Extension Educator, Horticulture
A rose by any other name would smell as sweet, but might not get as many diseases. Rose problems can become rampant this time of year. Here are a few of the possibilities.
Blackspot is a common fungal disease of roses. It can completely defoliate roses by fall, which weakens the plant and shows the plant's not-so-attractive prickly legs. Blackspot causes (you guessed it) black spots on the leaves, starting at the bottom of the plant and moving upward. Infected leaves eventually turn yellow and fall off. Infection can also appear on canes as reddish-purple spots.
The best way to combat blackspot is to select resistant varieties; however keep in mind even resistant varieties may still be infected when weather is conducive to disease. As with most fungal diseases 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. If you choose to use fungicides such as Funginex, Daconil, or Mancozeb they should be started in spring as new leaves appear. Spraying now would protect only new leaves.
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 through sanitation by pruning out all dead and diseased canes and with fungicide sprays concentrating on coverage of the new growth. Once again plant roses in areas where they receive good air circulation.
When rose rosette disease first appeared it looked as though it would be a great natural control of the pesky weed, 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. Bury the carcass with proper ceremony.
It's hard to talk about rose problems without also discussing the dreaded Japanese beetles. If you have a few beetles, one tactic is a daily harvest of the beasts into a container (my favorite is a 2 lb. coffee container with the lid) filled with soapy water. If you have mass quantities of beetles, insecticide sprays of carbaryl sold as Sevin or synthetic pyrethroids such as permethrin and bifenthrin can be used. Keep in mind Sevin is toxic to beneficial insects as well. Systemic insecticides such as imidacloprid are most effective if applied in spring. Read and follow all label directions. Forget beetle traps. They attract more than they catch. For more rose information check out the UI Extension Our Rose Garden website. http://urbanext.illinois.edu/roses/
If your roses look wretched, take plant samples to your local UI Extension office http://web.extension.uiuc.edu/state/index.html or to UI Plant Clinic http://plantclinic.cropsci.illinois.edu/ at 1401 W. St. Mary's Road Urbana, IL 61802; 217-333-0519.