Aspen trees, scientific name Populus tremuloides, are medium-sized deciduous trees that can grow from 50 to 100 feet tall. They are referred to as quaking or trembling aspens in North America, because the leaves quake or tremble in the wind. These trees are also the most widespread in North America, and are famous for their vibrant yellow leaves in the fall. However, like most trees, they also suffer their share of conditions that cause discolored or unnaturally yellowing leaves.
Aspen tree leaves turning yellow: Common reasons
Chlorosis due to iron deficiency
Aspens can suffer from iron chlorosis if the trees cannot get sufficient iron from the soil to make chlorophyll. The condition manifests as yellowing leaves with green veins. Root stress and compacted soil could aggravate the condition, and this could damage or even kill the trees.
To fix the issue, chelated iron should be applied to the soil. Trunk injections or foliar iron sprays are also recommended. Check that the soil’s pH is correct, as this also affects iron uptake. Reduce traffic under the trees and ensure there is proper irrigation.
Fungal diseases like Marssonina leaf blight
Aspen tree leaves could turn yellow due to fungal infections like Marssonina leaf blight. The fungal spores tend to spread from the leaf debris of the previous year. The disease becomes prevalent when temperatures are warm, and manifests as yellow-ringed dark spots on the leaves that enlarge and merge.
To fix the problem, always clean and dispose of infected leaves, provide drip irrigation, and make sure the trees are spaced apart to maintain adequate air circulation.
Pests, such as oyster shell scale insects
The presence of pests like oyster shell scale insects could also be the culprit of the yellowing Aspen leaves. These insects suck the sap or juice of plant tissue, resulting in yellow and wilting leaves, stunted growth, and eventually death, if the infestation is heavy.
To exterminate these pests, spray insecticidal spray or neem oil onto the affected tree parts.
Other Aspen tree disorders and diseases
Cytospora
This is a canker-causing fungus that attacks the trunks or branches of stressed Aspen trees. The fungus enters the trees through wounds and creates oozing orange cankers that stain the white bark. Reduce the stress on the trees by ensuring that they are watered properly, and prune off infected branches to control the disease.
Other fungal problems that could affect Aspen trees include leaf spots and rust. These can defoliate trees.
Here are some steps you can take to prevent these fungal infections:
- Make sure to maintain moist, but not wet, soil.
- Irrigate in the early morning, but do not wet the foliage.
- Prune infected twigs and branches.
- Plant the trees with enough space between them for proper air circulation.
- Clean up and discard infected leaves, twigs and branches affected by fungal diseases.
- Fertilize to ensure sufficient nutrients in the soil.
- Abnormal plant outgrowths, due to poplar twig gall flies
These insects are the culprits if you notice that the Aspen trees have lumpy twigs or outgrowths of plant tissue that resemble tumors or warts. These small, dark small flies lay their eggs into new twigs during the spring season. The maggots do not harm the tree, but the woody galls that form can be unattractive.
These pests cannot be eradicated; insecticides are not very effective and pruning the twigs will not control them. You may have to rely on their natural predators like birds and parasitic wasps.
Leaf scorch
These are leaf spots caused by improper watering or too much fertilizer. It is sometimes also due to hot temperatures during summer. To fix the issue, reassess your watering schedule or your use of fertilizers and adjust accordingly. Provide shade during hot weather, especially for young Aspen trees.
Insects
Aside from scale insects and poplar twig gall flies, Aspen trees are also prone to attacks by aphids and spider mites. Aphids excrete honeydew that could result in black, sooty mold covering the leaves and branches. Spider mites could cause damage similar to fungal infections if the infestation is severe.
To fix this, use insecticides or, better yet, seek the help of tree professionals who have the right equipment, especially if there is a heavy infestation.
Conclusion
Yellowing Aspen leaves are mainly attributed to iron chlorosis, caused by iron deficiency in the soil or reduced ability to absorb the iron due to incorrect soil pH. However, it could also be due to fungal diseases such as leaf blight, or pest infestation such as aphids, spider mites or scale insects.
Image: istockphoto.com / Henrik_L