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Frequently Asked Questions

Below you will find information concerning common tree diseases.



Rhizosphaera Needle Cast vs. the Colorado Blue Spruce

Tree problems can be very sneaky. A lot of the time they creep in, and before you know it, the problem is staring you in the face, demanding attention. I've had several calls in the last two weeks from owners of Colorado blue spruce trees, all describing the same thing: the needles on their beloved trees are turning purple and eventually falling from the tree. In a word, the trees look horrible. Sometimes the problem is localized, sometimes it seems to be entire trees. The worst situation I've encountered this year is a windbreak of 35 Colorado blue spruce trees affected. Of course, everyone wants to know: what is the problem, and can it be fixed?


Diseases of Spruce Trees

Cytospora canker 
Leucostoma kunzei


  • 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 resin encrusted cankers on infected branches
  • Common on Colorado blue spruce, or very stressed spruces
  • More information on Cytospora canker...
  • More on Spruce issues.


    Oak Leaf Wilt


    Red and Black Oaks
    Oaks of this group, which includes red, black, pin, shingle, and many other oak species, are much more susceptible to oak wilt than are white or bur oaks. Infection of red or black oaks in early summer will generally result in tree death by late summer. The first symptoms of oak wilt occur in the top portion of the tree where leaves become bronze-brown in coloration and then wilt. These initial symptoms usually occur in June, but can appear as early as May. Wilted leaves have the bronze or dull tan tissue beginning at the leaf tip and edges. The bronze tissue progresses toward the leaf base, which is green. The separation between bronze and green tissue may be abrupt. Leaves that drop may be either totally green or the bronze and green described above. The streaking of the sapwood, typical of wilt diseases, can be found in the twigs. To find the brown streaks, remove the bark and examine the sapwood surface. The brown streaks may not be present in all trees; thus it is best to examine an actively wilting and defoliating tree. Defoliation may begin at any time after symptoms appear, and by late summer the infected tree may be bare of leaves. more...

    Oak wilt, caused by the fungus Ceratocystis fagacearum (Bretz) Hunt, kills oak trees. It has been found in 21 States, with considerable damage occurring in the Midwest. It was first recognized as an important disease in 1944 in Wisconsin (fig. 1) where, in localized areas (less than 100 acres (40.4 ha)), over half the oaks have been killed. Surveys in eight Wisconsin counties showed that about 11 percent of the annual growth increase of oak forests was offset by mortality caused by oak wilt.

    oak leaf wilt


    Right Tree in the Right Place - Wind Breaks and Privacy Hedges



    Differences Between Oak Decline and Oak Wilt


    Oak Decline. Within both red and white oak groups, oak decline is characterized by (1) progressive terminal branch dieback; (2) branch and bole sprout and staghead development; (3) sudden foliage wilt and browning, but no leaf drop; (4) fans and rhizomorphs of A. mellea often present beneath bark of roots and root collars on dying trees; (5) galleries and exit holes of A. bilineatus often present in stems of dying or dead trees (6) decline found throughout the range of oak; (7) mortality related to site features, tree stress, and affects of insets and diseases; and (8) tree mortality peaking 2 to 5 years following stress.

    Oak wilt in the red oak group. In the red oak group, oak wilt is characterized by (1) leaf wilt and drop over entire crown, (2) leaf portions bronzing or browning, (3) rapid tree mortality, (4) no progressive branch dieback, (5) formation and then death of short-lived sprouts in the season after infection, and (6) vascular streaking (dark longitudinal streaks) found occasionally in outer growth ring.

    Oak wilt in the white oak group. In the white oak group, oak wilt is characterized by (1) leaf wilt on scattered branches that die back and form sprouts and (2) vascular streaking common in outer growth ring. Some trees may continue to wilt and die back until all branches and sprouts are dead, yet other trees may survive.

    In both red and white oaks, armillaria root rot and twolined chestnut borers are not usually associated with trees dying of oak wilt In oak wilt, unlike oak decline, isolated trees or small pockets of trees can be diseased with no history of environmental stress. The infection centers are not related to site features. Laboratory diagnosis is usually required to conclusively identify oak wilt-diseased trees.


    Dothistroma Needle Blight of Pines




    Forest Insect & Disease Leaflet 143 

    U.S. Department of Agriculture Forest Service

    Glenn W. Peterson1


    1 Plant pathologist, Rocky Mountain Forest and Range Experiment Station, U.S. Department of Agriculture, Forest Service, Lincoln, Nebr.


    Figure 1
    Figure 1
    Dothistroma blight is a devastating foliar disease of a wide range of pine species. The causal fungus, Dothistroma pini Hulbary, infects and kills needles. Premature defoliation caused by this fungus has resulted in complete failure of most ponderosa pine plantings in States east of the Great Plains. In the central and southern Great Plains, D. pini damages Austrian and ponderosa pines in Christmas tree, shelterbelt, and landscape plantings. In California, Oregon, and Washington, the fungus damages plantings of lodgepole and Monterey pines. Infection by D. pini occurs sporadically in natural stands of lodgepole and ponderosa pines in Idaho, Montana, and Washington.


    The fungus has seldom been detected in young seedlings in nurseries in the United States. Yet, experience with epidemics in isolated new plantings in the central Great Plains indicates that trees infected in the nursery must have been responsible. The fungus is common on older transplants in nurseries that produce pines for landscape plantings. These nurseries are located in States.

    Dothistroma pini occurs in States shown in figure 1 and in southwestern Alaska. Twenty pine species and hybrids are known hosts in the United States. In the Central and Eastern United States, the fungus is found most often in plantings of Austrian and ponderosa pines. These two species are highly susceptible. The fungus has not been reported in natural pine stands in the Eastern and Central United States.



    Early symptoms consist of deep-green bands (fig. 2) and yellow and tan spots on needles. The deep green color of bands does not last and cannot be detected unless observed at the onset of symptom development. Later, the spots and bands turn brown to reddish brown (fig. 3). The bands are brighter red and more numerous on pines in California, Oregon, Washington, and Idaho, where this disease is often referred to as the "red band" disease. Map



    Figure 2 Figure 3 Figure 4
    Figure 2. - Early symptoms (deep green bands) on Austrian pine needles infected withDothistroma pini. Figure 3. - Spots and bands on Austrian bands) on Austrian pine needles infected pine needles infected with Dothistroma pini. Figure 4. - Typical appearance of infected Austrian pine needles: needle tips brown, needle bases green.


    The ends of infected needles progressively turn light green, tan, and brown, with the base of the needles remaining green (fig. 4).

    Needles may develop extensive necrosis (browning) 2 to 3 weeks after the first appearance of symptoms. Infection is typically most severe in the lower crown (fig. 5).


    Figure 5
    Figure 5. - Dothistroma pini damage severe in lower crown.
    Infected needles drop prematurely. Infected second-year needles are cast before infected current-year (first-year) needles. In some seasons, second-year needles are cast in the late fall of the year they became infected. In other seasons, loss of second-year needles is not extensive until late the following spring or early summer. Needles that become infected the year they emerge often are not shed until late summer the following year.


    Successive years of severe infection result in decreased growth and, ultimately, death. The disease makes pines in landscapes unsightly and pines in Christmas tree plantings unmarketable.

    Dothistroma needle blight can be mistaken for brown spot disease caused by the fungus Scirrhia acicola. The symptoms on needles are similar. With both diseases, trees are affected first in the lower crown. The dark stromatic fruiting bodies of D. pini and S. acicola look alike after they have erupted and split the epidermis. When a common host is involved; these two fungi can be distinguished only by microscopic examination of conidia. The conidia are somewhat similar in shape and size, and both have cross-walls. However, conidia of D. pini are hyaline whereas conidia of the brown spot fungus are colored, usually a greenish brown.

    These two fungi have several hosts in common. However, Scots pine, which is severely damaged by the brown spot fungus, is rarely infected and seldom damaged seriously by D. pini. A plantation of 36 geographic sources of Scots pine in eastern Nebraska has remained free of Dothistroma blight, but an adjacent plantation of Austrian pine has been severely damaged.




    What Is Thousand Cankers Black Walnut Disease?

    Thousand Cankers Disease

    Within the past decade an unusual decline of black walnut has been observed in several western states. Initial symptoms involve a yellowing and thinning of the upper crown, which progresses to include death of progressively larger branches. During the final stages large areas of foliage may rapidly wilt. Trees often are killed within three years after initial symptoms are noted. Tree mortality is the result of attack by the walnut twig beetle (Pityophthorus juglandis) and subsequent canker development around beetle galleries caused by a fungal associate (Geosmithia sp.) of the beetle. This disease complex has been namedThousand Cankers Disease (TCD).


    National Thousand Cankers Disease website

    USDA Forest Service Pest Alert


    Visual Symptoms of Affected Trees

    Thousand Cankers diseaseThousand Cankers disease: Curtis Utley, CSUE, Bugwood.orgYellowing and flagging of leaves on upper branches is usually the first symptom of the disease. This is followed by a progressive dieback that over time causes death of larger limbs and, ultimately, the entire tree. In Colorado, black walnut usually is killed within 3-4 years after initial leaf yellowing symptoms of thousand cankers disease are noted.


    The production of leaf yellowing/flagging of black walnut can have many other and far less serious causes. However, in areas where the disease is suspected to occur there should be additional examination to determine if thousand cankers disease is the cause.


    If you suspect you have Thousand Cankers and have ruled out other causes, please contact your local forester or state forest health agency to report the problem and confirm the diagnosis.

    Field Identification Guide by Seybold, USDA-FS, November 2009


    Report Possible Symptoms/Sightings of Black Walnut Trees with Thousand Canker Disease


    Canker Symptoms

    Thousand cankers disease kills trees by the production of numerous small, dark, dead areas (cankers) under the bark. The presence of cankers is detected by carefully removing the bark from symptomatic limbs. When peeling the bark to visualize cankers, be sure to not cut too deeply; the beetle galleries and fungus originally are found in the living bark (cork cambium) and not the wood cambium.

    Thousand Cankers disease

    Thousand Cankers disease: Kathy Keatley Garvey, UC Davis


    Individual cankers may originally be only a few millimeters in diameter, but ultimately can be 3 cm or greater and often assume an elongate oval form. Typically the appearance of a shallow tunnel produced by the walnut twig beetle will be present near the center of the canker.


    Thousand Cankers disease

    Thousand Cankers disease: Ned Tisserat, Colorado State University,


    The combination of a dark canker with the beetle tunneling is almost certain confirmation of thousand canker disease.  However, dark cankers occur under the bark following wounding and other injuries.  Culturing the Geosmithia fungus from the canker will allow positive confirmation of thousand cankers.

    Information from "Diagnosing Thousand Cankers Disease of Black Walnut", June 15, 2009


    Sanitation and Control Suggestions

    Very little is known about sanitation and control but research is continuing. If trees have the disease they should not be moved out of the area for at least three years. Full and complete debarking is believed to eliminate the beetle but that has not been 100% confirmed. Chipping of the bark does not eliminate the beetle. Fumigation or heat treating may be effective controls but tests are in process to confirm. To minimize the spread of this and a variety of other diseases firewood should never be moved.



    Quarantine in Bucks Aims to Halt Menace to Walnut Trees, August 15, 2011

    Thousand Cankers Disease hits ET, August 8, 2010

    Society of Environmental Journalists, August 19, 2009

    Fort Collins, CO, August 6, 2009
    Thousand cankers, a newly recognized and devastating disease that attacks black walnut trees, has killed a large number of trees across the western states and has now moved eastward into several communities along Colorado's Front Range, including Boulder and Colorado Springs and the Denver area.

    Denver, CO, July 13, 2009
    Black walnut trees in Denver are facing a very serious new threat called thousand cankers disease. This recently recognized problem has already devastated black walnut trees in Colorado Springs and Boulder and is believed to have been active in Denver for at least three years.




    *Photo by Kathy Keatley Garvey

    *Photo by Jim LaBonte, Oregon Department of Agriculture.

    *Photo by Jim LaBonte, Oregon Department of Agriculture.


    The beetles prefer wood larger than 2cm and feed on young branch tissue in the upper canopy early in the spring.  At these tunneling sites, cankers are diffus, brown to black and are often not visible until the outer bark is lightly shaved.  Later in the summer, adults move into the lower trunk to overwinter and continue to inoculate the phloem tissue with the fungus.  As the disease progresses, these cankers coalesce as well and can elongate to 2 meters in length.  In some cases, a brown or black stain will appear on the surface of these large trunk cankers.

    Walnut twig beetle and associated staining around tunnel.

    Coalescing branch cankers produced by Geosmithia.

    Adult twig beetle tunneling on excised stem after 24 hours.

    Large trunk cankers of black walnut.

    *Photos courtesy of Colorado State University




    Growing black walnut in pure stands is becoming more and more common. Although this practice has definite economic advantages, it can aggravate disease problems. This is especially true of leaf spot diseases, which multiply rapidly where large numbers of susceptible leaves are concentrated in a small area.

    Three such diseases are especially common on black walnut: walnut anthracnose, bull's-eye leaf spot, and white mold. All are becoming more prevalent as more walnut plantations are established. The symptoms produced by each of the three diseases are distinct and the grower can easily learn to tell them apart. 

    Gnomonia leptostyla

    Found wherever walnut is grown, walnut anthracnose causes premature loss of leaves, usually resulting in reduced growth and increased susceptibility to other diseases. Anthracnose also occurs on other nuts, causing the nutmeats to shrivel and darken.

    Look for: 

    Dark angular spots on the leaves ranging from pin-prick size to 1/2 inch (12 mm) in diameter.

    Very small black bumps (acervuli) within the spots, especially on the underside of the leaf near veins. You can easily see them through a hand-held magnifying lens. Acervuli are the fruiting bodies of the fungus;

    Prematurely defoliated trees. Examine some fallen leaflets. The presence of numerous spots with acervuli on the leaflets is a clue that anthracnose was the primary cause of the defoliation.

    crecent-shaped, bicellular spores (conidia) which are responsible for the spread of the disease.

    they contain many crescent-shaped, bi-cellular spores (conidia: 20 x 3 u), which are responsible for the spread of the disease.


    Trees not severely defoliated every year are not sufficiently harmed by the disease to warrant control measures. However, should control be required, spray benomyl or dodine fungicide on the foliage. (Benomyl is not registered for use on walnut trees grown for nut production.) Proper maintenance of soil fertility, especially with applications of nitrogen fertilizers, will also help minimize the damage.

    Cristulariella pyramidalis

    Bull's-eye leaf spot, like anthracnose, causes premature defoliation of black walnut trees. The range of bull's-eye leaf spot is not known. Recently, however, sever outbreaks of the disease have occurred in southern Illinois and Ohio, defoliating entire stands of walnut trees. Cristulariella also attacks maple, hickory, and many common weeds.

    Look for:

    Dark, round, dead, or dying areas with concentric white rings on leaves. These spots are more rounded than anthracnose spots and the rings give them a target-like appearance.

    The white Christmas treelike fruiting body of the fungus (approximately 350 x 100 u) on the underside of the leaf. No other pathogen of walnut produces a fruiting body resemblingCristulariella.

    Target-shaped spots on newly fallen leaflets, and for the Christmas treelike sporophore. An abundance of these prove bull's-eye leaf spot to be the cause of defoliation.

    No means of controlling Cristulariella has yet been developed. Fertilization with nitrogen should help minimize the harmful effects of the disease. Also weed control in and around the planting may help reduce early-season infection.

    Microstroma juglandis

    White mold, or downy leaf spot, is common wherever walnut is grown. The disease is more unsightly than damaging. Microstroma does not kill the leaf, nor is it known to cause defoliation. Any harm to the walnut is probably secondary due to the shading of a portion of the leaf surface and hence a decrease in photosynthesis.

    Look for:

    A whitish growth on the underside of the leaf, often concentrated along the veins, and for a yellowish discoloration on the top surface of the leaf.

    No control is recommended.

    Leaf diseases do not cause all defoliation of black walnut trees. Insects (e.g., the walnut caterpillar) and drought can also be the culprits. The fallen leaflets may show black specks of flecks but seldom spots typical of the three diseases described.