October 1, 2012 in Plant Diseases
Anthracnose, also referred to as blight, is the most serious disease of the native (Platanus occidentalis) sycamore in Delaware. This disease can infect London and Oriental plane trees as well. The disease is also common throughout the Northeast and mid-Atlantic area where it causes blighting of sycamores from mid-May through June.
A fungus, Apiognomonia venta, causes the disease, which may cause bud, leaf, shoot and twig blight.
The twig blight stage, which occurs before leaves appear in the spring, results in cankers that girdle and kill young twigs. Small black dots (fruiting bodies of the fungus) can be seen in the bark of the dead twigs. Buds on such twigs may be blighted and die before they open.
Shoot blight occurs as the very young leaves begin to unfold in the spring. This results in crinkling and browning of leaves–the most characteristic symptoms of the disease.
Leaf blight results from infection by spores produced in the twig cankers. About the time leaves are becoming fully grown, light brown, irregular dead areas occur along the vein. These areas may be small or large depending on the stage of disease development. Dark brown fruiting bodies of the fungus can be found on the diseased leaves. These infected leaves become completely blighted and fall prematurely.
When the disease is severe, cankers may even develop on large limbs. These limbs are killed and are quite obvious on older trees unless removed by pruning.
Factors Affecting Development
The anthracnose fungus overwinters in cankers on twigs and branches of diseased trees. In the spring, the fungus produces spores that are carried by rain to twigs and leaves. These spores start new infections when weather conditions are favorable for infection.
Although moisture is necessary for the spread of the fungus, temperature during early spring is the prime factor in determining how severe the disease will be. If the mean daily temperatures are 50-55 F between bud-break and early leaf emergence, anthracnose will be severe. However, if mean daily temperatures during this period are 60 F or above, the disease will be slight.
Although affected trees lose their leaves, they will produce a new crop that will probably be healthy since the weather is warmer and not very conducive for new anthracnose infections. Repeated defoliation by this disease can weaken trees and its effect can be seen as loss of vigor, dieback of large branches, and increased susceptibility to borers.
It is always a good idea to rake and destroy fallen leaves and twigs, since the fungus can overwinter on these plant parts. Branches with severe dieback or cankers should be pruned to remove the fungus. Maintain good tree vigor by watering during prolonged dry periods and fertilizing when necessary.
When planting sycamores or plane trees, select the most resistant species or cultivars that are available. Oriental plane is probably the most resistant followed by the London plane and the American sycamore. Because of the variability of the seedling London plane trees, we see great variability to anthracnose in the same planting of trees. Some cultivars of London plane such as ‘ Columbia’, ‘ Liberty’, and the older ‘Bloo dgood’ have a high level of resistance to anthracnose and would be preferred.
If trees need to be protected from anthracnose, several fungicides are available. Apply thiophanate-methyl (Cleary’s 3336), cupric hydroxide (Kocide), mancozeb (Fore) or chlorothalonil (Daconil), or Spectro 90WDG (thiophante-methyl + chlorothalonil) at bud-break and 10-14 days later. Sprays must be applied before the disease appears in the spring or control will not be effective. It may be necessary to have a reputable tree care company spray large trees.
For very large trees or trees that cannot be sprayed , an alternative is now available: systemic trunk injections containing the fungicide Arbotect 20S. This material, which can be used during the first two weeks of September, must be applied by trained arborists or someone familiar with trunk injecting and disease identification. Be sure to follow the label directions when applying fungicides.
If your tree does not show symptoms described in this fact sheet, it does not have anthracnose and fungicide sprays will not help it. Remember that certain insects, herbicides, drought or bacterial leaf scorch will also cause injury to sycamores that might be confused with anthracnose. Examine your tree closely and know what is causing the problem before applying control measure(s).
Caution: The information and recommendations in these fact sheets were developed for Delaware conditions and may not apply in other areas.
Bob Mulrooney Extension Plant Pathologist University of Delaware
PP-23 02/98. Rev. 4/04