Roses (Rosa spp.) are among the most finicky plants in gardens across U.S. Department of Agriculture plant hardiness zones 2 through 11. They've been intensely bred over centuries for outstanding flower size and color, often sacrificing pest and disease resistance in the process. Symptoms such as white fuzz and leaf curl are common problems for rose growers, who are ever on the lookout for new problems. These symptoms are generally caused by either sap-sucking insects or powdery mildew, both problems that are easy to cure.
Cottony Cushion Scale
The adult female cottony cushion scale (Icerya purchasi) sports a distinctive one-half-inch long fluted, white egg sac that gives the species their name. Younger cottony cushion scales excrete a white coating that they molt as they grow. These scale feed directly on the fluids of plants, causing distorted leaves, twig dieback and even defoliation in large numbers. They may also attract ants, who will farm the honeydew they secrete and protect them from predators.
Cottony cushion scale are a little more difficult to manage than most scale since they remain mobile throughout their lives. Imidacloprid is labeled for these scale, but time has shown that it does not actually kill them -- it does, however, effectively destroy their natural predators. Instead, begin control measures by managing ants. Place baits along the paths they travel to the affected rose, and apply a sticky band around the base of the bush if possible. Spray the bush when temperatures are below 80 degrees Fahrenheit with a horticultural oil to smother the insects as they feed.
Mealybugs are another sap-feeding insect that may trouble roses. They are small, segmented insects that often shed a white cottony or mealy substance that tends to collect on plants where they've been feeding. Mealybugs range in size from five-one hundredths to one-fifth inch. Large populations cause similar symptoms as cottony cushion scale and attract ants as well.
Spray mealybug-infested roses with a strong blast of water from a garden hose to dislodge these insects wherever possible. Once dislodged, they cannot reattach before they starve to death because they move so slowly. As with cottony cushion scale, baiting and excluding ants can help encourage predator populations. Insecticidal soaps are also effective at eliminating these insects, but it should not be applied when temperatures exceed 80 F.
Powdery mildew often survives on roses from year to year in buds or as fruiting fungal bodies hiding on the plants' bark. Once actively growing, powdery mildew can appear as a widespread powdery coating or as white, fuzzy patches on the surface of affected leaves. New growth is often distorted or dwarfed and will drop prematurely.
Powdery mildew is encouraged by poor air circulation within the plant's framework. Open up the inside of the affected rose by removing up to one-third of the branches where they attach to the crown. Prune a severely overgrown plant slowly, though, or you risk killing it. Watering the leaves of powdery-mildew infected plants can kill spores, but do so at a time when the water will dry quickly, or you risk inviting other fungus. If cultural management doesn't seem to be working, try a safer fungicide such as neem oil, sulfur or the biological fungicide Bacillus subtilis.
- Fine Gardening Plant Guide: Genus Rosa
- University of California Statewide Integrated Pest Management Program: Rose
- University of California Statewide Integrated Pest Management Program: Cottony Cushion Scale
- University of California Statewide Integrated Pest Management Program: Scales
- University of California Statewide Integrated Pest Management Program: Mealybugs
- University of California Statewide Integrated Pest Management Program: Powdery Mildew on Ornamentals
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