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Disease and Insects afflict a lot of flowering plants, but they can be devastating to rose bushes, especially black spot. Hardy rose bushes are not immune to the problems. In northern and midwest gardens, rose bushes covered by snow may be protected from the cold, but the moist conditions in spring caused by slow melting snow can contribute to the problem of spreading disease which has over wintered in the ground and on the rose bush.

Controlling Rose Diseases

Many of the new hybrids, as well as some of the old fashioned rose bushes, are quite resistant to the viruses and fungi that can plague rose gardens. With a well selected rose bush, and proper care and attention, you may be able to avoid the worst of these problems. Should a disease strike, there are many chemicals on the market that can help correct the problems, and some that will inhibit the diseases from striking in the first place. They are generally sprays, or canisters of powder that is “puffed” out in a powdery mist onto your bush. There are of course non-chemical or organic solutions and prevention of rose disease, which should be your first line of defense. Some of the rose diseases can be devastating to your plants, so check them diligently and work to prevent them in the first place. Most fungus disease can be spread by wind, water, insects and GARDENERS, so clean up after you have handled infected plants. And rain or sprinklers splashing water will spread fungus, 2 to 3 inches of mulch around your plants will minimize water splash. The spores can be carried on your hands, garden gloves, tools and clothes.  In Northern and Midwest gardens, leave winter protection in place to keep the plant dormant until the ground is well thawed and temperatures are warm. Fungus that has overwintered will not be presented with such moist conditions, as the ground at the base of your plant will thaw and then dry out more rapidly in the warm temperatures. Proper pruning to keep the centers open will allow better air circulation which can deter fungus from growing. Preventing fungus from striking is the best defense.

Black Spot: Diplocarpon rosae, a fungal disease, is the most common and most rampant rose disease. Nearly every rose bush is susceptible to some degree, hybrid tea roses are generally the most susceptible. Small dark spots appear on the leaves, growing and eventually covering the leaf. They then turn yellow and promptly fall off. Often the bush will lose it’s leaves entirely. When a rose bush goes into winter in this state, the buds are very vulnerable to winter injury. Black spot may continue to spread to the canes, and leaf and flower stalks. Warm damp weather will encourage the disease. Dry hot weather, over 85 degrees, will kill spores. Infected leaves should be immediately removed from the bush, or the ground, and thrown in the trash, with the hope of minimizing the spread. NEVER throw them in the compost heap. There is nothing you can do about rain and humidity keeping your leaves wet, but you can do supplemental watering with a soaker hose (drip method) or a long, slow trickle from the garden hose into the soil. Once black spot has infested your rose bushes, a fungicide that contains Chlorothalonil or triforine applied every 7 to ten days will help get the disease under control. Before winter, spray the stems and ground surrounding the plant to reduce black spot problems returning in the spring. The spores are harbored in the canes and ground, waiting to strike again in spring.

Powdery Mildew: Sphaerotheca pannosa, also quite common, can be nearly as devastating to a rose bush as black spot. Easily identified, the plant is coated with a white powdery substance almost like talcum powder. The leaves, leaf and flower stalks, and the petals may be attacked. Young flower buds may fail to open or produce malformed blooms. Under watering and undernourishing can contribute to susceptibility. High nitrogen fertilizer can also contribute (in addition to causing the bush to produce leaves instead of blooms). Poor air circulation and cool nights help spread the disease. Interestingly enough, wet leaves can inhibit the growth of spores, whereas most fungi will thrive on the wet leaves.

Canker: This fungus enters a healthy plant through pruning wounds and like other fungus, is spread by splashing water, insects and dirty tools. Canker appears as dead or discolored areas on the rose canes, or appearing to “hollow out” the cane while growth above it seems fine. Once the canker has consumed the girth of the cane, growth above it will die. Do not fertilize or prune a plant infected with canker. Both with stimulate new growth, which is most susceptible. A copper based fungicide will help control the fungus, as well as the fungicides that control black spot. To help prevent canker, prune out canes that are crossing each other, and always prune to a outward facing bud node. Canes will die back to the node anyway, inviting canker.

Rust: Phragmidium mucronatum produces rust colored spots on the leaves, or may appear as little pods of orangey spores on the leaves, stems and stalks in it’s later stages of development. Leaves will turn yellow and fall off.

Downy Mildew: Peronospora sparsa is not as rampant as black spot, but it certainly can be deadly. Red or purplish blotches appear on leaves, stems and flower buds, then turn brown with a gray mold forming. The blooms may turn brown. Dead spots may develop on the plant, and within a few days it may drop all it’s leaves. Cool wet weather followed by warm, then cool and wet again may encourage the development of downy mildew. Four weeks of hot dry weather can prevent reproduction of the spores, but the fungus survives, ready to reproduce again next year. If it didn’t kill your rose bush in the first season, good chance it will in the second season.

Bacterial Infections and Viruses

Bacterial infections in rose bushes usually cause blackening of canes, wilting, or root tumors. Viruses display a yellowing mosaic pattern on leaves (leaf mosaic virus), or distorted abnormal leaves (rose rosette). These afflictions are not as common as fungi. 

Treatment: Selecting disease resistant rose bushes and proper rose care is the best defense, but will not prevent these diseases. As mentioned, there are chemicals available to help prevent and treat the diseases, but there are also many popular homemade or organic treatments. Try the following sprays for prevention, and spray all surfaces, including the underside of the leaves.

• One tablespoon baking soda dissolved in a gallon of water for black spot and powdery mildew. Or a similar recipe, one teaspoon baking soda, 1/2 teaspoon oil (horticultural oil, such as Sunspray Ultra-Fine) and a quart of warm water. Mix and apply with a spray bottle every 1 to 2 weeks. Note: overdosing and high concentrations of baking soda will burn the foliage.
• A deficiency of potassium can make rose bushes more vulnerable. If needed, add 150 grams of sulfate of potash or sulpher dust to each plant to control powdery mildew and black spot.
• Steep a whole crushed garlic in a gallon of water, strain, and spray every couple of weeks.
• Use antitranspirants made for evergreens (Wilt Pruf or Vapor Guard) to form a coating over the leaves to protect from fungus.
• Silicates, marketed as ProTeKt, may control fungus.
• Rose Flora is a bacterium that combats black spot fungus. Apply to the plant foliage and to the soil once while the rose bushes are dormant, once right after the leaves appear, and about once a month thereafter.
• Wilt Pruf controls mildew, and perhaps rust. Apply with the same directions, or same time as, Rose Flora.
• Neem oil, marketed as Defense, kills bacteria, viruses, fungi and pests. Apply carefully according to directions.
• Rose bushes that are consistently diseased should be removed and destroyed.

Insects

Aphids: There are a tremendous number of aphid varieties, and not all attack roses. Rose aphids and potato aphids are the most common offenders. The adult lays hundreds of eggs on the rose canes. The aphid pierces tender plant material then sucks the liquid out. They do not tend to seriously damage the plant, but certainly cause aesthetic damage, often leaving leaving your rose petals dried and shriveled. Plants attacked by aphids do not grow as vigorously and the aphids often carry disease. You may most often notice their arrival when new growth and buds appear, but they generally hang around all summer and eggs are able to survive winter. Since they love the tender growth, first, stop fertilizing to minimize the food source. There are many predators that feed on aphids, so avoid chemicals that will kill the predators too, give the predators a chance to do their work. The aphids can be sprayed off the plant with a garden hose which usually drowns them. Do this every other day for a week or so, then keep a close eye out to see if any survive and return. If the aphids still are not under control, spray with insectidical soap or neem oil. Be aware that when sprayed in hot weather, these too can damage your plants.

Rose Chafers: These beetles appear in early June and live about 3 weeks, feeding on fruits, rose blossoms and flowering plants. They are a 3/4 inch long, thin, tan colored beetle with long legs and a reddish brown head. Often a very large number will appear on a single plant. Leaves are often completely consumed. You can hand pick them off or try botanical insecticides with a little horticultural oil added, that will help the insecticide stick to the leaves. Organic pyrethrum and rotenone, or chemical Sevin can be applied every 3 to 4 days.

Rose Bud Borers: The two borers that afflict rose bushes are Rose curculios (or rose weevil) which are about 1/4 inch long and bright red with black beaks, and Rose leaf beetles which are about 1/8 inch long, and are shiny blue or green. They both bore into the flower buds preventing bloom. You may simply pick them off the plants and remove and destroy infected buds, or spray with insecticidal soap mixed with pyrethrin. This will also help prevent spread to other plants.

Japanese Beetles: These destructive beetles are about 1/2 long, metallic green, with coppery bronze colored wing covers. They chew small holes in leaves and flowers, often feeding in groups. They begin on the top of the plant and work their way down, you may find them burrowed head down into flower blooms. Milky spore, or a granular grub control insecticide applied to the lawn/ground will attack the grubs. Milky spore can control the problem for years. Chemical grub control should be applied late in summer, and it is important to follow directions exactly. Chemicals to control the adult beetle include Sevin and Diazinon, and must contact the beetles to be effective.

Spider Mites: Spider Mites are quite common, and several species will attack roses. They are generally found in colonies, living on the underside of leaves. Signs of infestation are yellowed, dry looking leaves with small white dots, and sometimes you will find webbing on the stems and leaves. They are very tiny, so you probably won’t see them. Apply dormant oil or lime sulfur, but only when the rose is dormant. If you need to apply something in the growing season, use insecticidal soap mixed with pyrethrin.

For non-chemical control of rose disease and insects, try the products from PlanetNatural.com

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