Dear Friends and fellow Gardeners,

I recently gave a lecture on rose diseases to the Woodland Rose Garden Club.

The following are a few thoughts and ideas that may be helpful in the fight against rose pathogens.

This last year was a particularly bad year with cool cloudy weather. The conditions were right for the spread of the black spot, powdery mildew, and rust. In order to stop the spread of these fungus diseases, it is important to know the optimum conditions for infection.

Powdery mildew. The most important plant fungus problem in the interior valley of California. The spores are produced and spread the best during periods of cloud cover with temperatures in the 80s and relatively low humidity. The new growth is attacked first. You will notice raised swelling of tissue followed by white powdery fungus. Severe spread of the fungus can cause a serious decline in the growth and bloom of roses

Methods of control. Mildew cannot attach itself the leaf that has a barrier so washing the leaves in the afternoon with high pressure will remove the spores and help in the prevention of new spore attachment. Particularly helpful in washing the leaves is the surfactant "Safer Fungicide, Miticide." You can also use antitranspirants such as "Wilt Pruf" to keep a barrier on the leaf surface.

Prevention. During the winter, after pruning, spray the entire plant and soil with lime sulfur. The spores overwinter on the rose canes around the leaf buds, and in the soil. Cover the soil with 2-3 inches of mulch since this will help prevent the soil spores from occupying the plant.

Black Spot. This fungus is usually more of a problem in the south since the best conditions for spreading are warm, moist weather. Severe cases of black spot (irregular small black spots) can cause defoliation and death if not treated. Black Spot is difficult to control, "Safer" can be used as a preventive because the spores have a more difficult time with a barrier on the leaf. Antiranspirants probably work better. You can use lime sulfur during the growing season as long as the temperature is below 80 degrees.

Prevention. Prune off infected foliage and remove diseased canes during winter. You may want to prune the rose into complete dormancy with severe infections. Winter spray with lime sulfur both plant and soil.

Rust. More of a problem along the Pacific Coast since Rust likes cool moist conditions to spread its orange to yellow pustules. Severe infestations can cause leaf drop and bloom reduction. Spores are spread by wind and water. If using water or surfactants to clean the leaves make sure it's done at the time of day that allows for the leaf to thoroughly dry.

Prevention. Much like the other fungus diseases. Prune and spray with lime sulfur during the winter.

Cultural considerations. Select varieties that are disease resistant, provide sufficient nutrients during growing season, deep water during the growing season, plant roses in full sun, keep fallen leaves and debris cleaned up around the plants, mulch and spray with lime sulfur during the winter, and light pruning of infected parts during the growing season.