Bleeding Black
Hypoxylon canker in Texas red oaks
Hypoxylon canker rots a red oak from the inside out. Photo: courtesy Dell Hood
As the disease advances, the bark looks as if burned to ash. Photo: courtesy Dell Hood
Click on either photo to view it in more detail.
Dell J. Hood
It’s a hard fact of life in Central Texas. The oak trees we treasure in our landscapes suffer no less than we do from disease, damage, and decline over the years. We hear reports of environmental stresses, especially drought and summer heat, the effects of which can be magnified by our poor land management practices. We learn about oak wilt and how to prevent its spread, but there are many other organisms which infect oaks and virtually all other woody species.
Over the past year I have noticed that a distressing number of red oaks (Quercus buckleyi) on our land have become infected with Hypoxylon canker (Hypoxylon atropunctatum, more recently named Biscogniauxia atropunctata), a fungal disease widespread across the U.S. that attacks oaks and other hardwoods. Hypoxylon is Greek for “under the wood.” The Latin prefix atr- can mean dark, black, or even bloody. So atropunctatum means, most gently, “black spotted.” Translations give us clues!
The first clues we had were a peeling of bark on the trunk and thinning of the canopy as limbs died and were broken off by strong winds. This was followed by orangish brown splotches scattered on the trunk. The fungus infects the inner bark and produces hyphae—threads—which form dense mats called stromata, which then produce the spores by which the fungus spreads to other trees. Once the spores have entered the sapwood, the disease rapidly deteriorates it, producing dark decay lines running through it. By this time the tree is essentially dead.
No treatments are available to stop the disease.
As the infection grows in the sapwood, the stromata grow larger, and more limbs die as the tree’s ability to send water and nutrients to the canopy is lost. Toward the end of summer, I saw the most recognizable and definitive evidence: stromata covering as much as a quarter to a third of the trunks of infected trees. Eventually, the lighter color stage of the fungus becomes black, giving the tree the appearance of having been charred by fire.
Arborists and forest service agencies agree that the Hypoxylon fungus is widespread from the eastern forests through the South and Southwest to the Rocky Mountains, where it is a severe problem for aspens. Demian Gomez, Regional Forest Health Coordinator of the Texas Forest Service based in Austin, wrote that his office has seen reports across central and north Texas of its affecting multiple hardwood species in the past year.
It most frequently infects trees that are stressed by one or more environmental and physical factors. Locally, the most damaging stresses have been the multi-year drought, ice storms, and extreme cold accompanied by strong winds, causing broken limbs and exposed sapwood.
The best prevention for this canker is to maintain tree health by preventing human-caused injuries and ensuring enough water is available to keep the tree vigorous. The fungus does not readily develop and reproduce in healthy tissues. If canker is limited to branches, they should be pruned off and the wound sealed to prevent infection by the other oak nemesis, oak wilt. If you have infected red oaks, they should be removed as quickly as possible. The fungus will die out if the spores do not find damaged or stressed host trees. Until a specific anti-fungal agent becomes available, we will have to live with repeating cycles of growing infestations followed by retreat.