This article will help you identify and treat the most common ailments that might befall your roses. They fall into two categories: insect pests and microbial diseases.
Some insects are beneficial to your roses, but many like to chow down on the leaves and buds. In most circumstances, it’s best to treat the plants with insecticides only when you see the pests or their effects; otherwise, you can weaken the plant by killing the good insects as well as the bad. The following section discusses how to identify and treat infestations of common rose-attacking insects.
Aphids are tiny green bugs that like to cluster on new leaves and buds to suck out their juices. These little vampires can drain away nutrients that the plants need to survive, causing foliage and buds to wither, distort, and die. In many cases, a black mold is associated with aphid infection.
If you prefer to use chemical free-methods of aphid control, you have two options: you can blast the bugs off your plants with a water hose (a temporary measure at best), or you can introduce ladybugs into your garden. The ladybugs won’t eradicate the aphids, but they’ll keep them under control. Chemical methods include spraying with horticultural oil to kill the eggs; application of disulfoton granules to the soil around the bushes; and treatment with various insecticides and industrial soap. The mold infection will disappear with the aphids.
2. Cane Borers
Cane borers are tiny caterpillar-like larvae that enter rose canes through tiny wounds and pruned stems, and then proceed to set up housekeeping. On occasion, they may enter the by actively boring into the cases. Whatever their point of entry, their activity stops new growth and causes existing leaves to turn yellow and die, resulting in sparse, stunted foliage. This sort of infection can be a hard one to spot; the best evidence is the presence of tiny bore holes in the canes.
Prune the canes below the infected areas. All infected canes should be discarded and, if possible, destroyed. To prevent new infection, spray the pruned ends of the living canes and any visible wounds with plant-wound paint.
3. Japanese Beetles
These shiny green or coppery beetles are particularly fond of rose blooms, leaving small round holes in the petals and occasionally in adjoining leaves. They rarely exceed 1/2 inch in length, but have large appetites.
If only a few beetles are present, they can be picked off the plants by hand and subsequently destroyed. Otherwise, you can try a variety of commercially-available insecticides. For those who prefer organic solutions, the application of milky spore disease to the soil around the bushes will eradicate the larvae.
4. Leafcutter Bees and Caterpillars
If you find large holes in your rose leaves, they’re probably the work of caterpillars or leafcutter bees. If the holes are irregular and occur within the central portions of the leaves, or if entire leaves have been eaten, then your plants are plagued by caterpillars. Caterpillars are the larvae of various insect species, and as such they eat leaves for food. They can cause a great deal of damage to a growing plant. On the other hand, semicircular sections taken out of the edges of rose leaves are a sure sign of leafcutter bees. These half-inch insects don’t eat the foliage they cut; instead, they use it to build their nests.
Like beetles, caterpillars can be picked off by hand and destroyed. They can also be controlled using various chemical sprays. The organic alternative is the use of sprays containing Bacillus thuringiensis, an organism fatal to caterpillars but perfectly safe for plants and other animals. No real treatment is necessary for leafcutter bees, although you might want to prune away the damaged leaves. Don’t use insecticides on these particular bugs, since they aid in pollination.
If your buds are bending over near the tops of their stems and dying before they open, you’ve probably got thrips. Thrips are near-microscopic bugs that attack leaves and buds, often leaving the latter with bumpy brown outer petals. They mostly prefer plants with pale-colored blooms.
Prune away and destroy all infected canes and buds. Once this is done, spray the tops of the bushes with insecticide or industrial soap. You should also spray the soil around the plants, in order to kill the immature thrips that live there. Keep a close eye on your roses; thrips are a serious pest, and may come back more than once.
Most rose diseases are fungal in nature, though some are viral or bacterial. Fungicides, unlike insecticides, can be used whenever you see fit; however, if you discover a fungal infection on your plants, it’s often too late to control it with fungicide. In such cases, more stringent measures are necessary. This section describes the most common rose diseases, and suggests treatments for each.
1. Black Spot
Black spot is one of the most serious and widespread of rose diseases, and it strikes the yellow strains of roses especially hard. A fungal infection, black spot is characterized by the appearance of large black spots on leaf surfaces. Eventually the spots spread and join, and the infected leaves turn yellow and fall off. If enough leaves die, the plant will be unable to photosynthesize the nutrients it needs to live, and may itself die. Although black spot doesn’t visibly affect the canes, the spores of the disease often cling to them.
To prevent black spot from appearing, spray or dust your roses with fungicide every 7-10 days, and water them from above early in the day; a wet rose is an invitation to fungal infection. Make sure individual rose plants are spaced well apart, and trim away any dead canes in order to enhance air circulation. If a plant is already infected, prune away all diseased leaves, canes, and flowers and destroy them. Make sure this is done well away from the plants, as the spores can easily become airborne. Check the plant repeatedly for re-infection in the following days, and immediately remove any diseased vegetation that may appear.
Canker is a fungal disease characterized by elongated, sunken spots on the canes and stems; this may be accompanied by cracking. Eventually the spots will encircle the infected canes, killing them.
Prune way the canes below the points of infection and destroy the diseased vegetation. There is no effective chemical treatment or preventative for canker.
3. Gray mold
Gray mold, another fungal disease, causes rose vegetation to turn black; buds may also blacken. Infected buds usually do not open, but if they do, they will be discolored and the petals will be furred with a fine grayish growth. This disease is especially common in cold, damp weather.
For gray mold, use the same treatment you’d use for black spot.
4. Powdery Mildew
Powdery mildew can appear at any time of the year, and is not exclusive to rose plants; it may infect other bushes or trees as well. It manifests as a dusty white powder on leaves, stems and buds. If left untreated, it will kill the leaves and deform or kill the flowers.
Spraying or dusting with fungicide every 7-10 days can help prevent the disease, as can a lime-sulfur spray administered very early in the spring while the plant is still quiescent. Otherwise, make sure the plants are properly pruned in order to enhance air circulation. If the mildew appears, remove all diseased vegetation and spray the entire plant with fungicide in order to keep the disease from spreading.
Rust, like all the diseases discussed here, is a fungal disease. This one makes the topsides of rose leaves turn pale green or yellow, while spotting the undersides with dots of powdery orange. It may not kill the infected foliage, but will cause the leaves to wilt and/or curl up. This disease is especially prevalent during cool, humid weather.
Preventatives include treatment with a lime-sulfur spray in early spring, followed by a regular regimen of fungicide every 10 days, beginning in mid-spring. To eradicate the disease, remove any infected stems, canes, or leaves, and discard them; keep doing this on a regular basis and continue with the spraying regimen until the rust is gone.