Symptoms of leaf scorch due to environmental causes include browning of the leaf margins and\or yellowing or darkening of tissues between the main leaf veins. As the condition progresses, entire leaves may dry up, turn brown and become brittle. Leaves sometimes wilt rapidly, and when this happens, they may remain a pale green color, even though dried out. Damage is usually more pronounced on the upper, windward, or southern side of trees. Plants may lose many leaves prematurely during late summer and exhibit some twig dieback.
Leaf scorch can be caused by adverse environmental conditions, such as soil compaction, transplant shock, nutrient deficiency, drought, salt toxicity, and weed killer injury. Leaf scorch is common in Montana due to cold soils and slow root growth. The loss of leaves is seldom fatal, but conditions causing leaf scorch should be corrected if possible because over time, they can cause the decline or death of the tree or shrub. Plants under stress are subject to secondary problems such as attack by insects or diseases.
To help prevent leaf scorch, prune sprouts and diseased areas. Maintain vigor through proper watering and fertilizing. Water deeply to encourage deep root systems that enable trees to withstand environmental stress such as drouth and winter desiccation. Check soil moisture at least 12 inches down; if it is rather dry, water trees slowly and deeply, allowing water to penetrate at least two feet. Deep-water the entire area under the canopy, one and one-half to three times farther than the branches. Because 95% of the roots of most trees, including tall evergreens and large deciduous trees, are found in the top 18 inches of soil in this extended area. Sufficient moisture will help keep the trees vigorous enough to withstand pest attacks, as well as help prevent winter injury. It is very important to deep-water trees and shrubs at least twice a year, especially in areas where the water table is far below the soil surface or on sites exposed to wind; water every fall, after leaves turn autumn color, but before the ground freezes (perhaps at the same time fall fertilizers are applied if needed), and again in spring, as soon as the ground thaws to replenish dehydrated roots. Avoid frequent, light waterings, and watering only at the base of the tree trunk. Trees suffering from drought can be selectively pruned to reduce transpiration, which is the loss of water due to evaporation through the leaves. Drought occurs when transpiration exceeds the plant's ability to supply water through the roots. Prior to planting, reduce foliage of transplants by pruning to minimize early injury due to water stress.
To determine if trees need fertilizing, have the soil tested and supply any missing nutrients. Avoid overfertilization, which can also harm trees and ornamentals. In many parts of Montana, additional tree fertilizers are not needed, especially when planted in turf areas that receive fertilizers. Unless trees are showing symptoms of nutrient deficience, such as chlorosis, stunted growth, or deformed foliage, refrain from fertilizing them unless soil tests indicate otherwise.
Compacted soil can be loosened over time by the annual use of core aerators, which remove a plug of soil up to four inches long, and then adding an inch or so of finished compost. The compost will soon work its way down into the soil, loosening it over time. Using dark-colored permanent mulches under the entire canopy area can help prevent compaction once soil is loosened, and help warm soils at the same time.
When using herbicides near trees do not allow mist to settle onto trees, and avoid spraying branches, foliage, or trunks with the solution. Apply pesticides only on windless days to avoid problems with drift.
Harris, R.W. 1992. Arboriculture; Integrated
Management of Landscape Trees, Shrubs, and Vines. Prentice- Hall, Inc.,
Englewood Cliffs, NJ. 674 pp.
North Dakota Tree Handbook. 1995. Agriculture and Natural Resources, North Dakota State University, Fargo, ND. 228 pp.
Pirone, P., J. Hartman, M. Sall, and T. Pirone. 1988. Tree Maintenance. Sixth Edition. Oxford Univ. Press, New York, NY 10016. 514 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
Categories: Disease, Tree, Leaf Scorch