Black spot could be an omen of death for rose bushes during humid periods

Backyard Q&A

May 26, 2002|By Dennis Bishop | Dennis Bishop,Special to the Sun

Q. My rose leaves get black-spot disease every spring. Because I do not like to spray a lot of pesticides, I am concerned about the effects of the disease on my plants. How much harm does black spot do to roses?

A. Black spot is a fungal disease that thrives in warm, humid conditions. If rose foliage is damp, it is very difficult to control black spot without regular sprayings of fungicides. Under these conditions, even resistant varieties may become infected, and susceptible varieties can be severely damaged.

Black-spot disease typically starts in the foliage; however, it can spread to the stems and, in the worst cases, kill entire plants.

Q. Our trees put on a lot of growth this spring and look very healthy. However, many of the branches have become weighted down and it is difficult to work underneath them. Is this an OK time to cut the branches back?

A. This is a good time to prune your trees. Be sure to use good pruning techniques. It is always preferable to take out whole branches rather than to cut in the middle of a branch. When branches are cut in the middle, an unsightly stub is left, and because the stub never heals properly, it can become an entrance for diseases.

Also, think about the future and cut out a few extra branches so that you will not have to prune next year.

Q. We planted several new trees this spring; however, we will be gone for part of the summer and they will not get watered frequently. Can you tell us what we might do to help the trees?

A. First, I would build a ring of soil 3 to 6 inches high around each new tree. This enbankment should follow the outside perimeter of the root ball but not cover it. It will help collect rainwater for the roots.

Second, I would put two inches of compost inside the ring. This will help keep in moisture.

Finally, I would cover the area with 6 inches of loose straw. The straw will shade the root ball and further help to retain moisture without suffocating the tree.

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, / users / hgic.


1. Do not use sawdust or peat moss as mulch. They will become packed over time and will form a layer that is impervious to water.

2. Keep roses thinned out. This will allow air to circulate and prevent the development and spread of diseases like black spot.

3. Looking for a small, inexpensive composter for your rowhouse? A trashcan with one-inch holes drilled in the side works well. It will keep the compost in but allow air into the pile.

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