Quaking Aspen Diseases
Diseases in the leaves of aspen can reduce the aesthetic value of these trees. Typically, leaf diseases on aspen are not going to kill the plant, but it certainly diminishes the value of the tree itself. Should a tree lose leaves during the early summer months and the tree is otherwise healthy, it should reproduce new buds which in turn become leaves. As with any tree, when a tree loses leaves too early in the late summer months, it also loses its ability to store nutrients.
Probably the most common leaf disease in our area is Marssonina leaf spot. Marssonina leaf spot, also called Black Leaf Spot, is a common disease in quaking aspen and can often be found in some varieties of cottonwood. This disease is most often recognizable by small, dark brown to black spots which appear on leaves affected by the disease. At times there may be yellowing on the outer part of the leaf. As late summer arrives, these spots tend to fuse together and form large dead patches on the leaves. Leaf diseases intensify throughout the summer and progress from the base of the plant and proceed upwards until reaching the top of the tree. Fall colors are sometimes less intense when infected with leaf diseases. Marssonina leaf spot is typically most noticeable in July and August. The disease has a tendency to spread rather quickly when given the right conditions and may travel great distances when the wind allows such. The fungus survives the winter in the fallen leaves and other weed debris on the ground and as it warms up during the spring, it releases the spores and starts the cycle over again.
Being that quaking aspen have shallow roots and loves lots of moisture, it’s the ideal situation for developing Marssonina leaf spot disease. If the tree doesn’t get enough water, the tree suffers, too much water and the tree develops Marssonina leaf spot. Typically Marssonina leaf spot flourishes when the temperature is between 60 to 80° F.
There are several ways of controlling Marssonina leaf spot. First, control the amount of moisture applied by fairly deep watering once every 7 to 10 days as this forces the roots a little deeper in the soil. Second, NEVER water at night! This is not the only fungus that will develop when you water at night. Third, air circulation is critical. It helps dry-out areas that might otherwise stay moist and induce fungus spores. Fourth, place approximately 3-inches of mulch below the tree. This helps preserve moisture around the root system while keeping leaves and other debris relativity dry. Fifth, when you do develop Marssonina leaf spot or any other leaf fungus on Quaking Aspen, start treating with Natural Guard ‘Copper Soap Liquid Fungicide’ at the first sign of a fungus. Follow the directions closely as copper fungicide must be applied on a regular basis to be effective. Start treating as soon as you recognize that a problem exists.
The steps above is certainly a great line of defense for quaking aspen, but as a general rule for this plant as well as all other plants in the landscape, removal of leaves, fallen branches, dead foliage and other dead or decaying material should be removed from the site.
While there are several other blights (Ink Spot, Leaf and Shoot Blight, Septoria Leaf Spot, and Leaf Rust) that may affect quacking aspen, we have found that Marssonina leaf spot to be the most prevalent fungus in our area.