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How to treat roses when fungus is among us

Ann Dozier is a UC Master Gardener.
Ann Dozier is a UC Master Gardener.

Q: My roses suffered from all kinds of diseases last fall. Is there anything I can do now to have healthier bushes this year?

—Alicia Amezquita Garcia,

Los Osos

A: Let’s face an unpleasant truth: Many rose varieties are high maintenance — and fungal diseases are among the most common problems.

It may be helpful for you to recognize conditions that cause fungal diseases. Different types of fungus, for instance, cause black spot, rust and downy mildew, and all of them multiply on damp leaves. These diseases will be more prevalent in coastal areas than further inland. Powdery mildew, however, occurs in warm inland climates.

Fungal spores are spread by splashing of water — therefore, consider an on-the-ground watering system, rather than sprinklers. If you must sprinkle, do it early in the day, so leaves have the opportunity to dry. Some of these fungal diseases will be splashed up onto young growth when it rains.

Discourage diseases by removing all remaining leaves from the plants after pruning in the winter. Rake up and dispose of all leaves on the ground. Re-mulch to prevent splashing of water. During the growing season, removing leaves with rust, raking up leaves that have fallen and using a horticultural oil spray will reduce the amount of fungal problems. Fungicides are more helpful as prevention, rather than cure. The type of treatment depends on the specific disease. You can’t prevent these diseases but you can reduce them.

Perhaps your best recourse is to replant with more resistant varieties if your roses have continual disease problems. Consult with a nursery in your area and also with neighbors to find rose varieties that work best in your micro-climate. For information on rose diseases online go to www.ipm.ucdavis.edu/index.html or call your local Master Gardeners.

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