There are several diseases that can cause black spots on leaves or twig dieback, or both, on aspen. One, BACTERIAL BLIGHT, causes the entire leave to turn black and die, along with the twig it is attached to. New foliage and twigs are the most susceptible. Cankers, or oval, sunken areas, can also develop on the south side of trunks and branches in trees with this disease. The disease is not usually fatal to trees, especially mature ones, but can cause stunting.
To manage the disease, prune infected tree parts, making the cuts about 8 inches below the damaged parts. Sterilize your pruning tools after each cut in a solution of 1 part household bleach to 3 parts water. Spray the entire tree with a fungicide. Burn or dispose of the pruned material. Rake up and dispose of leaves and debris in the fall. If bacterial blight is a chronic problem in your aspen trees, begin treatment in spring when leaves start to emerge. Spray the tree 3 or 4 times, once every ten days, with Bordeaux mix or other product labeled for bacterial blight.
SEPTORIA LEAF SPOT is another disease in aspen, this one caused by a fungus. Symptoms are the brown or black mostly circular spots on the leaves. Damage does not usually extend into the stem. The fungus overwinters in dead leaves and twigs. Severe infections can result in partial defoliation of the tree, but is usually not fatal to the tree, which will generate new leaves. However, cankers can be produced that can girdle and kill stems or branches, or serve as infection courts for other diseases.
To control septoria leaf spot, remove and dispose of fallen leaves and stems. In spring, spray trees 2 or 3 times with a Bordeaux mix or other fungicide labeled for septoria leaf spot to prevent infection. If you purchase aspen trees, ask for disease-resistant clones. Plant trees with plenty of room to allow for good air circulation, and prune with the same goal in mind.
VENTURIA LEAF AND SHOOT BLIGHT is also a fungal disease that causes black or brown lesions that kill the leaf and shoots on aspen. Within a few days, the blackened tissue turns olive green as the fungus continues to grow. In wet years all terminal shoots may be killed, but damage does not spread to the rest of the tree. Young saplings are more prone to damage than older trees. Prune infected shoots, disinfecting pruners between cuts. Rake and remove fallen leaves to help prevent reinfection next spring.
A fourth cause of black spots on aspen leaves is WINTER DAMAGE. Environmental factors, such as cold, drought, or wind can cause damage in the winter months that does not become evident until the trees are in leaf. To check for this possible cause of leaf spots, cut a twig from the branch, and split it lengthwise. If the cause is winter damage, the inside of the twig will be brown or black. To help reduce winter damage, deep-water all trees (both evergreens and deciduous) in the fall after leaves of deciduous trees have turned fall color, but before the ground freezes. Water the extended root area 2-3 times farther than the branches extend, to a depth of two feet. Repeat the deep watering in spring as soon as the ground thaws. Test the soil to determine if additional nutrients are needed, and supply them if they are. This will enable the tree to go into the winter months with enough vigor to withstand many of the stresses it can encounter.
Aspen trees are relatively short-lived, usually 20 to 40 years for individual trees. As they become older, they will become more prone to problems, including the diseases mentioned here. New shoots generated from the roots of the older trees can be encouraged to grow into mature trees if your aspen trees are aging. Don't confuse the shoots with suckers, which can arise from the base or trunk of some kinds of trees. Suckers do not give rise to the strong, healthy trees, but root-generated shoots, which arise a distance from the parent make very satisfactory replacement trees. Remember, however, the new trees will have the same susceptibility to problems as the parent stock.
Cool, wet spring weather in spring can create ideal conditions for ANTHRACNOSE diseases on ash and maple. On ash, look for irregular, rapidly expanding, water-soaked spots on new leaves. Young lesions are greenish-brown to dark brown. Severe infections may lead to girdling and wilting of young shoot bases. This damage may resemble frost injury. Leaf drop and twig blight can occur also. Symptoms of maple anthracnose include brown to black areas that run along the leaf veins. There may be crinkling and distortion of severely infected leaves. Leaf drop and twig cankers may occur.
Generally, anthracnose alone cannot kill a tree. However, severe disease levels can inflict stress which may predispose trees to other disease, insect, and environmental problems. Fungicides such as chlorothalonil and mancozeb are most effective if sprayed in early spring at bud break, followed by regular intervals during wet weather. This means by the time symptoms become widespread on the leaves, it is usually too late to get complete control. Dry weather is the best control for this disease. Raking and destroying fallen leaves and pruning severely infected branches may help somewhat.
References:
Pests of Landscape Trees and Shrubs; An Integrated Pest Management Guide. 1994.
University of California. Oakland, CA 94608-1239.
Pub. #3359. 327 pp.
Pirone, P., J. Hartman, M. Sall, and T. Pirone. 1988. Tree Maintenance.
Sixth Edition. Oxford Univ. Press, New York, NY 10016. 514 pp.
Plant Health Guide. 1995. Meister Pub. Co. Willoughby, OH 44094-5992.
178 pp.
Sinclair, W., et al. 1987. Diseases of Trees and Shrubs. Cornell Univ.
Press, Ithaca, N.Y. 576 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
Categories: Tree, Disease, Aspen, Ash, Maple
Date: 04/02/02