Why Don’t the Trees Look Better?

Given the unusually cool weather and ample rainfall over most of the state this summer, one might expect to see trees looking full and healthy. However, a fair number of trees from a wide range of species do not fit that description. I have observed many crabapples and red maples with sparse foliage; ornamental pears in serious decline; and planetrees, cottonwoods and ashes losing leaves in late summer. Following are some theoretical explanations for the less-than-impressive appearance of some of these species.

First, I should state that this has been the coolest and wettest summer that I can remember in my 25 years in Missouri. While it has reminded me of Oregon, it is important to note that even Oregon summers tend to have dry spells. One theory that is emerging in my mind to explain the poor growth of some trees is related to the effects of soil drying on drainage. In most summers, there are significant periods when precipitation is much less than evapotranspiration, leading to moisture deficit and soil drying. When a clay soil dries, it tends to form cracks, often leading to improved aeration and drainage. When the soil stays continually moist all summer, there is no opportunity for cracks to develop. And during especially rainy periods, this could lead to substantially lower oxygen levels in the root zones of trees than in a more “normal” year. Another problem associated with poor gas exchange is accumulation of carbon dioxide to phytotoxic levels. So, while we usually complain about the cracks in our lawns in July and August, perhaps they are a necessary evil if we want to maintain gas exchange in the root zones of trees growing in clay soils.

While we normally see development of foliar diseases on certain trees each year, they usually subside with the hotter, drier weather of mid- and late summer. This season, the cool temperatures and frequent rainfall have greatly increased the number of hours that leaves have been covered with a film of free water. Some of the “Bloodgood” planetrees on campus have been almost completely defoliated in August by what appears to be powder mildew. Although it is common for planetrees to develop mildew, I have never observed complete defoliation of this tree in late summer. Anthracnose can cause significant defoliation of planetrees and sycamores in spring, but it is has subsided and trees are refoliated by August. Similar development of unusual disease patterns due to unusually cool, wet conditions might explain the defoliation that I have observed on cottonwoods, ornamental cherry and perhaps some ash.

The seemingly accelerated decline of many ornamental pears this summer is puzzling. For the past five years, Extension Specialists all over the state have described pears slow to leaf out, developing sparse foliage with poor color. Often, these unthrifty trees are right next to trees that appear healthy and vigorous. One theory that has been forwarded to explain this phenomenon is that of delayed graft incompatibility. All of the cultivars of Pyrus calleryana, such as Bradford, Aristocrat, Redspire and Cleveland Select are propagated by budding. In some cases, a bud union may form and a tree may grow vigorously for a number of years. Then, for some reason, the bud union may begin to deteriorate, leading to lack of continuity between the vascular systems of the roots and trunk. Since transport of water and nutrients in such a tree is impaired, there may be a gradual decline. If this explanation is correct, however, a cool, wet summer would be expected to minimize the effects of the impaired vascular connection. Perhaps we can fall back on theory number one (poor gas exchange due to no cracks in the clay) to explain the apparently accelerated decline of some pears this season. Another possibility (highly theoretical) might be that there is a pathogen (fungus, bacterium or virus) that might be attacking the bud unions of ornamental pears and that wet conditions may be favoring the development of the pathogen. Post mortem examination of a number of declining or dead pears would be very useful in testing the above theories.

Regardless of the causes of the poor appearance of some trees in Missouri landscapes this season, we can be assured that next year will bring new surprises. Most landscape plants look good, and most of those that don't will probably recover from whatever ails them. Although much of the state had too much of a recovery from recent droughts, the soil moisture is charged up, and plants should respond well to that next year (when the cracks will likely return in July). When transplanting a large spruce tree in my landscape a few weeks ago, I observed that the soil was uniformly moist down to about 4 feet deep.

Chris Starbuck, Woody Ornamental Horticulture, UMC, (573) 882-9630