Hypoxylon canker of shade trees is a weak ascomycete fungus that negatively affects growth and can eventually lead to the death of already dying or diseased host trees. There are many different species that affect different trees. For example, Hypoxylon atropunctatum, a common species, is found on oak trees, Hypoxylon tinctor affects sycamore trees, and Hypoxylon mammatum infests aspen 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 because it is easier to infect. Thus, Hypoxylon had an endophytic stage. An endophyte is an organism such as a bacterium or fungus that spends some of its life not causing disease to the host while being on it. When a stress or injury occurs, the fungus will infect and begin living off the nutrients of the tree. Hypoxylon infects trees throughout the United States, but a few notable wide-scale problems have occurred in Arkansas, North Texas, Oklahoma, and the Mountain West.
Hypoxylon canker of shade trees is a secondary disease: a primary factor, such as drought or physical damage, usually causes the dying and decaying of the tree first before the Hypoxylon canker pathogen infects. This will cause the tree to be in stress and potentially start to decay. The pathogen infects the living tree while it is in stress; it will not infect a tree that is healthy or already dead. In order for a tree to become infected, the tree must be already dying or at least must have branches that are dying. The pathogen will therefore kill the tree if the tree is unhealthy.
Hosts and symptoms
Hypoxylon canker of shade trees has three primary species, through which the pathogen can infect a variety of hardwood shade trees. Hypoxylon atropunctatum is most commonly found on oak, Hypoxylon mammatum is a significant pathogen on aspen trees, and Hypoxylon tinctor infects sycamore trees. 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 to a dark grey color. The bark becomes brittle and flakes off, and black and grey cankers will appear. These changes are symptoms, indications of a disease. They demonstrate that the pathogen is present and infecting or has already infested the host.
Hypoxylon also produces characteristic signs, which are either going to be direct products of the pathogen or the appearance of the pathogen itself. The visible signs from Hypoxylon are conidia and ascospores. Conidia are produced during the asexual stage, while ascospores are produced during the sexual stage. One can sometimes see the conidia with the naked eye if they are structuralized, but it is much easier to see them under a dissecting microscope. When looking through a compound microscope, it is possible to see the ascospores. They will be located on perithecia. The microscopic signs of the perithecium are the flask-shaped sexual fruiting body that contains ascospores.
Lastly, the pathogen is going to be found primarily just beneath the bark of the trees, located throughout the tree and if the pathogen spreads. The pathogen can also spread to the bark of surrounding trees. If one sees symptoms on the main trunk, the tree is likely too diseased to save, since the trunk is the spot where the nutrients and water are transported. Once Hypoxylon is active 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. Fortunately, if Hypoxylon is found on the branches or limbs there is a good chance the tree will live if the fungus does not spread. Pruning out the limbs and branches that look like they have the disease on it or are dying will solve the problem.