We get many questions at the Extension office all year long about pecan trees. Leaves having bumps on them, limbs falling, pre-mature nut drop, leaves and shuck turning black, etc. I will try to describe a few of the pests associated with pecans and maybe this will help you understand what is happening to your trees.
One of the first pecan calls I get in the spring is pecan phylloxera. This is a tiny insect that feeds on the young new growth including shoots, shucks, and leaves. As the leaves and new growth mature you will notice many knot like structures. There are different species of pecan phylloxera; some will only feed on the leaflet which is not a major problem. The ones that feed on the shuck and leaf petiole are the bad ones. When they feed, the plant forms a gall in reaction to the feeding of the insect. The insect continues to feed and lays eggs when they are mature. If galls are on the petiole the leaf may drop, if galls are on nutlets, they may not mature.
Fall webworm builds the web structures on ends of pecan branches as well as other trees. These insects are not a major pest that is hurting your pecan production. It is possible for high numbers to decrease pecan production, but it is not a pest a home pecan grower would be able to manage. A pecan farmer who maintains a spray program may not have this problem, but it is not something that should concern a home producer. Please do not try to burn these out of trees. They look bad, but management is not recommended to a small grower.
Twig girdlers are a very interesting pest. These insects will lay eggs on a branch about pencil size or a little larger, and then girdle the branch. The branch will eventually break off during a wind and fall to the ground. Other insects can girdle the branch, but the twig girdler makes a clean beveled edge and is very common. A good management practice would be to pick up the fallen branches and destroy them.
Other insects that damage the shuck or nut include hickory shuckworm, stink bug, nut curculio, pecan nut casebearer, and pecan weevil. Black and yellow pecan aphids can be found feeding on the foliage along with several other insects that I can provide information about if anyone is interested.
Pecan scab is the biggest pest of pecan trees in the southeast. This is a fungal problem that causes spots on the leaf, leaf petiole, and pecan shuck that causes early nut and leaf drop. A pecan farmer would have a spray program designed to manage this pest, but I do not recommend a home gardener try to spray. The pecan farmer has an air blast sprayer capable of reaching the top of the tree and will spray multiple times each year. If you are planting new trees, I can recommend some trees that are showing resistance to scab. For older plantings, I would suggest using cultural practices to make the tree as healthy as possible. Pecan trees can have other disease problems, but scab is the most common. Scab will be worse during rainy seasons compared to drought years. Although difficult to accomplish, it will help if all the leaves, stems, and undeveloped nuts that fall from the tree were hauled off site and destroyed.
Since I do not recommend a home gardener spray a pecan tree, what could a small home gardener do to increase production? The answer is maintain healthy foliage to increase carbohydrate reserves. What that means is keeping the leaves as healthy as possible will increase production. Pecan trees that consistently drops their leaves early in the fall would not be as productive as other pecan trees that hold leaves until much later in the season. The best ways to maintain healthy foliage include weed control, mulch, irrigation, and proper fertilization. I would start a pecan fertilization program in April, and the Extension System can provide information on fertilizer.
The weed control, mulch, and irrigation should be done at any time of year that it is needed. Irrigation may be needed many times of year when the tree is actively growing; however, it is most important during the months of August and September. If you are planning to plant pecan trees in the future, I would plant them no closer than 60 feet apart and I prefer 70 or 80 feet for good sunlight and air circulation when they become mature trees.
If you have any questions about pecan trees or most anything else, give us a call at your local County Extension Office.
Dr. Chip East
Alabama Cooperative Extension System