Aphids, blackspot, dieback, powdery mildew, and rusts are common problems associated with roses. Regular inspection is required and immediate treatment is recommended should problems be discovered. Heading off problems early can make a big difference in the long term health of your plants. As a general rule, modern roses are more prone to pests and diseases than most old garden roses.
The best way to avoid disease and insect problems is to maintain healthy growing conditions for your roses. This starts with the selection process. Select cultivars and species that are hardy and disease resistant. Practice good maintenance and care practices and procedures. Make sure each plant is properly planted and examine or scout your plants on a regular basis.
Disease: Generally, most disease problems surface and become prevalent during periods of moist weather. This is a time to be most diligent in scouting your roses and maintaining good health management practices.
Dispose of fallen and diseased leaves, make sure there is good air circulation and properly water and mulch your plants. Many fungi overwinter in fallen leaves and stem cankers. Raking and removing leaves in the fall may provide some control. Avoid wetting leaves when watering. By providing good air circulation around the rose bushes helps reduce the problems associated with wet or damp conditions. Mulching around the bushes helps keep water from splashing up onto the leaves. If you choose to use fungicides such as Funginex, they should be started in spring as new leaves appear.
Rose diseases can be classified as affecting foliage or leaves, flowers or buds and canes. Other symptoms may be caused by chemical, animal or environmental factors. If you see evidence of disease on your plants, it is essential that the problem is properly diagnosed in order to use the correct treatment. It is always best to identify the specific problem and follow specific treatment recommendations. If you are stumped, call your Extension office for help. You may be asked to send a sample of the diseased part of your plant to the office for help in diagnosing the problem.
Insects: Insect pests that attack roses and affect flower growth and quality should be a concern to most rose gardeners. Pests are not likely to kill the whole plant, but can kill or stunt parts of the plant, affect flowering, and cause unsightly visual damage. Regular inspection is important to minimize damaging effects of insects.
For leaf chewing or piercing or other suspicious damage, it's important to properly identify the culprit before trying to treat. There are several categories of insect defoliaters or leaf feeders, piercing and sucking feeders, bud and shoot feeders, gall makers and stem borers that you need to be aware of in making your diagnosis.
| Problem | Description |
Solution |
| Black Spot |
The spots have a fairly uniform black color with fringed margins. The spots enlarge from pinhead size to the diameter of a dime and coalesce to form large irregular blotches. The entire leaf may become yellow or there may be a yellow area around individual spots. Infected leaves usually drop off after becoming yellow. The leaf spots are unsightly, but the most serious effect on the plant is premature defoliation. The early loss of leaves reduces the food manufacturing capacity of the plant and predisposes it to injury from other diseases, insects, and environmental conditions. |
Spray foliage every 7 — 10 days throughout the growing season with a chlorothalonil fungicide. Don't water with sprinklers. Rake up and destroy fallen leaves. Spray entire plant with lime-sulfur dormant spray in late winter. Apply 1" — 2" of fresh mulch atop the rose bed each spring. Practice plant sanitation, especially in autumn. Chemical pesticides may be available. If you choose to use a chemical pesticide, contact your local Cooperative Extension office for specific recommendations. Use according to label directions. Avoid Black Spot with a Preventative Spray: Even during years of drought, humid climates can expect some Black Spot on roses. A dormant spray of a fungicide applied after pruning will help greatly in offsetting the problem. A rose fungicide containing either lime of copper is a good low toxic choice. |
Rose Virus |
Rose Mosaic Virus shows up as yellow mottling on leaves and deformed new growth. It can stunt growth or it can be a mild infection. If there are only a few affected leaves, the plant may continue growing and blooming fine. The really good news is that it won’t spread to your other roses. |
Rose virus is transmitted during grafting or budding at the nursery. There's no successful way to control this virus with chemicals. If the plant becomes too weak to bloom properly, remove and destroy. Once a rose is infected with Rose Mosaic Virus, there’s not much to be done except check with the nursery for a replacement. |
Powdery Mildew |
First symptoms of powdery mildew are a slight curling of the leaves followed by white powdery fungal growth. Buds may not open, those that do produce malformed flowers. The leaves become blistered and have a reddish cast. Powdery mildew is more severe on the succulent growth that frequently results from excessive use of nitrogen fertilizers.
|
Spray foliage with fungicide at first sign of mildew. Spray again at intervals of 7 — 10 days if mildew reappears. Rake up and destroy fallen leaves. |
Japanese Beetle |
|
Remove small numbers by hand and destroy. If the infestation is severe, spray flowers and foliage with carbaryl once a week from June until September. It's best to spray during evening to avoid killing bees. |
Thrips |
|
Remove and destroy infested blooms and buds. Spray flowers, buds and foliage with acephate insecticide three times at intervals of 7 — 10 days. |
Circular black spots with fringed margins and possibly yellow edges appear on leaves. Leaves may drop
prematurely.
Yellow spots or patterns on foliage and malformed new growth. 
Thin
layer of grayish-white, powdery material forms on young plant parts.
Infected leaves may distort, curl or turn yellow or purple and drop off. 
Holes appear on leaves and buds. Beetles are red, green-spotted, brown or metallic green
and up to 1/2" long. 
Leaves, flower buds and flowers distort and may be flecked with yellow or brown streaks and spots.
Shake infected flower over white paper. Tiny yellow or brown insects will be easily
seen against white background. 