Tree cankers -- often sunken, discolored areas -- can spread and overwhelm the entire plant

Backyard Q&A

In the Garden

April 04, 2004|By Dennis Bishop | Dennis Bishop,Special to the Sun

A tree service recently looked at one of our trees and said it had cankers. Can you explain what a tree canker is and what causes them?

If you have a dead area of tissue on a tree leaf, it is called a leaf spot or blight. A comparable area on a tree branch or trunk is called a canker; it is often sunken and discolored.

Diseases cause most cankers, but there may be other reasons. For example, if a tree trunk were hit by a lawn mower, some of the tissue would be killed. This damaged area would technically be a canker. While the initial damage may appear insignificant, it could be just the beginning. After trees are wounded like this, they are vulnerable to bacterial and fungal diseases. Should a fungus attack, the canker would expand and might seriously injure the trunk or kill the entire tree. In this case, the initial cause of the canker was the lawn mower, but most of the damage came from a secondary cause, the fungus.

Because disease can spread from cankered branches to other areas of the tree, the affected branches should be removed in most instances.

Can you tell me when the 17-Year locusts will be coming to Maryland? And what should I do to protect my plants?

The insects we often call locusts are actually cicadas. They do not have to come here because they already live here -- in the ground.

Some emerge every year and are called annual cicadas, while others emerge only occasionally and are called periodic cicadas. The brood of cicadas that lives in Maryland is called Brood X. They spend most of their life sucking on roots but emerge once every 17 years. The Maryland brood will begin emerging in May and will be out for about two months.

They will be a nuisance, but should do very little damage to plants. The damage occurs when the female slits the bark on branches of small trees and shrubs to deposit her eggs. It is absolutely not recommended that you spray with chemicals. This would have little or no effect on the cicadas, but it would damage the environment. If you would like to protect your shrubs and small trees, cover them with plastic netting. The holes in the netting should be no larger than a quarter-inch; otherwise, the insects will be able to slip through. Be sure to tie the netting at the base.

Dennis Bishop is an urban horticulture educator for the Baltimore office of the Maryland Cooperative Extension Services. If you have a gardening or pest problem, you can call the Home and Garden Information Center hot line (Monday through Friday, 8 a.m. to 1 p.m.) at 800-342-2507. You can also e-mail questions, order publications and diagnose plant problems by visiting the Web site


1. Harden off broccoli, cauliflower and other cool-season vegetable transplants by gradually exposing them, over five to seven days, to sunlight and cool temperatures before planting them.

2. If you have not done so already, plant peas now and continue to plant succession crops of spinach, beets, lettuce and radishes.

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