Posted by Brent Wilson on 10/4/2016 to Pest & Disease Control Tips
Every summer we get a few questions about blemished or discolored leaves on Japanese Maples, which are most often symptoms of leaf scorch, a noninfectious disease or disorder that occurs most often after prolonged periods of dry and windy hot weather or bright sunshine, when the roots of a tree, especially young or recently planted trees which have yet to develop a substantial root system, are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves.
Leaf scorch is identified by browning or greying of leaf margins and/or yellowing or darkening of the areas between the main leaf veins. Leaves may dry, turn brown, and become brittle. In mild cases of leaf scorch, the leaves remain attached to twigs or stems, and little damage results. In more severe cases, premature dropping of leaves and twig dieback may occur during the late summer, though such plants do not die.
Unfavorable locations, such as sandy or gravelly soil, near obstructions or pavement that restrict root growth, or exposed windy slopes can promote scorch. Anything that affects the plant’s ability to take up water, including insect and disease problems, can result in leaf scorch. Herbicides, pesticides and water sprayed on foliage can also contribute to scorch.
Regarding leaf scorch, there's good news and some bad news.
THE BAD NEWS FIRST
If the leaves on your Japanese Maple are looking scorched and raggedy towards the end of summer there's nothing you can do to make them look better during the current season.
THE GOOD NEWS
If your Japanese Maple has leaf scorch, but the tree is still alive, there's most likely nothing to be too worried about. As previously mentioned, the scorched leaves are a temporary condition caused by dry and hot weather, especially towards the end of long summer. Japanese Maples usually recover fine from leaf scorch and will produce beautiful fresh new leaves the following spring.
Causes of Leaf Scorch
If the leaves of your Japanese Maple are grey or brown around the edges, and are scorched looking and curled, but the branches are still flexible and alive, it is most likely leaf scorch. That said, it's always a good idea to first rule out a soil moisture problem.
Japanese Maples prefer a moist but well-drained soil. Constantly soggy or wet soil due to over irrigation or poor drainage in the planting area can lead to deadly diseases such as root rot. Symptoms of root rot are browning or blackening of leaf edges, which can look very similar to symptoms of leaf scorch. If this occurs to the leaves of your Japanese maple, to rule out a soil moisture problem, check soil moisture with your finger or a moisture meter. If the soil is soggy or wet reduce watering to maintain a damp to slightly moist soil or improve drainage in the area where your tree is growing.
If the soil around the roots of your tree stays constantly soggy even after reducing watering, and you suspect a soil drainage problem, it is best to conduct a simple soil drainage test, as described below.
To test soil drainage, dig a hole 12" wide by 12" deep just beyond the root ball of your newly planted tree. Fill the hole with water and let it drain. Then, after it drains, fill it with water again, but this time clock how long it takes to drain. In well-drained soil the water level will go down at a rate of about 1 inch an hour. A slower rate indicates poor draining soil and is a caution you need to improve drainage, replant your tree in a raised mound, or relocate and transplant your tree to a well-draining site. It is best to dig and relocate established trees during winter, after the tree has gone dormant and lost all its leaves
If the soil has been too wet for too long, and a majority of the stems and branches on your Japanese maple have turned black with shriveled bark, there might not be much you can do to save the tree. You can use the scratch test on branches and stems of your tree to check for signs of life. To perform the scratch test, use a knife or the edge of a coin to scrape away a very small section of bark on leafless stem or branch. If the underbark is green there is life, if brown or tan color the branch has died and should be removed. Continue this process throughout the tree. If over 50% of the tree is dead there's not a good chance of long term survival. This doesn't mean you can't attempt to restore the tree to health by replanting and or relocating it.
Causes of Leaf Scorch
Too much sunlight. In their native habitat you'll find Japanese maples growing as understory trees in the dappled shade or filtered sun of woodland borders. With the exception of a few cultivars that have demonstrated good tolerance to full or all day direct sun exposure, most Japanese maple cultivars prefer some shade, especially in the mid-afternoon hours when sunlight is most intense. If your Japanese maple is growing in a spot where it receives direct afternoon sunlight and year after year the leaves become scorched during the summer, chances are you have a variety that prefers some shade.
As for a long-term remedy for leaf scorch caused by too much exposure to direct afternoon sunlight, you might consider to relocating and transplanting your tree to a shadier location, which as previously mentioned is best done while the tree is in winter dormancy. The only other alternative would be to plant a larger tree to the west side of your Japanese maple that would eventually provide shade during the afternoon hours.
Excessive heat. Most Japanese maple varieties will tolerate the heat in USDA Zones 5 to 8b. That said, there are a few Japanese maples that will grow in Zone 9. South of zone 9 the summer heat is usually just too much for Japanese maples. That said, even in zones 5 to 9 an unusually hot summer, in which there's a much warmer and longer than usual heat wave, can cause some leaf scorch, especially on trees growing in a location that receives direct sun during the mid afternoon hours. If and when this happens it's usually towards the end of summer so there's not too long of a wait for your tree to drop all of its leaves in fall with the return of fresh and beautiful new leaves the following spring!
Water on foliage. Water on the leaves of a Japanese maple, especially when the sun is shining during the hot afternoon hours, can scald leaves. To avoid scalding, be careful to water your tree at the base rather than splashing water on the leaves. If you have a sprinkler system set it to run during the early morning hours, which will allow any water to dry from the leaves before the sun gets too hot.
Note: When your tree has leaf scorch, be careful not to over-water it. Trees with scorched leaves won't be drinking as much water. Provide only enough supplemental water when needed to maintain a damp to moist soil
Chemical applications. Japanese Maples are very sensitive to many types of chemical insecticides and fungicides. Because they don't have any serious problems with insects or disease, chemical applications should be avoided or only used when absolutely necessary. If you must spray for control of insects, such as the Japanese beetle, use a mild solution of Sevin (Carbaryl) spray or Neem oil at half the strength suggested on the product label. To reduce the possibility of leaf scorch, make sure to spray only in the early morning hours.
If you think your Japanese maple has a serious problem, and you're not sure if it's leaf scorch or some other disease, it's a good idea to consult with your local arborist, professional nurseryman, or local Extension service agent. Also feel free to email photos to us at [email protected] and we'll do our best to provide a diagnosis and remedy if necessary. You may also send photos using the form on our Contact Us page.
Hope this information was helpful.
Plant Long & Prosper!
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