For The Answer
Saturday, August 25, 2001
There are a number of plants that are reacting to our long period of hot weather and lack of rain this summer. Pecans in the landscape are typical of that group.
This spring was wonderful for pecans. The cool weather and well-timed rains resulted in full crowns of large lush leaves. In addition, pecans have a tendency to be alternate bearing. Last year was an off year for most trees in the San Antonio area. This year trees in most neighborhoods are loaded with nuts.
It is not unusual for wet conditions to change to dry conditions in San Antonio. For a species to survive such cycles of extreme requires mechanisms to adjust to the new conditions. Pecans do it by dropping leaves and nuts. The goal of the tree would seem to be to retain leaf cover in balance with the roots capability to bring up water to support leaves and the nuts. The adaptation is not perfect. In addition to dropping some nuts, as the drought persists, some of the remaining nuts may not completely fill out because of lack of water.
If you examine the nuts that are falling, some will probably have black spots. Stink bugs inject the young nuts with some of their digestive juices and then feed upon the dissolved area. Even more noticeable are the holes left by birds, mostly grackles, obtaining water from the maturing pecans. Pecans have a water stage when the interior is accessible source of moisture for thirsty birds.
Nuts that are small and brown when they fall have usually been hollowed out by pecan nut casebearers. In years past, we attempted to spray for them but, in an urban setting, it is not practical.
If the leaves dropping to the ground have angular patches of yellow, your pecans may be infected with black aphids. Look carefully on the underside of the leaf for the pinhead-size insect. Only three or four may be visible, but it does not take many to affect the leaves. Yellow aphids may also be present on the leaves. They also make their living by sucking the juices from pecan leaves, but do not have the impact of the black aphids.
Like for pecan nut casebearers, it is not practical to spray insecticide for aphids in an urban setting. The sprays that are available are more effective at killing beneficial insects than they are aphids. Within a few weeks the aphids are back in greater numbers than before, along with honeydew (syrupy excrement) on the driveway and cars.
Leaves and nuts falling can be messy but are not much of a threat; unfortunately, branches also fall from pecans loaded with nuts in droughty periods. Some trees are especially prone to the problem. Pecans with branches over the house or play areas should be pruned on a regular basis by a professional.
In summary, expect your pecans to drop leaves and nuts this summer. Some varieties may also not fill out well. The main reasons are the drought and heavy crop. Aphids may also contribute to leaf-drop, but it is not practical to spray for them. A deep watering on the drip line once per month may help the nuts fill out. To reduce the danger from branches breaking, ask your arborist to do any necessary pruning.