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Summary
Hypoxylon canker of shade trees is an ascomycete fungus that negatively affects growth and can eventually lead to the death of the tree (3). There are many different species that affect different trees. Hypoxylon atropunctatum, a common species, is found on oak trees. Hypoxylon tinctor, on the other hand, affects sycamore trees. Although the fungus is found on the majority of tree bark in an infested area, it only harms the tree when the tree becomes stressed or injured (2), meaning that it is an endophyte. An endophyte is a bacterium or fungus that spends some of its life not causing disease to the host while being on it (12). When a stress or injury occurs, the fungus will infect and begin living off the nutrients of the tree. Wide-scale problems have occurred in Arkansas, North Texas, and Oklahoma (2,3,5). Hypoxylon canker of shade trees is a secondary disease: a different factor usually causes the dying and decaying of the tree first before the Hypoxylon canker pathogen infects (4). This factor could be something so simple such as not giving the tree enough water. This will cause the tree to be in stress and maybe start to decay. The pathogen infects the living tree while it is in stress; it will not infect a tree that is already dead. Also, it will not infect a healthy tree (3). The tree must be already dying or at least has branches that are dying. The pathogen will then kill the tree if the tree is not being properly taken care of.

Domain: Eukarya Kingdom: Fungi Phylum: Ascomycota Genus: Hypoxylon

Hosts and Symptoms
Hypoxylon canker of shade trees usually affects oak trees as the species Hypoxylon atropunctatum, but it can infect many other trees including the sycamore tree as H. tinctor (2,3). In order to tell if the pathogen is affecting the tree, the bark must be closely examined, but the symptoms are distinct. Early stages of the disease show a light brown and tan color that looks dry and dusty. Later on, as the pathogen goes through the sexual stage, the bark will turn into a grey color. The bark becomes brittle and flakes off (2) and black and grey cankers will appear (3). Because these changes are indications of a disease, we can say that these are symptoms. They show us what is happening when the pathogen is present and infecting or has already infested the host. On the other hand, pathogens have signs. Signs are either going to be what is produced by the pathogen, such as ooze in bacterium, or they are a portion of the pathogen. The signs we can see from Hypoxylon atropunctatum are conidium and ascospores. Conidium are produced during the asexual stage, while ascospores are produced during the sexual stage (4,7). You can sometimes see the conidium with the naked eye if they are structuralized, but it is much easier to see them under a dissecting microscope. When looking underneath a compound microscope, you will be able to see the ascospores. They will be located inside or around the perithecium, which is the flask-shaped fruiting body that contains ascospores. Lastly, the pathogen is going to be everywhere on the trees and on many trees. If you start to see symptoms on the main trunk, then you’re too late and the tree is either already dead or almost dead. Since this is the spot where the nutrients and water are transported, the odds of the tree surviving are slim to none. Once the disease is in action on the main trunk, there is no other way for the water and nutrients to get to the rest of the tree and thus it dies. On the bright side, if it is found on the branches or limbs there is a good chance the tree will live. Pruning out the limbs and branches that look like they have the disease on it or are dying will solve the problem (2).

Disease Cycle
Hypoxylon canker of shade trees is an ascomycete fungal disease (1,6). The fungal pathogen must take advantage of wounds in their host tree or unhealthy tree tissue to infect and colonize the host, as normal, robust trees are not very susceptible to infection (1). However, many hosts of Hypoxylon canker, such as red and post oaks, are especially sensitive to external factors; thus, they are easily stressed and can become susceptible hosts (2). It is often noted that Hypoxylon canker does not kill a tree--it merely capitalizes on trees that are already dying due to stress or other means (3).

Hypoxylon canker of shade trees has both an asexual (anamorph) and sexual (teleomorph) life cycle (2). The fungus can infect seedlings and typically colonizes under the bark of the trees (3). Since the fungus infects under the bark, the signs of the pathogen aren’t visible until the bark sheds off in response to the physical pressure imposed by the growthing pathogen; this typically happens in the transition between spring and summer (3). This shedding reveals large brown or black fungal mats, called stroma, which are the reproductive structures that house the asexual spores, conidia, on conidiophores (4). At this point, the reproductive structures will release conidia (3). These spores are distributed by wind or rain splash (3). If these spores land on a susceptible host, successful infection will take place if there are humid and warm conditions for 48 hours (1).

While the spores can colonize and slowly grow under the bark of trees, they do not demonstrate any pathogenic effects or grow in great numbers until the tree is water stressed; for this reason, the year following a significant drought is often when Hypoxylon canker is seen in shade trees (4). Thus, before stress, Hypoxylon only infects its host in a local, benign manner, with limited propagation. Once the stress occurs, however, the fungus colonizes the inner parts of the bark and vascular system; while the infection is not fully systemic, it is certainly less localized than before stress (4). After the conidia have been dispersed by late summer and fall, the stroma turn very dark and crusty (3). At this point, the stroma produce sexual fruiting bodies. These fruiting bodies, the sexual spores--ascospores--are produced in perithecia and can also be distributed in a manner similar to the conidia (7, 14).

It is important to note that the Hypoxylon atropunctatum can remain dormant in healthy trees for many years before becoming pathogenic due to plant stress from temperature, water loss, physical and chemical diseases, and other plant pathogens (5).

Environment
Hypoxylon canker greatly depends on dying tree matter, which is why it generally attacks only damaged and stressed trees. Normal, healthy trees are able to fend of infection with their natural defense mechanisms (2). Furthermore, it is known that the Hypoxylon fungi already live on the majority of surrounding trees and are only waiting for a possible infection route to be created. When damage or stress occurs, the fungi will infect and begin living off the nutrients of the tree, flourishing in the dying material (2). Natural stresses such as defoliation by insects, animal damage, soil nutrient depletions, drought, overcrowding, and other disease can weaken trees, enabling Hypoxylon canker to infect (9). Human-caused problems can also lead to infection; soil compaction and excavation, construction, and physical injuries are known stresses to trees (9). Drought is one of the major indicators that Hypoxylon will become a problem in the following months to years. Droughts account for a large majority of tree-related problems as the water deficit has a negative effect on the tree’s natural developments (9). Symptoms usually develop within a year after a drought, but with extreme conditions can develop in a matter of months. (4). A major factor in the survival of Hypoxylon canker is the moisture content within the tree wood. When the stresses mentioned above begin to weaken the tree, moisture content of the wood decreases. As the wood further dries, the Hypoxylon fungi will feed and further rot and decay the tree, potentially causing death (9).

Management
Although there is no known cure for Hypoxylon canker, proper management practices and care can significantly reduce problems that may occur. Generally, avoiding tree stress and maintaining an overall healthy tree is the most ideal way of avoiding the canker. Avoiding any physical injuries to the trunk, limbs, or roots is important as these injuries can disrupt growth processes and xylem/phloem transport. Disruptions to the soil surrounding the tree should also be minimized. Any major disruptions, such as landscaping or irrigation modifications, in the surrounding environment can develop significant problems for both young and mature trees (2). Proper herbicides, pesticides, and fertilizers should be used in further tree protection. Additionally, adequate propagation along with good drainage is extremely important in sustaining a healthy tree. Studies have found that Hypoxylon canker favors lower tree density stands in addition to only stressed trees (10). This could be due to the mode of spore dispersal, the inability of spores to germinate on older bark, the fungal method of entry, and the specific tree age (11). If possible, minimize forest thinning unless recommended by an arborist or qualified specialist. When dead or dying tree branches do appear, selective pruning should be performed to remove these potential infection sources. The removal of dead or infected regions can prevent or considerably reduce advances of Hypoxylon (3).

Importance
The most significant impact of Hypoxylon canker of shade trees is that the ascomycete fungal pathogen can contribute to the death of many species of tall, mature shade trees (4). While this can have an economic impact, since many of these types of trees are used for aesthetic landscaping in residential and commercial areas (1), there is also an ecological impact. Specifically, Hypoxylon canker is most devastating to forest aspen trees (1). Historically, aspen trees, especially in the American West, are in decline due to a variety of factors; in fact, Aspen distribution in Utah, in National Forest System lands, has decreased by 60% (8). While Hypoxylon is not the major cause of this decline, the fact that the fungus can take advantage Overall, today, the loss of aspen or any other species of tall, mature shade trees can have a large impact on the surrounding ecosystem; these trees can serve as habitat for a variety of animals (5). Furthermore Hypoxylon canker of shade trees is found throughout the United States, in regions such as the mountain West, coastal South, and Midwest/Great Lakes region (5, 8, 1).