crepe mrytle

i would like to help my neighbor with her crepe mrytles. the landscaper planted them on the shady side of the house. her husband waters way too much, every day the system comes on. i have mentioned it but they dont get it. each year the cm get this black stuff all over the leaves. i know it is some type of mildew probably, what should they use to rid the leaves of this?

In our area, crape myrtles are very susceptible to powdery mildew. It appears as a gray substance on the leaves which causes the leaves to curl. Crape myrtles require very little water when mature - that's one of the advantages in growing them in hot, humid areas like central Alabama. I would be very cautious about foliar spraying. Hope this helps.

why would you be so cautious about spraying? if they were mine i would cut them way back and start anew with little water since they are one the shady side of the house

I wasn't clear - I was referring to your neighbor over watering the plants. Spraying water on the foliage of the crape myrtles can cause problems. I was not referring to chemicals - I don't use them.

I might also recommend that they transplant them unless they like the crape myrtles just for foliage--they tend to bloom much better (and probably suffer less from mildew too) if they're planted in the sun. I'm curious though why they didn't listen to the suggestion of watering less...are the sprinklers there to water a lawn as well, or is it just for the trees? If it's watering the lawn too then they're unlikely to change their watering habits which means eventually these trees will die, and hopefully you can help them pick out replacements that are more suitable for damp shady conditions! As far as something they could spray on the leaves, I don't know that anything is really going to help unless they change the conditions that they're keeping the plants in--you might get rid of it temporarily but as long as they're overwatering like that and probably splashing water everywhere with the sprinklers the mildew is just going to come back.

They are on the list as a Texas xeriscape plant...might want to let your neighbor know they need very little water.

They don't like shade either.

I would not cut them "way back" as someone suggested. That is called crepe murder and leaves the trees looking terrible.
We have that black stuff in southeast Louisiana especially the years when it rains a lot at one time. It also gets on my camellia bushes that are underneath the crepe myrtles. I wash it off the camellia leaves but the crepe myrtles just seem to do o.k. with it. Lady bugs attack it every once in a while and that helps some.

The problem with your neighbor's crepe myrtles is caused by honeydew from aphids dripping on the leaves where sooty mold then grows. Powdery mildew is a different mold. It is not a serious problem, but it does look bad and interferes somewhat with photosynthesis. The lady bugs liveoaklady is referring to don't attack the mold, but rather the aphids feeding on the undersides of the leaves. Spraying the crepe myrtles with horticultural oil, making sure the spray reaches the underside of the leaves, will help control the problem with little harm to the environment.

I don't think you can kill a crepe myrtle even if you tried.