Maple is a genus of trees and shrubs of the family Sapindaceae. There are at least 132 species, mostly native to Asia, Europe, Africa and North America. They have recognizable palmate leaves and winged fruits. Maple trees and shrubs are an important group of ornamentals for lawns, parks and streets. However, like most plants, they can suffer from conditions that cause yellowing leaves.
Maple Tree Leaves Turning Yellow: Common Reasons
Phyllosticta leaf spot
If you notice yellow spots edged with a purple border on the leaves of your maple tree, the culprit is a fungal disease called Phyllosticta leaf spot. The spots measure about a quarter-inch in diameter, and have a distinctive blackening in the center. The disease usually appears during spring and summer.
The spores appear in spring and are carried from tree to tree through the wind. The trees should be partially defoliated in case of a severe infection. You should also apply copper-based fungicides at least three times at two-week intervals to control the fungus. Rake the fallen leaves, since the fungus can still survive on the ground during the winter months.
Maple tree leaves can also turn yellow due to leaf scorch or sunburn. This means the trees are not receiving the correct amount of light. Leaves turn yellow or brown if they receive too much or too little sunlight along with insufficient moisture. Leaf scorch commonly occurs during the summer months.
To treat the problem, add mulch around the base of the trees to keep moisture near the roots and avoid unnecessary moisture loss. Make sure that the trees are watered enough; they need approximately 10 gallons of water for every inch of tree trunk diameter, measured at knee height, according to the National Gardening Association.
Bacterial leaf scorch
Maple tree leaves turn yellow when suffering bacterial leaf scorch, which is caused by a bacterium. The infected leaves eventually become brown and drop off. The branches will weaken and the trees’ lives are threatened.
This disease usually occurs during late summer or early fall, especially if there has been a period of drought. Leafhoppers help spread the disease, but they can be eradicated with insecticides. You should also water the trees just enough, and ensure the soil is fast-draining.
Scale and aphids could invade maple trees and feed on the sap or juice of their leaves. This makes the leaves yellow, wilted and distorted. The discolored leaves may drop off.
You can fix this problem by treating the tree with insecticidal soap, neem oil or horticultural oil. Apply these thoroughly to control and eradicate the pests. You can also use predatory insects like parasitic wasps and mites that feed on aphids and scales.
If a maple tree has yellowing leaves with green veins, it could be due to iron chlorosis. The tree is unable to absorb the available iron in the soil due to high levels of clay and alkalinity. It will experience an overall decline and the canopy will die back.
There is no long-term cure for this condition, and you have to remove and dispose of the trees. Treating the soil with chelated iron can also help solve the problem.
Maple tree decline
Maple leaves could also turn yellow due to maple tree decline. This is a progressive condition typically signaling the end of a tree’s lifespan. An early symptom of the condition is premature leaf yellowing, which occurs before the leaves normally change color for the fall months that start in July or August. Other signs of the condition include early defoliation, death of small twigs and branches, a lack of leaves on the upper branches, and brittle roots.
Maple tree decline is a progressive condition due to causes ranging from insect infestation to a lack of nutrients. If treated early the maple trees can be saved, and it is not contagious, so does not endanger the nearby trees.
Carotenoids are pigments that are present in maple tree leaves and hidden by chlorophyll during the growing season. Chlorophyll is the pigment that makes leaves green, and is a requirement for photosynthesis. When the winter season sets in the chlorophyll starts to die away, revealing other pigments in the leaves.
Sunny days followed by cold nights can speed up the process and the leaves will turn yellow faster. Warm and wet summers can also trigger the development of yellow pigments in leaves.
Maples make up a large genus of shrubs and trees that are popular as ornamentals. Like most trees and plants, they suffer from conditions that cause yellowing leaves. The common reasons for yellowing leaves include leaf scorch or sunburn, and Phyllosticta leaf spot. It could also be due to maple tree decline, insect infestation or iron chlorosis.
Image: istockphoto.com / Татьяна Санина