Hypoxylon can be identified by the dark-colored, crusty fungus tissue on the dead, cankered areas. Light-colored spore masses are frequently seen on the branches and stems of severely affected trees. Large pieces of bark may slough off, exposing the fungus underneath. Bark may slough off in patches but, characteristically, it sloughs off in strips up to several meters long. Yellowing or wilting of oak leaves may signal the onset of hypoxylon, but these symptoms are typical of many stress factors and are not necessarily caused by this disease.
Willow and water oak trees appear to be most susceptible, followed by red oak and occasionally post and live oak. Although found throughout North America, this disease is primarily a problem in the Southern United States.
Hypoxylon is a fungal disease. Spore masses may vary in color from tan to bluish-gray to black. These spores, transported by the wind, cause new infections on wounded or stressed trees. Hypoxylon fungi are commonly present on healthy trees. Under conditions of extreme stress, the fungi can switch from being benign into an aggressive pathogen.
Keep the trees healthy using proper tree care practices. Prune out any branches showing early infection, since this reduces the amount of spore inoculum. If grade changes or construction are anticipated, contact a certified arborist for recommendations. Insect populations should be managed to reduce stresses that result from insect feeding.