Question
Late last season my 3 crepe mrytles - Tonto reds - became infested with aphids and the foliage darkened by the resulting black sooty mold.  The trees were healthy all summer and bloomed beautifully.  I have a dog that regularly urinates in this area and I think the resulting high nitrogen content is a causative agent in the problem.  Early in the season I noticed aphids and treated for them with neem oil and insectical soap with good results.  The trees were doing great and then as I waited for the foliage to turn lovely colors in the fall, the leaves instead started to turn black.  This all happened so fast - everything was looking great and then all of a sudden - Black leaves.  There was definitely an infestation of aphids, with the resulting mold.  I sprayed the trees with a combination of sevin, neem, and insectisidal soap and got rid of the aphids.  It was so late in the season that the leaves were done for and just dropped with the black mold on them.  The bark of the trees was blackened.  Now as spring approaches what should I do to continue to treat and to prevent any future problems.  I will do my best to keep the dog out of this area.  Can I add anything to the soil to break down the nitrogen if that fails? What about the black bark?  After the severe cold of the winter I had hoped I could start off on a clean slate.  The area is mulched with pine straw - should I remove the old before adding new?    Much thanks for any advice.  I  try to spray and treat sparingly using the treatments that are the least toxic to the environment.

Answer
First the aphids. The aphids suck the plant juices from the leave and twigs and secrete a substance called honeydew. This is high in sugar content and a sooty mold will grow on the honeydew. This is the black you are seeing on the bark and twigs and leaves. When you control the aphids the sooty mold will slowly go away since it has no food supply. This spring use an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is poured around the tree and the roots take it to the leaves and when the aphids suck the juices they are killed. The product says it will last for 12 months but for sure it will last through the growing season and protect the trees from aphids. It is much safer than sevin or the insecticidal soap. The active ingredient in this product is imidacloprid and is listed as an LD50 of 5,000mg/kg Table salt has an LD50 of 3,000mg/kg which means that this product is safer then salt. Since it is applied to the soil it will not be blown to other areas and it will break down fast so it is not a problem with the environment.

No the old pines straw should not be removed -is will decay and the added straw will add to the weed control around the trees. Since the trees are blooming good I would not fertilize them. One sign of over nitrogen fertilizer is the reduction of blooms and this is not the case here. I would keep the dogs away to reduce the amount of urea that could harm the root systems. Watering will dilute the urine and help reduce and effect to the tree.