Garfield County Cooperative Extension

Leaf Scorch on Trees and Shrubs

Rick Nelson Extension Educator, Ag/4-H & CED

Leaf scorch is appearing on various trees and shrubs in and around the Enid area. This is not a disease but rather a physiological problem associated with damaged roots, limited soil area, or hot, dry winds. Moisture is lost so quickly from the leaves that the roots cannot absorb and transfer water quickly enough to replace what is lost. Though scorch is usually associated with droughty periods, it can appear even when the soil is moist.

Scorched leaves turn brown or, in some cases, turn black from the edges and between the major veins. If severe, the leaf may drop. Leaves may be affected over the entire tree or may be affected only on one side. Some pine species are also prone to this condition due to the delicacy of the needles. Though rare, we have also seen damage on Colorado blue spruce this year with about an inch of new growth turning brown. These tips will eventually fall off though some homeowners knock them off to improve the appearance of the trees.

Though scorch can be due solely to the weather, the condition of the roots of plants can make them much more susceptible to this condition. Shallow soils such as those over hardpan or rock lead to a limited root system that may not be able to absorb all the water needed. Trees may be more sensitive to scorch this year because of the heavy rains many areas received earlier this spring. Though soils were recharged, in many cases so much rain was received that oxygen was driven from the soil resulting in root damage. That root damage is now making it more difficult for trees to provide all the water needed for the leaves. Also, root damage due to disease, insects, poor drainage or construction can cause poor water uptake. One can help alleviate damage due to dry soils or limited root systems, water once per week if there is no rainfall. Mulching small trees or shrubs will help conserve moisture.

 Iron chlorosis is a common problem which will display a leaf scorch because of the high pH in some soils. Though these soils normally contain adequate amounts of iron, the high pH ties up iron so that it is unavailable to plants. Classic symptoms of iron chlorosis are yellow leaves with a network of dark green veins. In severe cases the entire leaf turns yellow and the edges of the leaf scorch and turn brown.

One of the best methods of avoiding iron chlorosis is by planting tolerant trees. Trees that are susceptible to iron chlorosis include pin oak, sweetgum, and dawn redwood. Moderately tolerant trees are ash, cottonwood, linden, elm, hawthorn, most oaks and ginkgo. Even closely related trees can differ markedly in their resistance. For example, pin oak is notorious for sensitivity to iron chlorosis while most other oaks are moderately tolerant. Also, red, silver and Amur maples are susceptible, but Norway maples are much less so. There are several methods used to correct iron chlorosis in trees. Not all methods work in all situations. The following are the most common.

Soil Treatment: The idea is to acidify a small quantity of soil so that the tree can absorb the iron it needs from these areas. This will only work on non-calcareous soils.  A mixture of equal parts of iron sulfate and elemental sulfur are mixed together, and the mixture is placed in holes made under the dripline of the tree. Holes should be 1 to 2 inches in diameter and 12 to 18 inches deep. Space them 18 to 24 inches apart. Each hole should be filled with the iron sulfate-elemental sulfur mixture to within 4 inches of the soil surface. This method is labor intensive and results are sometimes variable.

Iron chelates can also be used as a soil treatment. The only chelate that is effective above a soil pH of 7.2 is Iron EDDHA. It can be found in the products Sequestar 6% Iron Chelate WDG, Sequestrene 138 and Millers FerriPlus. Use these products in the spring before growth starts. Dry chelate can be sprinkled on the soil and watered in or dissolved in water and applied as a drench under the dripline of the tree. Normally, soil-applied chelates last only one year.

Foliar Treatment: Leaves are sprayed directly with iron chelates or iron sulfate early in the season. Response is quick, but leaf burning is possible. Response can be spotty and temporary. Repeat applications may be needed.

We do worry when leaves are lost in the middle of summer, especially if the tree tries to throw out a new set of leaves. The tree uses a great deal of energy to releaf and does not have much time to make it back before winter arrives. One also needs to worry when leaves turn completely brown and do not drop from the tree. This may mean the tree has died. In such cases, bend the twigs to make sure they are supple and not brittle. Brittle twigs mean that part of the tree is dead.

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