Post oaks have arguably the most sensitive roots of all native Central Texas tree species. On multiple occasions throughout my career, I have been called out to diagnose declining post oaks in and around the Austin area. With this particular species, it is extremely important to note that ANY root disturbance at all can lead to a very fast and most often irreversible decline. The trees quickly yellow, slowly defoliate and then eventually die. This can occur quickly or over several years, depending on the extent of the root damage.
Most post oak decline, until recently, was associated almost entirely with construction damage. Driveway and new home construction, additions and remodels, swimming pools, etc. In these instances, we were often unable to reverse or even halt the decline and bring these trees back. It is extremely Important with post oaks to try to eliminate any construction around the roots at all, or to protect as much of the root zone as possible PRIOR to construction.
As if that weren’t enough, we now have a much bigger problem. Hypoxylon canker, Hypoxylon spp., is a fungal pathogen that is now moving into the Austin area. This fungus is not new to Texas, but is now starting to wipe out post oaks in Bastrop and closer in to Austin and in rural Georgetown areas. Although this disease seems to affect mostly weakened trees, the recent droughts have exacerbated the problem by stressing post oaks everywhere, and now it is rampant. It is also an extremely fast-moving fungus, and kills and infects rapidly.
The fungus is transmitted by beetles and airborne spores from tree to tree, not by the roots. However, there is currently no cure for this disease. The only preventive measure is to invigorate the trees by invigorating them by watering them during drought conditions, and removing broken and damaged limbs from uninfected trees to help prevent/reduce avenues for infection. Removing infected trees is also not considered effective due to the extensive nature of the fugal spores in infected areas. With post oaks, once the fungus has infected a tree, eventually it kills the tree or large parts of the tree, and the bark falls off, exposing the brown fungal spores. If you touch the exposed wood at this point, you can actually see the brown very fine spores cloud up into the air. If the tree is not completely dead at this point, it soon will be.
Lightning damage can also lead to infections. It is currently still under debate as to whether the disease infects healthy trees as opposed to only stressed, unhealthy trees, although the Texas Forest Service has mentioned that increasing the wood water levels to ‘normal’ amounts can help reduce infection. Either way, I see it as a catastrophic killer of post oaks and I think that it is well on the way to eventually wiping out all Central Texas post oaks if nothing changes. I am also concerned about the susceptibility of our three oak wilt resistant species, bur, chinquapin and Monterey live oaks as they are also white oaks. I have already seen it kill a 50 year old bur oak in Austin last year.