Dallas Morning News - September 20, 2019
Hypoxylon Canker in Trees
Hypoxylon canker in trees – scary sounding thing, right? It is a fungal disease that is common on many hardwoods. An opportunistic fungus, Hypoxylon atropunctatum, causes it. Red oaks are more susceptible than trees in the white oak group like bur, chestnut, chinquapin and white oak. It can also be found on elm, pecan, hickory, maple and sycamore. It is usually manifest as black or gray splotches where bark has been sloughed away.
Hypoxylon canker on a limb
Hypoxylon cancer on bark
Here's the most important part of the story. Hypoxylon canker is never the cause of problems in a tree. It is basically unable to cause serious disease in healthy trees but can quickly colonize weakened trees, especially those with dying bark and wood resulting from other issues.
Hypoxylon sets up shop in sick trees that are weakened by drought, root disease, mechanical injury, soil contamination, construction damage or being too deep in the ground. These true causes of stress enable this opportunistic fungus to produce cankers on branches and trunks. Perfectly healthy trees can even develop this canker on lower limbs that have been shaded out by dense canopies, but that’s not a serious treat to the overall health of the tree.
Some arborists think that there was an increase of this fungus in recent years and that more oaks and pecans have been dying across the South. That could be the case due to environmental stresses, poor site factors, drier weather, air pollution and various forms or soil contamination. It's not however because sick trees are infecting other trees.
Following the droughts of the early 2000's for example, many trees got in trouble, got sick and were severely damaged. Post oaks were especially hard hit. Hypoxylon canker showed up on many of these trees. That's its job – to push sick trees over the edge to death.
It is said – by people who should know better - that there is no effective control for this disease and that if over 15% of the crown is infected, the tree should be cut down, ground level, burned or be from the site. It is also said that since the fungus can remain active in dead wood, it should not be chipped and used for mulch or burned as fuel.
I don't agree with any of that. Hypoxylon canker in the tree trimmings is not a risk and does not creep out of the pile to attack healthy trees. But when a tree is severely weakened, it can move in and be the final knife in the heart of the tree. So - keep your trees healthy by avoiding high-nitrogen synthetic fertilizers, maintain dramatically exposed root flares and apply the amendments of the Sick Tree Treatment as time and budget allow. Hypoxylon canker will not be bothering your trees.