Leaf Scorch on Japanese Maples and Others

Leaf scorch seen on area trees

Brian Jervis: Ask a Master Gardener

Saturday, September 3, 2016

Q: The leaves on my Japanese maple turned brown on the edges this summer. What is this? Could it be a disease? A. G., Tulsa

A: Brown leaves on plants have many causes. The diagnosis as to the particular cause depends on the type of plant, the season, the environment and the amount of water available.

Japanese maples are notorious for developing brown-edged leaves during summer. This is called “leaf scorch.” It is seen in many plants unable to keep up with their leaves’ water demand during hot summers. Sometimes this is due to lack of water, but with Japanese maples, it often is due to their genetic makeup. They evolved in regions that are cooler and wetter than ours. Their circulation, which was adequate in those areas, often has difficulty coping with our heat and hot winds.

Water flow in any plant begins in the roots where it is absorbed. It is then carried up the trunk through the plant’s cambium layer, located just under the bark of trees. Cambium is the tree’s circulation. As it travels up the tree, leaves are the last stop and the last place water reaches after entering the outer edge of a leaf. So when water is deficient in a heat-stressed tree, the outer parts of leaves initially dry out and turn brown. With worse stress, the whole leaf may die.

While it may not seem logical, too much water in the soil can block delivery of water to leaves. Excess soil water forces oxygen from the soil, damaging the small roots that are needed to absorb water and nutrients. This results in water deficiency in leaves.

Trees that are not ordinarily stressed by our summers often develop brown leaves after planting, before mature roots are able to develop. In young trees, the demand for water exceeds the ability of the immature roots to deliver. That is why it is suggested that newly planted trees be watered regularly, throughout the year for three years after planting.

Other causes of water deficiency in leaves are one of many conditions that damages the tree’s circulatory system. Fungal root rot, systemic infections clogging the flow of sap and trauma to the trunk. Lawn equipment and sometimes rabbits and squirrels may damage the cambium and interfere with circulation.

Diseases may cause browning of leaves as well; the most common of which are fungal diseases. These diseases, for the most part, do not just limit the browning to the edge of a leaf. They often create browning with some yellowing in the form of spots, circles, streaks or holes in random patterns within the leaves. Leaves with disease often turn yellow and drop from the plant rather than curl up at the edges and remain on the tree as often seen with leaf scorch.

In most cases brown leaf scorch on any tree is not fatal, the tree will survive after the stress has passed. It is important to irrigate trees, especially the younger ones.

Garden tips

§  Now is a good time to submit a soil sample to the OSU Extension Office for testing. Do this before reseeding fescue or creating a garden bed this fall. Call the Master Gardener office at 918-746-3701 for instructions.

§  Now is the time to apply a pre-emergent herbicide to lawns to control winter weeds such as henbit. Do not use on fescue if you plan on overseeding it this fall. OSU suggests the pre-emergent herbicide brands containing either dithiopyr, pendimethilin, or prodiamine would be an excellent choice.

§  Tall fescue should be mowed at 3 inches and up to 3½ inches if it grows under heavier shade. Don’t fertilize fescue lawns until it cools later this month, then fertilize again in November.

§  Begin to reduce the amount of light on outside tropical houseplants by placing them under shade trees before bringing them indoors for the winter.