Answer: What you describe doesn't sound good! With any luck at all you're seeing weed whacker or lawn mower damage. If so, the tree will overcome the injury over time. On the other hand, the symptoms you describe are typical of a disease called crown canker. Crown Canker, also known as Collar Rot of dogwood is caused by the pathogen Phytophthora cactorum.
This disease causes injury to flowering dogwood (Cornus florida) and may kill the affected tree or weaken the tree and make it more susceptible to attack by other organisms.
The most outstanding symptom and the real cause for death of the top of the tree is the slowly developing canker on the lower trunk near the ground line, referred to as the "crown" region of the tree. The pathogen kills the bark just above the ground level. Often at an early stage of canker development, infected areas of bark will ooze dark-colored fluid. At this time, infections can sometimes be found by gently removing thin layers of bark in the affected area. If the disease is present, the inner bark, cambium, and sapwood show discoloration.
Over a period of months to years, this killed area of bark becomes sunken, dries, and falls away leaving the wood exposed. The canker is then visible and its seasonal progress is not hard to see. As the tree becomes weakened, it is more susceptible to attack by the dogwood borer and is more severely affected by short, droughty periods during the summer.
It is too late to control crown canker after the fungus has invaded a large part of the tree base. However, if the infection is confined to a relatively small area, it may be possible to stop the spread of the disease at the root collar or trunk base by excising the canker. This simple surgical operation can be done by removing all the discolored bark and sapwood in the canker area and, in addition, removing about an inch of healthy bark and sapwood around the canker with a sturdy, sharp knife.
Wish I had better news for you!
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