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Rose Diseases

by Don Janssen, UNL Extension Educator

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Seeing spots before your eyes can mean serious problems -- especially if you see them when you look at your rose bushes.

Leaf spots and discolorations are the first signs of several common rose diseases. Others are characterized by spots or swellings on stems or roots.

The most common rose disease is black spot. As the name implies, the most obvious sign of the disease is black spots that appear on the leaves. Infected leaves turn yellow and drop off prematurely. Severely infected plants may be completely defoliated by midsummer.

Plant foliage exposed to the black spot fungus must remain wet for at least six hours to become infected. In Lancaster County, at least, moisture was available in late May and June, so gardeners should see plenty of black spot in July.

The disease organisms overwinter on infected canes and fallen leaves. A thorough fall cleanup and weekly spraying or dusting with a fungicide beginning when leaves are half-open are needed to prevent and control the disease. Control is important because severe defoliation time and again weakens the plants and makes them more susceptible to winter kill and other diseases. It also reduces the number of flowers produced.

Powdery white masses on young leaves, shoots and buds are powdery mildew. Like the black spot fungus, the spores of powdery mildew overwinter on fallen leaves and infected stems, and the disease may cause premature leaf drop. It may also cause young shoots to be swollen or distorted. Control powdery mildew with fungicides labeled for use on roses.

Blotches, rings, angular spots and other sorts of variegation on rose leaflets are signs of a virus disease. These diseases are spread when roses are propagated. Most common rose viruses are known to be mechanically transmitted. There are two that do appear to be naturally transmitted but no one is sure how. Infected plants may be slightly to severely dwarfed.

Once plants are infected, there is no cure. The only control is prevention: buy plants free of the signs of virus diseases. Flower production is reduced on virus infected plants.

Cankers first appear as small reddish or brown areas on the stem. They slowly grow larger, eventually girdling the stem and killing the entire cane.

Cankers commonly occur in plants weakened by black spot, poor nutrition or winter injury. Keeping bushes free of black spot, fertilized and well protected in winter are the best controls for cankers. Also prune out all canker-infected canes, disinfecting pruning tools with alcohol between cuts.

Crown gall in roses is caused by a bacterium. Galls begin as small swellings, usually around ground level but sometimes higher up on the stem or on the roots. They increase in size slowly, stunting and sometimes killing the plant.

Control crown gall by prevention. Buy plants free of galls and plant them in soil that has been free of crown gall-infected roses for at least two years. If the disease appears, remove and burn infected plants.

This resource was added June 2008 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement