Karen Cambre, Sharpe, Kenneth W.
News article for July 23, 2018
Crape myrtles are having an excellent blooming year which seemed to start earlier than most years. I am also noticing a lot of premature leaf drop and leaf color change and it is way too hot to be autumn already.
The problem that I am seeing is a fungal leaf spot known as Cercospora leaf spot. Symptoms include leaves that turn yellow, orange and red and are falling from the tree. This leaf spot on crape myrtles can cause severe defoliation during wet periods in the summer. I have seen it much worse, but we also have a lot of hot weather to go this year. The severity will usually depend on how much rainfall you receive. Since rainfall has been spotty, I would expect this outbreak to be spotty; however I have seen it in multiple locations.
Treat leaf spots with a systemic fungicide which contains propiconazole (such as Fertilome Liquid Systemic Fungicide II and Bonide Infuse Systemic Disease Control) or tebuconazole (such as Bayer Advanced Garden Disease Control for Roses, Flowers and Shrubs).
Another prevalent problem in crape myrtles is sooty mold, which is a black crusty mold that gets on leaves and is very unattractive. Sooty mold is actually the results of sucking insects such as aphids, which leave a high sugar secretion or honeydew on leaves. Sooty mold is always in the environment and will attach itself to the honeydew. The way to control sooty mold is to control sucking insects.
If you are using an Acephate band to control sooty mold, which works well, the end of July to the first part of August should be your timing for the second and final application for the year. If you did not make an early treatment, it is not too late to start. The insecticide treatment is to mix 4 parts of Orthene 75S or Acephate 75S with 1 part water to make a paste. Use the paste to paint a band around each trunk of the crape myrtle tree several inches above the base of the trunk. The band should be twice the diameter of the trunk. So, if you have a 1 inch diameter trunk, paint a 2 inch band around it. Everything above the painted band will be insect protected.
Each crape myrtle variety has different growth and flowering characteristics. Some varieties will start blooming as early as mid-June and others will not start until mid-July. The flowering period for some varieties will only be 70 days and others will flower for 110 days.
If blooming is waning, in addition to insect and disease issues, never overlook the obvious. The obvious is hours of sunlight. The best flowers are produced in full sun and at least 8 hours of daily sun is necessary for good flower production. Many people fail to realize that as shade trees mature, they allow less sunlight for understory plants and rob small trees like crape myrtles of needed sunlight. Without proper sunlight, plants cannot manufacture enough energy through photosynthesis to produce vibrant floral displays. Evaluate your sun patterns.
Fertilization is another key to good flower production in crape myrtles. I would not fertilize now as it is too late in the year to stimulate growth, but be ready to fertilize with a complete fertilizer next spring.
Last on the list for good flower production is do not commit crape murder. Crape murder is a term for excessive pruning. It can hinder flower production, especially when drastic pruning occurs in early spring. Severe pruning will promote so much foliar growth that you get very little flowering. Crape myrtles require very little pruning when the right variety is planted in the right place.