Tree Diseases

Like all other plants, trees do get diseases from time to time. Some are quite specific in terms of the type of tree they affect, while others will attack any tree.

The most widespread diseases that attack deciduous trees non-specifically are:

• Powdery mildew,
• Sooty mold,
• Verticillium wilt,
• Canker,
• Leaf spots, and
• Heart rot.

Common tree diseases

The organisms that cause tree diseases are pathogens and the most common type are fungi that get their nourishment as parasites. Some are so tiny you can’t even see them, while others, like some mushrooms, have bright fruiting bodies that look quite attractive attached to the tree. Apart from fungi, some tree diseases are caused by viruses and by bacteria.

Powdery mildew is a common disease that attacks the leaves of trees and other plants. It has a powdery appearance (as its name suggests) that is created by the millions of tiny, little spores that form on the underside of the leaves. Practically every type of tree on earth is susceptible to powdery mildew, particularly those growing in high-humidity areas.

Sooty mold is a black fungus that grows on the surface of leaves, especially those that are shaded. This type of fungus isn’t a parasite, because it grows on deposits made by various insects, like aphids. It looks very ugly but doesn’t do much damage to trees.

Verticillium wilt is a disease that is caused by a fungus in the soil that results in leaves of trees becoming light in color and looking dull. They then start falling, without even wilting. In severe case an entire branch might die. If trees are affected over any length of time, growth is usually stunted, because the fungus invades the entire root system. More than 300 tree species are susceptible to verticillium wilt, particularly maples, elms, catalpa and stone fruit trees.

Canker is caused by fungi that injure the bark tissue and enable the canker to enter and spread. There are various types of canker, for instance the annual type that grows for one season as well as far more destructive perennial cankers that recur.

Leaf spots infect leaves with spores of various fungi and bacteria, and there are numerous different types of leaf spot diseases. Leaf spots are more common in shaded areas when trees are crowded together or planted in low areas of the garden. They are also more likely to occur in late summer. This is not a dangerous disease, and it can be avoided by simply following good gardening practices.

Heart rot is another tree disease that is caused by a fungus. It starts when the tree is damaged in some way, and then invades this part of the tree, living off it. Fruiting bodies may be seen in the form of mushrooms or ‘conks’.

You will find some useful information on specific tree diseases on this web site: http://www.ag.ndsu.edu/pubs/plantsci/hortcrop/pp697-1.htm For more information about common tree diseases you can download a pdf file that covers diseases of shade and ornamental trees: http://www.utextension.utk.edu/publications/spfiles/SP546.pdf

Fruit tree diseases

The most common fruit tree diseases include:

• Leaf curl,
• Downy mildew, and
• Powdery mildew.

Leaf curl results in young leaves becoming puckered and distorted. They also tend to develop a slightly pink tinge. The disease is at its worst when the weather is cool and wet.

Downy mildew results in pale yellow, translucent and oily spots developing on the upper surface of leaves, while a downy white fungus forms on the underside of leaves. The disease is at its worst in humid weather.

Powdery mildew in fruit trees is the same disease that attacks other trees. The growth generally appears on young leaves and the growing tips and then spreads to older leaves and to the stems. Low rainfall, high temperatures and quite high humidity encourage the spread of the disease.

Management of tree diseases

Chemical control with fungicides is still the most common way to overcome tree diseases.

Since some diseases are spread by insects, controlling pests is a good way to prevent at least some diseases – and you can use organic means to do this.

Unfortunately not all diseases can be controlled, for example verticillium wilt. In this case, the only thing to do is to prune affected branches and then fertilize and water well to encourage healthy growth of the tree. The problem with this particular disease is that it is caused by a fungus in the soil and so it can easily recur and also affect other trees and plants.

If trees defoliate then get rid of the diseased leaves, preferably by burning. If you need a permit to burn, make sure you get one.

Prevention of tree diseases

The best way to prevent tree diseases is to plant disease-free trees that are also disease-resistant. Always plant in a suitable position, determined by the type of tree you are planting, and make sure that the soil is suitable as well.

Unhealthy trees, like unhealthy people, are more likely to be affected by disease of one sort or another. This means that you need to make an effort to keep your trees healthy, well fed and watered. Mulch and keep young trees well watered to prevent them being infected by canker-causing diseases.

When you prune trees, do so carefully to avoid exposing large areas of wood beneath the bark. If branches are damaged by wind or storms, remove any stubs and clean up the trunk.

If a tree has been removed because of soil-borne disease, don’t plant the same species (or another susceptible tree) in the same spot.

Top choices for landscaping, disease resistant trees

It is always wise to plant disease-resistant plants and trees. But also remember that different areas have their own set of common insect and disease problems. The other thing to remember is that while a tree may be resistant to disease, it may also be an alien to a particular region (or country), and then it isn’t a good idea to plant it – or it may even be illegal. So check that out as well.

Here are some trees that reportedly are resistant to certain diseases.

• Birch, hornbeam, hawthorn, honey locust, flowering crabapple, willow, mountain-ash, pine, maple and spruce are said to be resistant to verticillium wilt.
• Various dogwoods are said to be resistant to dogwood anthracnose and powdery mildew.
• Even though Dutch elm disease affects elm trees, the American elm, Chinese or lacebark elm and the hybrid elm are all resistant to this disease.

The pdf referred to earlier has some interesting information about particular shade and ornamental trees that are resistant to disease.