical component of tree production.

Our goals are 1) to provide an overview of Christmas tree production and 2) to briefly discuss insect and disease management in Christmas tree production. Many effective and economically viable pest management practices can be readily integrated with other aspects of tree production. Information presented here will largely be drawn from our experience in Michigan and other Lake States, one of the largest Christmas tree producing regions in North America.

The Christmas Tree Business

Approximately 38 million natural Christmas trees are purchased each year by American consumers. These trees represent the annual harvest of an estimated 20,000 individual growers who annually plant, trim, protect and otherwise manage trees on roughly 500,000 acres of plantations. More than 15 different conifer species are grown for Christmas trees.

Christmas tree production is a significant industry in the Lake States, the Pacific Northwest, North Carolina and areas in the Northeast. Traditionally, growers in these regions supply about 90% of all trees offered for sale at retail outlets. Approximately 10,000,000 trees are produced each year in the Lake States of Michigan, Wisconsin and Minnesota. Michigan is the largest producer, annually harvesting 5,000,000 trees, valued at up to $100,000,000 in some years.

Christmas trees are produced by two principal types of operations; "wholesale" or "choose-and-cut" plantations. Most trees offered for sale at retail outlets such as garden centers, nursery stores, high volume discount stores and lots operated by service clubs, were produced by wholesale growers. These are individuals or companies that operate and manage large plantations of trees. They may sell trees either directly to retail outlets or to "brokers" who in turn market trees to retailers. Some large operations may sell 100,000 trees or more each year.

Choose-and-cut operations are usually smaller plantations that manage and produce trees for direct sale to consumers. These operations invite customers out to their fields and will either cut the trees for the purchaser or allow the customer to cut his/her own trees. Many choose-and-cut operations enhance the experience of getting a Christmas tree by providing a variety of recreational activities for customers. Choose-and-cut farms are found in every state and are often concentrated near large population centers. These operations vary considerably with regard to the species and quality of trees produced, and the services offered.

Christmas tree production is a long-term process. The average 7 to 8 foot tree generally requires 8 years of growth after planting. Some species require even longer production times. Likewise, larger-sized trees which are becoming increasingly popular, require more years to produce.

A major challenge for Christmas tree producers is to determine which tree species will be popular with consumers several years in the future. Although several species are utilized as Christmas trees, the majority of trees sold at retail markets today are Douglas-fir, Fraser fir, noble fir and Scotch pine. Species such as white spruce, Austrian pine and red pine are no longer as popular as they were in the past. Producers who are not sensitive to shifts in consumer preference will have difficulty marketing even high-quality trees of less preferred species.

Christmas Tree Production Practices

Each component of Christmas tree production must be addressed from the standpoint of maximizing both productivity and quality. Failure to appreciate or deal appropriately with any aspect can result in management difficulties, pest problems, or production of low-quality trees which are difficult to sell in competitive markets.

Site Selection

Not all species used for commercial Christmas tree production grow equally well on all sites. Factors such as soil texture and fertility, water availability and air drainage will affect growth rates and tree quality. In general, true firs and Douglas-fir require better quality sites than pines and spruces. Species that are planted "off-site" will frequently experience stressful conditions. These trees are more likely to be attacked by insect or disease pests, and will be less likely to tolerate or recover from pest damage, than healthier trees.

Site Preparation

In the past, Christmas tree plantations were sometimes established with little or no site preparation other than removal of competing woody vegetation. Survival and success of these plantation were mixed. Where site preparation was minimal, pines were easier to produce than spruces or firs. Modern Christmas tree operations often spend much effort in site preparation activities including tillage, use of cover crops and soil fertility enhancement. These efforts pay off in higher seedling survival, fewer pest problems on young trees, more rapid initial growth and an overall increase in tree quality.

Rotation Length

Most conifers planted for Christmas tree production require 7 to 10 years to reach maturity, depending on the species, the size of trees produced, and the intensity of management. Pines generally can be produced faster than spruce or fir trees, although size of planting stock, soil fertility and water availability can significantly affect rotation length. Damage from insect or disease pests that affect tree appearance, form or growth rate, can increase rotation length and production costs. Even after a pest population is controlled, trees may require 1 to 3 years to outgrow or recover from the damage.


Most Christmas tree plantations are established using planting machines. Seedlings may be planted by hand on adverse sites or to fill in plantations where mortality occurred in previous years. Nearly all growers plant 2 to 4-year-old seedlings or 3 to 5-year-old transplants in early to mid-spring. A few growers may produce their own planting stock, but the majority of producers purchase stock from private seedling-transplant nurseries. Pests, particularly diseases, may be transported on infested nursery stock. Purchasing stock from reputable dealers or buying inspected and certified stock can help prevent establishment of new pest problems.


Photograph: Workers shear Scotch pine trees in a Christmas tree plantation. Shearing develops a thick uniform canopy and the characteristic tapered shape that appeals to Christmas tree consumers.

Christmas trees are typically sheared or shaped each year, beginning 2 to 3 years after planting, and continuing on through harvest. Shearing accomplishes two goals. First, shearing develops the characteristic tapered shape associated with high-quality Christmas trees. Second, shearing controls the amount of annual growth and in some species, increases bud set. This results in greater density and uniformity of the foliage. Timing of shearing depends on the species of conifer. Pines are usually sheared during June and July; shearing of other species usually begins in August and continues until finished in the fall.

Shearing can affect pest infestation or damage. For example, tight shearing can result in very dense crowns with little air circulation. This situation can lead to problems with needlecasts or other foliage diseases. Effectiveness of insecticide or fungicide applications may be poor if sprays do not penetrate the dense outer canopy. On the other hand, shearing can remove much of the damage caused by shoot-boring insects, Pales weevil (Hylobius pales) and other pests. A good understanding of potential pest problems as well as consumer preferences, should help growers develop suitable shearing practices.


In late summer, trees with that will be harvested that year are identified and marked with tags or flagging. Many growers spray trees with a water-soluble green latex pigment in August or September. The green paint is applied to mask the characteristic yellowing of foliage which commonly occurs in some varieties of Scotch pine, eastern white pine and Douglas-fir. Needles begin to turn yellow in fall, in response to shorter photoperiod and cooler temperatures.

Actual harvesting begins in late October and will continue on through mid-December. After cutting, trees are shaken to remove dead foliage and debris, then baled with string or net. Trees are then transported from the plantation to a storage or loading yard where they will be stored until shipment. Shipment to retail centers and stores is usually well underway by November 18 to 20.

Major Christmas Tree Species

Although several conifer species have been used for Christmas trees, most commercial production involves four genera. These are: pines (Scotch, Austrian, red and white), spruces (Colorado blue, white, and the Blackhills variety), firs (balsam, Fraser, Concolor and Noble), and Douglas-fir. A few other individual species may be produced in specific regions or local areas, but these species rarely account for significant numbers in the national market place.

Douglas firs

Major Pests

Rhabdocline needlecast (Rhabdocline pseudotsugae), Swiss needlecast (Phaeocryptopus gaumanni), Cooley's spruce gall adelgid (Adelges cooleyi)

This species is native to western regions of the United States and Canada and is widely planted in the Lake States and northeast. Because Douglas-fir breaks bud early in the spring, it must be planted on sites with good air drainage to prevent injury from late spring frosts. Douglas fir does best on well-drained, loam to sandy loam soils and will grow rapidly once it is established. It will not tolerate heavy soils which are poorly drained. Needle retention is generally good, although trees that have not experienced freezing temperatures before harvest may loose some needles during the display period.

True Firs

Major Pests

Balsam twig aphid (Mindarus abietinus), Spruce spider mites (Oligonychus ununguis), Balsam gall midge (Paradiplosis tumifex), Lirula needlecast (Lirula sp.)

Fraser fir - Closely related to Balsam fir, Fraser fir is native to high elevations in the southern Appalachians, but has been widely planted in other production areas. Fraser fir requires ample soil moisture and fertility, and a soil pH of less than 6.5. It will not grow in very wet or dry locations. Fertilization and sometimes irrigation are frequently used in well-managed plantations. Because of its excellent needle retention, attractive aroma, straight stem and dark green­silvery blue needles, the popularity of this species among consumers has increased tremendously in recent years. It is now widely recognized as a premier Christmas tree species and is one of the most popular species among both growers and consumers.

Balsam fir - Native to the northeastern United States and adjacent Canada, balsam fir has been long used as a Christmas tree. Like all firs, its soft, fragrant foliage is prized by consumers. For many years wild balsams were harvested from natural stands and marketed throughout the northeastern United States. Most trees are now harvested from plantations located in New England and much of Quebec and the Maritime provinces of Canada. Growth is best on loam soils which are well-drained. This species responds well to fertilization and intensively managed plantations regularly supply trees with nitrogen fertilizer to promote growth and enhance foliage quality. Balsam fir foliage is often used for wreaths, garland, and other Christmas greenery.

Concolor fir - Native to states in the Rocky mountains and intermountain west, Concolor or white fir is an important timber species throughout much of its range. However, because of its attractive bluish-green needle color and soft foliage it has been planted as both an ornamental and a Christmas tree. Concolor fir is adapted to a wide variety of sites and soils. It will tolerate fairly dry sites, especially after it is well-established, and also grows well under varying soil pH conditions.

Noble fir - Noble fir has become a popular Christmas tree and is produced primarily in the Pacific Northwest. This species is known for its excellent needle retention and symmetrical branching habit. Within its native range, Noble fir grows at elevations ranging from 2000 to 5000 feet above sea level. Efforts to grow this species outside of its native range have met with limited success, especially in the Lake States and northeastern United States. Some efforts are underway to identify seed sources which might prove hardy outside the region. Because of its popularity among consumers, many trees of Pacific Northwest origin are sold throughout the central and eastern United States.


Major Pests

White pine weevil (Pissodes strobi), Cooleys spruce gall adelgid, Pine needle scale (Chionaspis pinifoliae), Rhizosphaera needlecast (Rhizosphaera kalkhoffii), Cytospora canker (Cytospora kunzei)

Colorado blue spruce - This species is planted extensively throughout much of the United States. Although native to the central Rocky Mountains, it is adaptable to a wide variety of sites and soils. Colorado blue spruce is more popular in choose-and-cut operations than in large wholesale plantations, largely because of lack of consumer demand at the retail level. While its foliage is attractive, the stiff, sharp needles make for difficulty in handling and display. Needle retention is generally better than other spruce species.

White spruce - For many years white spruce was a favorite species of many eastern growers. It is adaptable to diverse planting sites throughout much of the Lake States and northeastern United States. Its tendency to grow into a tree with a "natural" Christmas tree shape contributed to its popularity because only a minimal amount of shearing and shaping was required to produce a quality tree. However, its popularity has declined in the last several years due primarily to its reputation for poor needle retention. The small, short needles have a tendency to dry rapidly when the tree is displayed in a heated room with low humidity. A variety known as Blackhills Spruce has better needle retention characteristics and continues to be planted by a few growers.


Major Pests

Pine root collar weevil (Hylobius radicis), Zimmerman pine moth (Dioryctria zimmermani), pine needle scale, white pine weevil, Lophodermium needlecast (Lophodermium seditiosum), Dothistroma needlecast (Dothistroma pini), Cyclaneusma needlecast (Cyclaneusma pinus), Sphaeropsis (Diplodia) shoot blight (Sphaeropsis sapinea)

Scotch pine - For many years this species enjoyed the position as the nation's most popular Christmas tree. Native to Europe and Asia, Scotch pine grows well on a wide range of sites and soils throughout much of the United States and Canada. Many varieties were imported from Europe and Asia and exhibit considerable variation in such traits as needle length and color, stem straightness, branch angle, and hardiness. Scotch pine continues to be widely planted, especially in the Lake States, where it has long been viewed as the "bread and butter" species of the Christmas tree industry. Scotch pine adapts readily to plantation culture and responds well to management practices such as shearing. It has excellent needle retention and a long shelf life following harvest, even in warm and dry locations. However, more than 40 insects and diseases attack Scotch pine, making pest management a major aspect of Scotch pine production. Although still a significant component of the national market, the popularity of Scotch pine has declined over the past several years. Increased sales of both Douglas fir and Fraser fir have reduced market share in several locations. It remains a favorite of cut-your­-own growers in many areas of the central and eastern United States.

White pine - The name white pine may refer to several species, including western white pine and southwestern white pine. However, when used to describe Christmas trees, it commonly refers to eastern white pine. This species is native to the northeastern United States, adjacent Canada and higher elevations in the Appalachians. It is also an important timber species and was a notably valuable lumber species in the late 1800's. It grows on a wide range of soil types, including loamy sands to heavier clay loam soils. When planted to produce Christmas trees, it responds well to traditional shaping and shearing practices. It can be developed into a tree with thick foliage and uniform symmetry, and it exhibits excellent needle retention.

Austrian pine -This European native was a fairly popular Christmas tree species several years ago, but presently enjoys limited popularity among consumers. It tolerates a wide variety of soil and site conditions and is often planted along roadsides and as an ornamental. When grown as a Christmas tree, it generally is restricted to cut-­your-own operations, most often in locations where poor sites prohibit production of other species. It is relatively unpopular among consumers because of its long needles, stiff branches and generally "coarse" appearance.

Red pine -Red pine is native to states in the north central and northeastern regions of the U.S. and is a valuable tree for the production of wood fiber and timber products. It has been locally planted for Christmas tree production, but does not enjoy widespread popularity among consumers. It grows best on well-drained loam or sandy loam soils of low to medium fertility. It has characteristics similar to those of Austrian pine, although its needles are usually somewhat shorter and not as stiff.

Pest Damage

Damage from insect and disease pests can lead to dead branches or terminal leaders, and some pests will kill trees. Loss of needles or shoots due to disease or defoliating insects will reduce tree growth, resulting in longer rotation times and economic loss. Aesthetic injury is probably the most common damage caused by pests. Christmas tree value is based largely on the appearance of the tree and its attractiveness to potential customers. Missing or dead foliage, a crooked stem or tiny white scales on the foliage will reduce the value of the tree or even make the tree unsalable.

Principal Classes of Insect and Disease Injury

Needle loss - Defoliation

Needlecast diseases or defoliating insects such as sawflies cause needle loss. This injury can lead to reduced growth rates and thin, unattractive canopies. Loss of current-year needles, where nutrients are concentrated, usually has greater effects on tree health and appearance than loss of older foliage.

Death of branches, terminal leader or lateral shoots

Photograph: A terminal leader killed by white pine weevil feeding. Trees may require corrective pruning and 2 to 4 years of growth to recover from white pine weevil damage.

White pine weevil will kill 2 to 4 years of terminal leader growth, affecting rotation length and stem shape. Zimmerman pine moth, Cytospora canker, and gall rust pathogens (e.g. Cronartium spp. and Endocronartium harknessii) can kill large branches, affecting tree appearance and sometimes growth rates. Several shoot-boring insects (e.g. Rhyacionia spp. and Eucosma gloriola), Pales weevil (Hylobius pales), and some diseases such as Diplodia kill individual shoots.

Sap-Feeders - aphids, scales, mites

High populations of aphids, scales, mites and other sap-feeders often seriously reduce the appearance of Christmas trees and can affect tree health. Some species, such as Cooley's spruce gall adelgid cause unsightly galls to form on lateral shoots of spruce trees. Pine tortoise scale (Toumeyella parvicornis) excretes large amounts of sugary honeydew. A black sooty mold grows on the honeydew, turning needles and shoots black. White armor covering pine needle scales gives pine needles a white, flecked appearance. Feeding by the balsam twig aphid will cause needles of fir trees to twist and curl tightly.

Tree mortality

Some pests, including pine root collar weevil and white pine blister rust (Cronartium ribicola), kill Christmas trees. Pales weevil, and root diseases (e.g. Phytophora spp.) may kill young seedlings.

Economic Injury Level and Action Threshold

In IPM, the Economic Injury Level (EIL) and Action (or Economic) Threshold (AT) are used to determine if and when pest control measures are needed. Unfortunately, few thresholds have been developed for Christmas tree pests. The difficulty of establishing an EIL or AT for any given pest arises from several factors. These factors include the difficulty of 1) quantifying aesthetic injury, 2) determining the economic costs of that injury and 3) relating pest density to levels of aesthetic injury.

Photograph: These galls on a blue spruce Christmas tree were caused by Cooley's spruce gall adelgid. Although the galls seldom affect the health of the tree, they detract from the tree's appearance and reduce its value.

However, the concept of an Action Threshold remains valid and is incorporated into pest management recommendations wherever possible. One factor that affects pest management decision-making is the type of damage caused by the pest. For example, in Scotch pine fields, the AT for a tree-killing insect such as pine root collar weevil is lower than for a minor defoliator such as sawflies (Neodiprion spp.).

Another important factor is how soon the trees will be harvested. A light infestation of pine needle scale on Scotch pine or Cooley's adelgid on Douglas-fir is much more critical when trees are within 1 to 2 years of harvest, than if trees are in the middle of their rotation.

A third factor affecting an AT is how long it will take the tree to recover from pest damage. For example, shoots killed by European pine shoot moth (Rhyacionia buoliana) or other shoot borers are often removed anyway during shearing. However, white pine weevil attack typically results in the death of 2 to 4 years of growth on spruce or pine trees. Although tree form can eventually be restored with corrective pruning, harvest of weevil-damaged trees will be delayed for at least 2 years.





Management Strategies

Cultural Control

Ideally, pest management should be incorporated into all aspects of Christmas tree production, from site preparation to harvest. It is especially important to select a tree species that is well-suited for the site conditions in the field. For example, Fraser fir requires relatively high levels of nutrient and moisture availability. In contrast, Scotch pine tolerates even sandy, coarse-textured soils where nutrient and moisture availability is low. Trees growing under stressful conditions are usually more susceptible to insect and disease pests and recover more slowly from damage. Irrigation and fertilization may effectively reduce stress and increase tree vigor. However, these practices will only be cost-effective for high-value species such as Fraser fir.

The importance of scouting, the practice of inspecting trees for evidence of pest infestation or damage, cannot be underestimated. Frequent and regular scouting will enable growers to detect signs or symptoms of pests before economic damage occurs. Signs include the physical evidence of pest presence such as insect frass or cast-off skins, pitch flows, or the fruiting structures of disease organisms. Symptoms refer to evidence that the tree has been affected by insect or disease attack. Branch galls, sparse crowns, or a wilting leader may be evidence of gall rust, needlecast disease and white pine weevil infestation, respectively.

Cultural control in Christmas tree fields should include frequent scouting to identify diseased, damaged or infested trees. Trees that are heavily infested should be "culled" and removed from the field. Several pests such as Zimmerman pine moth, an insect that feeds in the inner bark on stems and branches of pine trees, will complete their development even after trees are cut. Adult moths can then return to the field, lay eggs on live trees, and perpetuate the infestation. Therefore, culled trees should be destroyed within a few weeks by chipping or burning. Growers may also be able to mechanically remove and destroy pest-infested leaders, shoots or galls, before a new generation of the pest can emerge.

Trap logs have been used successfully to manage some insect pests such as pine shoot beetle (Tomicus piniperda), a recently discovered exotic bark beetle that is currently a quarantine pest (see below). Bark beetle larvae feed and develop under the bark of pine stumps, logs or recently cut trees. Trap logs consisting of 3 ft. long sections of pine logs, are set along the edges and lanes of Christmas tree fields to attract adult pine shoot beetles. The adult beetles colonize the trap logs and lay eggs. Growers then have a 4 to 6 week window to collect all trap logs and destroy them by chipping or burning, effectively eliminating the new generation of beetles.

Biological Control

Natural enemies often play an important role in reducing potentially damaging insect populations in Christmas tree fields. Fields managed on 6 to 12 year rotations are more likely to provide stable habitat for beneficial arthropods than agricultural systems where fields are harvested annually. Further, trees are more structurally complex than most agricultural plants. Trees can provide beneficial arthropods with resting or oviposition sites, and protection from adverse weather. Hedgerows along field edges that include flowering plants may enhance the fecundity or survival of parasitoids.

Common and important natural enemies in Christmas tree fields include spiders, flower fly larvae, lacewings and predatory mites. Ladybird beetle larvae and adults are especially important predators of aphid and scale insects. In addition, many insect pests are attacked by specialized parasitoids. Obviously, it is important to minimize insecticide use and drift to help conserve these natural enemies.

Biocontrol in Christmas tree production probably holds more potential than is currently being realized. There is a scarcity of information on effectiveness of biocontrol agents, and the proper timing and density for agents that will be released in fields. To-date, there are few specialized biocontrol agents for Christmas tree pests available commercially.


Use of insecticides, fungicides, herbicides and rodenticides is common in Christmas tree production. Application technology varies widely among growers, depending on the size of the operation. Many growers use air blast sprayers, but backpack sprayers, boom sprayers and airplane or helicopter application are not uncommon. Minimizing pesticide applications makes sense for both economic and environmental reasons, and makes it easier to integrate pesticides with other management strategies.

Timing of pesticide application can have major effects on efficacy. Growers are encouraged to use scouting to determine when the vulnerable stage of the pest is present. Using degree days, rather than calendar days, can also improve the timing of pesticide applications. Degree days accumulate rapidly during warm weather and more slowly when temperatures are cool.

Achieving adequate coverage is another concern when spraying conifer trees. Dense foliage, particularly on sheared trees, often makes it difficult to get good coverage. This is especially important when the target pest occupies the stem (e.g. Zimmerman pine moth) or older foliage near the stem (e.g. spider mites).

Quarantines - A Challenge for IPM

When exotic (non-native) pests are discovered, federal and state quarantines may be enacted to limit the spread of the pest. Quarantines are intended to provide a legal means of reducing the risk that exotic pests will be carried along when Christmas trees (or other plants) are shipped to areas not yet infested.

However, quarantines often represent a major challenge to IPM. Most quarantines restrict out-of-state shipment of trees if even a single pest is present. For example, state and federal gypsy moth (Lymantria dispar) regulations currently mandate that Christmas trees be sprayed at least once with an insecticide during the summer if the trees will be shipped out of state in the fall. Mandatory spraying, in combination with tree inspections at harvest time, was implemented to reduce the risk that gypsy moth egg masses or other life stages would be accidentally introduced into new areas. This situation can lead to unnecessary use or overuse of insecticides by growers who fear penalties or serious economic losses if any egg masses are found.

In contrast, university scientists and regulatory agencies are currently working together to develop an IPM program for pine shoot beetle, another exotic pest. Pine shoot beetle is a bark beetle that feeds in shoots of live pines and breeds in pine logs, cut pine trees, and stumps. It was first discovered in North America in 1992 and the infestation is centered in the North Central region of the U.S.

Under the terms of a proposed PSB Compliance Program, growers agree to implement cultural practices that will substantially reduce beetle populations. These practices include burning or chipping the cut trees and stumps the beetles breed in. Trap logs must be used to attract breeding adult beetles to prevent a new generation of beetles from infesting trees. Growers who agree to comply with the prescribed management protocol can then harvest and ship trees without a tree-by-tree inspection in fall. The Compliance Program will reduce the risk of introducing PSB to new areas, while at the same time promoting good management practices.


Growing Christmas trees is an intensive agriculture-forestry enterprise that requires the use of production techniques much like any other crop. Control of potentially damaging insect and disease pests is an ongoing part of the production process. IPM concepts such as scouting, use of thresholds and integration of various control strategies, are widely used by Christmas tree growers, although these practices are not as advanced as they are for other crops. Continued research is needed to develop EILs and Action Thresholds for specific pests, to identify workable biocontrol options for Christmas tree pests, and in other areas. A continued emphasis on IPM will ensure the production of high-quality trees with minimal economic and environmental impacts.


  • Benyus, J.M. 1983. Christmas tree pest manual. USDA Forest Service, North Central Forest Experiment Station and Northeastern Area State and Private Forestry. 108 p.
  • Johnson, W.T. and H.H. Lyon. 1988. Insects that Feed on Trees and Shrubs. 2nd edition. Cornell University Press. 556 p.
  • Kachadoorian, R., J.Cummings-Carlson, D.G. McCullough and D.O. Lantagne. 1995. Pesticides for use in Christmas tree production in the North Central region. Michigan State University Extension Bulletin E-2594. 52 p.
  • Koelling, M.R. and L. J. Dornbush. 1992. Growing Christmas Trees in Michigan. Michigan State University Extension Bulletin No. E­1172 (Revised), 12p.
  • Sinclair, W.A., H.H. Lyon and W.T. Johnson. 1987. Diseases of Trees and Shrubs. Cornell University Press. 575 p.