Common Disease Problems on Roses

Downy Mildew- Peronospora sparsa appears after consistent cool conditions (Jan-Feb), then a big temperature hike, followed by cool humid weather again. Symptoms include lesions on the leaves, stems and flowers, they appear purplish to brown, blocky and often accompanied by yellowing of the surrounding tissues. The Downy Mildew fungus forms a downy mass of spores on the underside of leaf lesions, difficult to see without a hand lens. Once conditions turn warm and dry, the disease is kept on check until the next cool, humid period.

Controls-Avoid overhead watering, increase air circulation by pruning out/thinning in the center of the plant. Remove infected tissue, place in a plastic bag and seal before moving it out of the garden. Kop-R-Spray can be used as a preventive measure, use as for grapes.

Powdery Mildew-Sphaerotheca pannosa var. rosae is recognized by its white to gray powdery growth on leaves, shoots, sepals, buds, and occasionally on petals. Leaves may distort and drop. Powdery mildew does not require free water on the plant surfaces to develop and is active during our warm, dry summers. Overhead sprinkling during midday may limit the disease by disrupting the daily spore-release cycle, yet allows time for foliage to dry.

Controls-The pathogen requires living tissue to survive, so pruning, collecting and disposing of leaves during the dormant season can limit infestations. Plants grown in sunny locations with good air circulation are less likely to have problems. Thinning out the center of the plant can help. Rose Defense or Serenade will also control Powdery Mildew.

Black Spot-Diplocarpon rosae produces black spots with feathery or fibrous margins on the upper surfaces of leaves and stems. Small black fruiting bodies are often present on the upper sides of leaves, there is no fungal growth on the underside of the leaves. The fungus requires free water to reproduce, so leaves should not remain wet for more than 7 hours.

Controls- Remove infested and fallen leaves in a plastic bag. Thin out the center of the plant for good air circulation. Prune out infected stems in the dormant season. Rose Defense or Serenade is effective in controlling the disease.