Cornell Extension Tree Diseases


Cornell University has the third-largest agricultural college in the United States. Through years of research and exploration, Cornell has been able to offer information about dozens of tree diseases affecting commercial growers and homeowners.


The leaves of a tree infected with anthracnose will be marked with brown, black and purple spots. The fungus can also grow along the veins of the leaf. A tree infected with anthracnose may lose all of its foliage. If this occurs early in the growing season, the tree will produce another set of leaves. Disposing of infected debris, raking and removing fallen leaves, and applying a fungicide help control the anthracnose fungus.

Black Knot

Cherry and plum trees are susceptible to black knot disease, a fungus that manifests itself as black masses on the branches. The disease is believed to be indigenous to North America. Black knot disease often goes unnoticed the first year because it may only appear as a slight swelling on a branch. The branch also may appear to be slightly split. By the second year, the growth becomes hard and black. The disease must be cut out of the tree. There are fungicides to control black knot.

Diplodia Tip Blight

In pine trees, diplodia causes the ends of the needles to turn brown and die. The disease spreads through the stems, affecting the entire branch and eventually the tree. Tip blight affects all pine trees, although some species are more resistant than others. Tip blight is spread by the release of spores from small fruiting bodies to the needles of the tree. The diseased tree shows signs of infection within hours. Treat the disease by removing the infected branches and clearing any debris from under the tree. Small doses of fungicide may be applied.

Dutch Elm Disease

Dutch elm disease can kill a tree in one season. Trees planted near the infected specimen risk contamination by root grafting (when two trees' roots grow together). By the time symptoms become visible, severe damage has been done to the tree internally. The fungus that causes the disease can be controlled with chemical treatments. However, the disease spreads not only by the spores of the fungus but by the beetles that infest the diseased wood. The infected tree should be removed to prevent damage to neighboring trees.

Fire Blight

The bacteria that causes fire blight in apple and pear trees is capable of overwintering in the tree and growing stronger when warm weather returns. Visible signs of this disease include new shoot growth turning brown or black and withering. The shoot will twist as it withers and begins to resemble a shepherd's hook. A sticky substance similar to resin is released by the bacteria and will drip from the infected sites as well as the fruit of the tree. Proper pruning and controlled spraying help control the bacteria.

Powdery Mildew

Powdery mildew is a coating on the leaves resembling a dusting of flour. The more humid the environment, the more susceptible plants are to powdery mildew. Trees planted near each other or in an area with a lot of shade are more vulnerable. Powdery mildew results in underdeveloped leaf systems or distorted growth of fruit. Untreated, the plant can die because the disease eats the tissue underneath the powder. Treat trees with a fungicide during normal spray schedules.

Keywords: tree disease, ailments in trees, tree problems

About this Author

Julie Richards is a freelance writer from Ohio. She has been writing poetry and short stories for over 30 years, and published a variety of e-books and articles on gardening, small business and farming. She is currently enrolled at Kent State University completing her bachelor's degree in English.