Plant Answers

Weekly Express-News Article
By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist
Saturday, September 1, 2007

“Leaf Drop and Wind Breakage”

Thankfully heavy winds from Hurricane Dean did not reach San Antonio; our trees are in a vulnerable state for wind damage. The crowns are full of leaves after the rainy summer and with the shallow root system in a soggy soil, some would have been blown over. Regular pruning (every five or seven years) helps reduce the likelihood of trees being blown over and it is always best if shade trees are not planted too close to the house, but this type of damage is hard to predict or prevent. The good news is that it is not common. If the winds do not knock whole trees over, what you will experience this summer and fall is leaf drop due to fungus, and if the rains slow down, even a leaf drop due to adjustment of crown to roots. If soils dry even a limited amount it is difficult for the tree to support all of the leaves that were produced during the long wet periods. Both the fungus and leaf adjustment drops are normal. Adding fertilizer, fungicide or water is not necessary.

Pecans are not the most vulnerable tree to blow over, but they are very prone to limit breakage. This year with the full crowns and large nut crop they have been breaking at an alarming rate. Some horticulturists believe you can reduce the chances of limit breakage by trimming off some of the pecans. Prune off the end nut clusters of as many limbs as you can reach with a pole pruner.

Limb breakage is only one of the reasons that pecans are not recommended as shade trees for San Antonio landscapes. They become huge trees up to 80 feet tall. They not only shade your yard, but the rest of the neighborhood as well. Another reason to think twice or three times before planting a pecan for a shade trees is their attractiveness to insects, particularily webworms and aphids.

Webworms are common in pecan trees in the spring and again in the fall. They are easy to kill with a Bt product or almost any other insecticide if you can get it high enough in the tree. That is the trick. It takes a professional spray to cover a 60 – 80 feet pecan and if you think it makes the neighbors unhappy that your huge tree shades their yard, they will be even more unhappy if you spray their house and landscape along with your tree. It is best to ignore the webworms. One effective treatment is to open up as many webs as you can reach with a long cane pole. The wasps will carry off large members of the caterpillars and the sun will fry most of the others.

Webworms can be ignored if you do not look up, aphids are more difficult to ignore. When their populations reach a high level in pecans, the ground under the trees becomes covered with honeydew. Honeydew is aphid excrement. It is like syrup and can make everything under the tree a sticky mess. As the “honeydew” accumulates it serves as a food source for sooty mold that grows all over the leaves and anything else that is covered. The aphid population eventually declines by natural causes and five or six weeks later the sooty mold also disappears, but for two or more months it is a mess.

The pecan crop is heavy this year, and you would expect the nuts to be full and succulent, unfortunately stink bugs and pecan scab have attacked many of the nuts. The result may be a large number of nuts with blackened, shriveled kernals. Some damaged pecans will be obvious, they will have holes in the shell and/or they may be very light weight. Others will not be obviously a problem until they are shelled. Refrigerate or freeze the harvested pecans as quickly as possible to preserve quality.