Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Just this past week, several people have brought me samples of ailing sycamore trees. On each of these samples there were two problems worth mentioning. One problem was an insect, the sycamore lacebug. It’s a tiny bug with lace-like wings. It sucks sap from leaves creating a speckled appearance. The lace bugs can be found on the undersides of leaves along with small black specks of their excrement. When feeding is severe, the leaves take on a brownish or dry look.
The lace bug has not become a serious annual problem on sycamores, but it does show up from time to time. The amount damage it causes is typically insignificant and tends to only become extensive in late summer or early fall. With damage occurring so late in the season, control is not needed.
The other problem evident on these samples was more noteworthy. The leaves were somewhat malformed and cupped downward with an excess of the leaves’ natural hairy fuzz. This is a sure sign of herbicide injury. It was very likely caused by applications of “weed and feed” products to the lawn. Dicamba or 2,4D, commonly found in “weed & feed” products, were probably the herbicides at fault. Sycamores are particularly sensitive to damage from these two herbicides.
The downward cupping of the leaves is not critical. After all, the tree will soon be losing these leaves in preparation for winter. The real damage from these lawn herbicides happens to the trunk of the tree. Both herbicides can lead to damage of bark tissues and the emergence of bark eruptions on the main trunk, usually more extensively on the lower portions of the trunk. The bark loses its beautiful mosaic pattern and smooth appearance. It becomes rough and fissured. Immediately beneath the eruptions, the bark tissue is pink and spongy. Beneath the eruptions, the inner wood becomes dark brown. This damage may later attract a borer, the American plum borer, which then causes even greater damage to the bark and trunk tissues.
Dicamba and 2,4 D can cause damage to sycamores even when applied at the correct rate, but greater damage occurs if they’re over-applied. Over-application often occurs when home gardeners don’t calibrate their drop-spreader or sprayer before applying a “weed and feed”. As a rule, 2,4 D and dicamba should not be applied in a general application to the lawn over the root zone of sycamores. Spot treating weeds is safer for the sycamores and more economical.
Yet another problem being noted by some sycamore owners is leaf scorch. This shows up as bright yellow to brown leaves scattered throughout the crown and accompanied by substantial leaf loss. The leaves most severely affected are the oldest... especially the inside leaves. The most likely cause of this problem is heat and drought. It’s been a hot summer. Sycamores are big trees and if they don’t get enough water they let you know. Lack of deep watering, girdling roots, and compacted soils can all be factors in a sycamore becoming drought stressed.