Problem type: Disease
Name of problem: Anthracnose
Plant name(s): Ash, maple, oak, sycamore, elm
Symptoms / Characteristics:
Irregular tan to brown blotches on leaf surfaces. Fruit, flowers and stem tissue can also become infected. In ash trees, spots appear as water-soaked areas then develop into brown, papery lesions. The leaf may have a distorted appearance but will usually remain attached to the tree. Oak and maple trees can develop irregular tan to brown spots along leaf veins and margins. Leaves may look blighted, become distorted and drop prematurely.
Anthracnose is a foliar disease caused by many different species of fungi. The fungus becomes active in the spring when cool, wet weather persists. Spores are released which then infect newly emerging leaves. Irregular tan to brown blotches will develop on leaf surfaces, but can also infect flowers, fruit and stem tissue. Infections can occur throughout the summer if periods of extended cool, moist or wet weather are present. Warm, dry weather stops the disease from spreading.
Symptoms can appear as minor spotting of leaves, to leaf blight and twig or branch dieback. Generally younger growth is more susceptible and as leaves and shoots mature, they become relatively resistant to infection. If infections are severe enough, entire leaves will turn brown, curl up and drop off. Branches with leaf loss usually produce new shoots by mid-summer without causing much stress to the tree. Other species such as oak and maple trees may develop irregular tan to brown spots along the veins or margins of the leaf. These leaves may also have a blighted appearance, becoming distorted and dropping prematurely. New shoots will develop by mid-summer. These symptoms can resemble frost injury as leaf tips can become shriveled, but commonly anthracnose is confused with heat and drought stress.
Control / Preventions:
Anthracnose overwinters in infected twigs and fallen leaves and branches. For effective control, raking up and removing fallen leaves in the fall will reduce disease survival. Infected branches should be pruned back to healthy wood if practical, to reduce the number of spores that are able to infect emerging shoots. Once injury to a tree is apparent, control is difficult and chemicals are usually ineffective.
Serious damage does not usually occur on well-established trees. Keeping trees healthy with proper watering and fertilizing when necessary, is essential for infected trees as they can lose a large portion of leaves and go on to produce a second flush of leaves. Prune branches to open up the crown, improving air movement, light penetration and faster drying of the leaves. Avoid wetting tree foliage with sprinklers by reducing the spray height, as anthracnose is spread during wet conditions.
Fungicide applications are usually not warranted, however during repeat infections, they may be necessary. Spraying can be done with a fungicide containing mancozeb, when the buds are swelling, followed by two applications, first when green tips have developed and second when the leaves are half grown (10-14 day intervals). Always follow instructions on the pesticide label.