Crape Myrtle issue
I am having a problem with a Miami Crape Myrtle. Two years ago one of the main trunps died. There was a split near the base of the trunk. Since then I have noticed white patches all over the bark (picture attached). Last year another trunk did not leaf out in the the spring. Again, there was a split near the base. Another two trunks had some bark problem near the ground. I suspected Crape Myrtle bark scale and applied a drench of Imidacloprid in early summer of 2018. The shrub appeared heathy and the appearance of the bark improved and the the damaged area near the base of two trunks appeared to be healed. This spring two additional trunks are dead, again with splits near the ground and white ares all over the bark. I doubt that it is weather related since I have grown the same variety of crape myrtle without problems for 20y. I can send a sample if needed but you may be able to diagnose from the two attached pictures. Thanks!
Montgomery County Maryland
In past years, drought has been a big problem in Maryland in the summer, and crape myrtle sailed through that. But, crape myrtles cannot tolerate wet, saturated soils. Usually, that meant not planting in a "wet spot", but with the abnormal rain fall last year, and the wet cloudy year before, many plants that have never had problems with the soil where they are planted have died or declined from root rots and drowned roots.
This may be what is going on with your crape myrtle. We see no obvious disease or insect issues otherwise. The white-gray patches are lichen, a harmless fungi/algae plant which only uses other plants for support. However, when you begin to see a lot of lichen on plants where it was not, that can mean that the plant is declining. Not due to the lichen, however. It's just an indicator.
Improve the drainage around your crape myrtle, if you can. Otherwise, you may need a plant there that tolerates saturated soils. Hard to say what the future holds, weather wise, with climate change and weather extremes predicted.
Thanks for your very helpful advice!
You are welcome.