Pecan Tree Injury

Observations on 2, 4-D Injury to Pecan Trees

Pecan Research

FRED R.BRISON*

IN EARLY MAY OF 1966, I observed a pecan orchard that had been damaged 2, 4-D. The chemical had been applied aerially to rangeland about 1 mile away and some apparently had drifted with the wind onto the pecan trees.

I inspected the grove in late May, 1966, June 20, 1966, October 20, 1966 and May 24, 1967.

These types of injury and abnormal growth were observed during these visits:

  1. Prompt killing of leaves and young limbs. By late May, 1966, the new limbs which began growth in early spring were dead and nearly all leaves were killed on certain trees. This type of injury was worse in tops of trees and on the sides.
  2. Nut clusters killed. Pecan clusters are borne terminally on current season shoots. When these are killed the pecans which they bear are also killed and there can be no crop that year. All of these current season shoots were killed on some trees, and some were killed on other trees throughout the grove. Damage of this character varied with varieties, with location and possibly with air currents that existed when the trees were injured. This injury was most severe in tops of trees and on sides of trees toward the 2, 4-D drift.
  3. Catkins. Catkins produce the pollen necessary for the development of the pecan nut. These are 3 or 4 inches long, willowy and fragile. It is normal for them to fall off after they have shed pollen. Instead of falling at the normal time, they were dead and still hanging as late as June 20, 1966 on the trees under observation. This was clear evidence of some abnormal influence.
  4. Yellow leaves. Leaves on some trees became deep yellow in late May, 1966. This was definitely an abnormal condition-not previously observed in this grove, and the uniformity and intensity of this yellow color had never before been observed on pecan trees. This yellow coloring was more noticeable on some varieties, particularly Schley and Frotscher, than on others. This color abnormality was temporary. By June 20, a month later leaves of affected trees had regained green color.
  5. Abnormal stem and leaf growth. New limbs on the trees had grown up to 8 and 10 inches long by May, 1966. Following the exposure to 2, 4-D, some of the limbs produced a malformed type of growth. Leaves on the older basal portion of shoots which had formed prior to the date when the 2, 4-D was applied were spaced normally, but very far apart on terminal portions of shoots that grew later. The later-formed leaves were chloratic, willowy and pointed somewhat like okra leaves. Leaflets were spaced wide apart on the central axis. The terminal leaflets of many of these malformed leaves were dead. This type of damage is characteristic of a response of cotton, for example, following exposure to 2, 4-D.
  6. “Copper colored" leaves. On June 20, 1966, six weeks after the application of 2, 4-D, many leaves had an abnormal green copper-like color, entirely different from the normal green color of pecan leaves. Many trees, which appeared to be normal in other 

     respects, had these copper-colored leaves. They occurred over the entire tree, and all leaves were affected on trees that showed this symptom.

    These affected leaves were also “cupped” in shape, the cup or convex formation on the underside, with the upper side oval. This characteristic was more pronounced on certain varieties, notably Success and Odom, than others. It occurred on native pignut limbs also.

    1. Deformed pecan clusters. The central stem of a pecan cluster normally grows straight and produces closely spaced lateral nutlets. On trees that were obviously injured, many clusters were observed to have developed at right angles
  7. Chemical Injury to Pecan Tree


    Fig. 1. Dying of top of Mahan tree during the year following exposure to 2, 4-D.

    to the original direction of growth before being subjected to 2, 4-D. Colored slides of these "deformed" clusters show the contrast between them and normal ones.

    1. Shedding of nutlets. Pecans are borne in a cluster known as a spike. It consists of a central stem and nuts borne laterally on it. Shedding of pecans from the clusters was general as early as June 20, 1966. This reduced the number and the ultimate yield of nuts. In some cases, all of the nuts in a cluster aborted. This condition was more common in some varieties than others. Most of the nutlets of the Schley variety dropped off prematurely.

    It is common for the last formed one or two nutlets near the terminal of the spike to abort. The shedding of the nuts from the affected trees did not follow this pattern. In all cases the lower nuts of the cluster were as likely to abort as the upper ones.

    Fallen nutlets were examined closely for evidence of insect or disease injury. None was found to account for the widespread shedding.

    1. Late summer shedding of leaves. The usual time for pecan leaves to shed is in late October and early November, usually after the first killing frost or freeze. Most of the trees in this orchard began to shed leaves in August and many of them were largely defoliated by mid-September. A slight infestation of black aphids (which, if severe will cause defoliation) was seen in June. These were effectively controlled by spraying in July, and none were observed on remaining leaves in October.

    The premature shedding of leaves limited the stored food necessary for a crop in 1967 and, as expected, the trees were largely barren of catkins and of nutlets in the spring of 1967.

    1. Nut quality. The trees matured only a small quantity of nuts in 1966, most of them having been killed or shed. Varieties which showed some promise in June produced no marketable nuts. This was true particularly of the Mahan.

    Representative samples of some varieties were analyzed for quality and the results show that the nuts were poor in quality.

    Pecans of the Odom variety were under size, 73 and 79 being required for a pound as compared with 45 to 50 for normal pecans. They are usually 1 inch or more in diameter, whereas over half of these were only 15/16” or less. The kernel percentage was 52 compared with a normal of 55 to 57. Kernels of nuts of this variety are regularly all fancy, whereas these were the entire lower grade --- ambers and "standard." Pecans of Stuart variety, when normally developed and matured will run from 42 to 48 per pound; the corresponding number for these was 77 reflecting small size or poor filling. Sixty-eight of these were 15/16” in diameter or less, whereas all should have been 1 inch or more in diameter. The kernel percentage was low 43.2 instead of a normal of 48 or 50.

    Relatively, the Success variety nuts were the poorest of the three varieties analyzed. Good nuts of this variety regularly run from 36 to 40 per pound. Ninety-eight of these weighed a pound. Nearly half were 15/16 inch in diameter or less. Good Success is well over 1 inch in diameter. The kernel percentage of good Success ranges from 52 to 56. These had only 40.5 percent kernels and they were all ambers instead of fancy.

    Normal green leaves are necessary for the development of high-quality pecans. It is reasonable to attribute the poor quality of nuts in this grove in 1966 to the late summer shedding of leaves described in 9 above.

    1. Delayed dying of tops. Many trees died back to the scaffold limbs, before they started growth in the spring of 1967. A photograph of such a tree is shown in the accompanying picture. This tree appeared in good condition in June 1966, (except for copper-colored and cupped leaves) and it had an estimated 200 pounds of pecans. It matured very few pecans. Severe and long-time injury to the tree accompanies the dying back of the main producing limbs of the tree. This condition was most common and severe on the Success and varieties, and least severe on Delmas and Stuart.

    It is significant that the full extent of injury was not seen immediately after exposure to 2, 4-D, and that signs of injury continue to develop for a full year after the initial damage.

*Professor emeritus, Horticulture Section, Department of Soil and Crop Sciences, Texas A&M University.