What Causes Aspen Leaves to Die Before Fall?

Several problems can cause premature leaf drop on aspen trees.

Aspens are medium-sized deciduous trees that produce vibrant fall foliage. Premature aspen leaf drop is a symptom of several fungal diseases and maintenance problems. Early identification of symptoms can help identify the problem and prevent further leaf drop. Provide proper spacing, preventative fungicides, proper watering and fertilization to ensure the vitality of aspen foliage.

Common Fungal Diseases

Aspen trees are susceptible to three common fungal diseases that cause premature defoliation: Marssonina leaf spot, ink spot and leaf blight. Marssonina leaf spot causes small brown spots with yellow halos on leaf surfaces, which enlarge as the disease progresses. Initial infections rarely cause serious injury. However, secondary infections can cause premature leaf drop. Ink spot symptoms consist of tan to brown ring-shaped spots, small holes or shot hole appearance on infected tissue and premature defoliation. Leaf blight affects immature aspens and causes blackened spots on foliage, distorted leaf growth and premature leaf death. All three common fungal infections favor water on leaf surfaces for infection and overwinter on fallen debris.

Serious Fungal Diseases

Anthracnose and Septoria leaf spot are serious fungal diseases of aspen that cause premature leaf loss and reduced tree vigor. Anthracnose infections initially appear as red to brown spots and blotches on foliage. As the disease progresses, foliage turns yellow to brown, which results in premature defoliation during mid- to late summer. Septoria leaf spot produces tan leaf spots with black margins and black fruiting bodies within the spots. Under favorable conditions, the disease can cause premature defoliation. Both fungi favor wet weather conditions and water on leaf surfaces for infection. Septoria fungi favor temperatures between 70 and 75 degrees F for infection.

Maintenance Problems

Improper watering and too much fertilization can cause leaf scorch on aspens. Leaf scorch causes browning on the margins of foliage, brown leaf spots near the main leaf vein and leaf death. Overwatering restricts the tree's ability to absorb oxygen, which restricts root development and the flow of water to new foliage. Synthetic fertilizers that have a high salt content reduce the tree's ability to absorb water, which results in leaf scorching when overapplied.

Prevention and Control

Reduce infections of Marssonina leaf spot, ink spot and leaf blight by pruning overgrowth to increase airflow, avoiding overhead watering and applying fungicidal sprays. Fungicides can stop all the fungal diseases from spreading to noninfected areas and prevent infections if sprayed early in the spring growing season. Provide aspens with deep irrigation that extends to the plants' roots, but infrequently enough that it does not drown the roots, and avoid overfertilization to prevent leaf scorch. There is no cure once leaf scorch occurs.

References

• Colorado State University: Colorado's Forest Types -- Aspen
• Colorado State University Extension; Aspen and Poplar Leaf Spots; W.R. Jacobi; March 2009
• University of Illinois; Anthracnose Disease of Shade Trees; Nancy R. Pataky; June 1997
• Colorado State University Extension; Leaf Scorch; S. Rose, et al.; August 2009