Landscape - Leaf Scorch in Trees and Shrubs

Leaf scorch in trees and shrubs is common this year because of the drought. However, there are other causes of leaf scorch that you need to be aware of. The following is a good article on the subject.

Leaf scorching in landscape trees and shrubs occurs when leaves lose water faster than the roots can supply it. When moisture is lacking, the margins tend to dry out first, hence the marginal scorch pattern typically associated with stressed plants. Symptoms of scorch may become evident any time water flow is disrupted within a plant.

Leaf scorching in plants is usually attributed to both biotic (living) and abiotic (environmental) agents. Why is it important to tell the difference, and how does one do it?

Abiotic (or environmental) agents that can cause scorching in leaves include drought, dehydrating winds, salt, flooding, chemicals, air pollutants, toxic metals, and nutrient extremes. Scorching can also occur when plants are placed in sites where roots do not develop normally (such as in planting islands or in soils with a hard pan layer), or when roots are physically damaged (such as occurs during construction). In most cases, this type of scorching is fairly uniform around leaf edges, affects newer leaves as well as older leaves, and will appear on vast expanses of the canopy. In addition, scorch symptoms may develop soon after a known stress (such as drought or an application of de-icing salt) occurs.

On the other hand, scorching can also occur in plants affected by living or biotic agents. Organisms that can cause this symptom include fungi, bacteria, nematodes, viruses, and insects. Leaf scorching due to biotic agents is not clearly defined. The scorch symptoms on leaves are often irregular in shape, and frequently a yellow or red “band” will appear between green and scorched tissues. In addition, symptoms may appear first on the older leaves of one or more branches, and then spread to other parts of the tree.

This growing season, the leaves of many landscape plants are exhibiting the uniform “scorch” consistent with an abiotic stress. Drought stress is the likely culprit for many trees, especially seedlings and shallow rooted and recently planted trees and shrubs that lack extensive root systems which cannot absorb water from deeper in the soil profile.

Some trees in the landscape, primarily oaks, are also affected by a biotic agent that causes leaf scorch. Bacterial leaf scorch of shade trees (BLS) is caused by the bacterium Xylella fastidiosa. Like leaf scorching due to other biotic agents, leaf scorch caused by Xylella is not clearly defined; scorch symptoms are often irregular in shape, and frequently a dull red “band” is apparent between healthy and scorched (necrotic) tissues.

Reprinted with modifications from "Leaf Scorching in Shade Trees – Biotic or Abiotic?" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University in the September 20, 2007 edition of the Plant and Pest Advisory, Landscape, Nursery, and Turf Edition, from Rutgers University.