Extension Educator, Horticulture
A healthy rose is better able to survive the winter. So take a few moments to evaluate your roses and consider replacements with disease resistant varieties.
I wouldn't plant a rose unless it was resistant to black spot. This fungal disease can cause almost complete defoliation of roses by fall, which weakens the plant. Blackspot has black spots (what are the odds?) on the leaves, starting at the bottom of the plant and moving upward. Infected leaves turn yellow and fall off. Infection can also appear on canes as reddish-purple spots. Infection is more serious during rainy weather. The fungus overwinters in fallen leaves and stem cankers. Raking and removing leaves in the fall may provide some control. Avoid wetting leaves when watering and give plants good air circulation. Mulching also helps keep water from splashing up onto the leaves. Many fungicides are available, they should be started in spring as new leaves appear and continue through the season if rains are persistent.
Powdery mildew is a fungal disease that affects young leaves, causing them to curl, twist and develop a purple coloration. As the disease progresses, leaves become covered with white powder. Blackspot is usually most severe on the lower part of the plant but mildew in roses affects the top part of the plant. Mildew is spread by wind and develops rapidly during periods of warm, humid days followed by cool nights. Infection can be reduced by pruning out all dead and diseased canes. Fungicides containing potassium bicarbonate, copper or chlorothalonil are effective for black spot and powdery mildew if they are applied early in the season.
Rose rosette disease looked like it would be a great natural control of the pesky multiflora rose. Unfortunately it didn't stick to wild roses and has now moved into cultivated roses. The symptoms are easy to spot. This is definitely a "wow, what's wrong with this plant" moment. Rose rosette causes plants to form very thick, multiple red stems with extreme thorniness. Leaves may also appear deep red, or a mix of red, yellow and green. Leaves are often distorted and stunted.
Rose rosette is a virus-like disease. Plants usually die within about 22 months of infection. The disease can be spread from rose to rose by a tiny mite, so small that 20 would fit on a pinhead (30 if they hold hands). Grafting can also spread rose rosette. Sorry, plants infected with rose rosette cannot be cured so rip them out roots and all. Burn or bury the carcass with proper ceremony.
Insects may also find your roses. The first noticeable symptom of spider mites is a gray or bronze color to the leaves from their piercing and sucking feeding. Mites reproduce rapidly in warm weather. High-pressure washing with water from a garden hose directed to the underside of the leaves every 2-3 days can manage mites as well as aphids, another pest of roses. Insecticidal soaps can also be effective.
Anything I have to say about Japanese beetles includes a lot of deleted expletives. One control measure is to use one of the systemic pesticides containing imidacloprid in March. It protects the leaves but the beetle will still attack the flowers. This time of year the Japanese beetles are on to their next life and the roses have some time to rebloom. Crown gall, rose mosaic virus and stem cankers are a few of the other problems which can besiege roses. However rose problems can be greatly reduced by buying healthy, disease resistant plants and providing good cultural practices. For more information on roses, check out our website atwww.urbanext.uiuc.edu/hort/ .Check out my "In the Garden" segment on WCIA TV Morning Show every Thursday at 6:50 am and 7:25 am.