Date: Thu, 15 Aug 2013 18:31:06 +0000
From: Al Cooke
Subject: Trees defoliating
I am going to attempt to address two plant related questions from Chatlist postings this week. The first regards crape myrtles from Savannah Savvy and the second regards maples from HW Queen.
But first let me address the issue of premature leaf drop involved with both of these plantings as well as numerous others. About August in most years I start getting questions about trees that are coloring prematurely and/or dropping leaves. The generic response is that the trees are under some kind of stress. Stress may be from too much or too little water, an unusually hot summer, competition from other plants, injury to trunk or roots by voles or string trimmers, nutritional deficiency, disease, insects, or a wide variety of similar things. With trees the causes of problems may go on for several years before the symptoms become obvious. It generally will take some more investigation to get more specific about the causes of premature leaf drop. It is generally not wise to take action without knowing the underlying cause. And while causes are often not easy to figure out, weather is sometimes a good catchall to ponder. A very wet period in June may have lead to root injury (lack of oxygen in the soil) that could be an explanation for any plant being under stress that may show symptoms many weeks later.
One place that does give more cause for concern is when trees lose leaves at the top. It’s normal for leaves and even branches to fall from the bottom or interior portions of the canopy where they are getting less light and making less contribution to plant health. When you see a bare branch sticking out from the top of a tree, it suggests the possibility of long term decline that may be worth hiring an arborist to investigate further.
Crape myrtle needs to be in full sun to bloom at its best. Blooming is part of reproduction and many female readers suggest that reproduction takes a lot of energy. It takes a lot of energy for plants too. And the source of that energy is the sun. Some plants are more efficient at using light and can bloom in the shade. Crape myrtle is not one of them. One thing that happens with landscape plants is that they grow enough to change the environment. Plants that were once in full sun may be in deep shade in a few years. In this case, the oak tree is likely also getting a lot of water and nutrients from the soil. As far as the crapes are concerned, the oak is competing for limited resources and probably is doing a better job. The resulting stress in the crapes can cause them to color and defoliate early. You could consider transplanting them. But as large as they are, that can be a big job involving hundreds of pounds of plant and soil per plant. It will also damage roots of the oak to dig up the crapes. Tough call with no easy answer.
As HW Queen suggests, there are many leaf spot diseases of maples. All of them are largely cosmetic and pose little hazard to the tree. I have never known of any knowledgeable horticulturist or plant pathologist who found these spots worthy of any treatment, the exception being nurseries that need the trees to look suitable for sales. (It’s hard to sell an ugly tree even if it is really actually quite healthy and well shaped.) If the leaves are falling off, it’s probably some stress as indicated above. Stress would also make the plant more susceptible to leaf spot pathogens. It’s quite likely that the spots are not causing defoliation but that the spots and the leaf drop are both caused by the same underlying cause. Maples in general have not had a great year and the amount of rain in early summer may be a part of the cause. Unless you are seeing dieback in the top of the canopy, this might be OK to wait and watch.
There is one other potential pathogen of maples that could be called a spot although it looks more like the leaves are scorched on the edges back to a distinct line where the foliage turns from brown to green. If you have this bacterial leaf scorch, it is fatal and no treatment is recommended. The good news is that it would not be likely to hit four trees at once. I have only seen this disease once and that was on an oak. It’s not common, but it is fatal.
alcooke
Al Cooke
Extension Agent, Horticulture
Chatham County Center, N.C. Cooperative Extension
PO Box 279, Pittsboro, NC 27312
919.542-8202; Fax 919.542.8246