Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Hot weather or drought conditions sometimes lead to a physiological problem called leaf scorch. Leaf scorch shows up as a browning and drying of the leaf edges. In severe cases the brown areas will extend into the leaf tissue between the main veins. In extreme cases, the entire leaf turns brown.
Leaf scorch is often more severe on the side of the plant where the heat stress was greater, usually the south or southwest sides. Reflected light from light-colored surfaces and heat radiated from cement or asphalt put additional heat stress on plants and can increase the severity of leaf scorch. Rock mulches also absorb and radiate heat causing plant stress.
Leaf scorch is associated with inadequate amounts of water being available for the plant to use, especially during hot weather when plants have higher water demands. However, the cause is not always a matter of heat stress and too little water in the soil. A number of other factors may contribute to the development of leaf scorch. These factors include:
Inadequate or poorly developed root systems due to recent transplanting or poor planting practices.
Girdling roots choking the tree.
Injury to the roots or trunk, especially mower and weed trimmer injury.
Root disease, especially root rot.
Interference with water uptake and utilization due to soil compaction, poor penetration of water into the soil, water run off on slopes; and high salts in the soil.
Excess water and saturated soils exclude oxygen from the roots and also interfere with root function and water uptake. The result of too much water can be the same as too little!
Leaf scorch is a sign that a tree is under some sort of stress. When scorch develops on tree leaves, it's important to analyze the cause and then take remedial action to alleviate the stress and keep the tree as healthy as possible.