CORVALLIS, Ore. — Mild winter weather will most likely coax roses into bloom early, which means dealing with the usual diseases and pests earlier, too.
When it comes to one of the country’s most popular perennials, the “big four” problems are powdery mildew, black spot, rust and aphids, according to Jay Pscheidt, a plant pathology specialist with the Oregon State University Extension Service.
Downy mildew, which looks strikingly similar to black spot, can also become an increasing problem.
“People are confusing them,” he said. “It’s difficult to tell the two apart.”
What you’re seeing on leaves
Black spot shows up as ragged black spots on leaves that often turn yellow and eventually drop.
The black spots caused by downy mildew are not as dark and can look angular. Under moist conditions, you might see a gray, downy-looking growth on the undersides of leaves.
As the name suggests, powdery mildew appears as a white powder covering leaves, but without the black blotches typical of downy mildew and black spot, Pscheidt said. It usually hits roses during the transition into summer’s driest time, especially when dry days are followed by humid nights.
Rust begins in spring and peaks in early summer. It appears as orange pustules that can blanket foliage in worst-case scenarios.
Aphids: simple, effective control
Insects are less of an issue with roses. Aphids are the main concern in Oregon and are fairly easy to control, according to Gail Langellotto, entomologist and professor in the OSU College of Agricultural Sciences.
Her first step is to squish a few aphids with her fingers to release a chemical signal that attracts natural enemies such as lacewings, ladybird beetles and parasitoid wasps. Follow with a strong spray of water from a hose to wash remaining aphids to the ground.
“Aphids are poor climbers,” she said, “and are less likely to reestablish because they run a high risk of getting eaten by ground-roving predators, such as spiders and beetles.”
Since aphid numbers can explode on nitrogen-rich plants, use an organic fertilizer or a slow-release synthetic fertilizer that provides less readily available nitrogen.
Choose resistant roses and prevent problems
As for diseases, the best strategy is to start with disease-resistant cultivars.
“Buy ‘fisherman roses,’” Pscheidt said. “The types that you can plant and then go fishing. Breeders are always coming up with new plants that resist pests. Local retail nurseries will generally have a good list for you.”
Older varieties can also resist disease. OSU Extension’s Pacific Northwest plant disease control handbook lists classics that fight black spot, rust and powdery mildew. Longtime favorite hybrid teas ‘Just Joey,’ ‘Chicago Peace’ and ‘Mr. Lincoln’ are included. ‘Gold Medal’ and ‘Queen Elizabeth’ grandifloras made the cut, as did well-known floribundas ‘Playboy,’ ‘Sexy Rexy’ and ‘Iceberg.’
Early-season care: simple steps that help
- Don’t crowd plants. Space roses so sun penetrates and air circulates to dry foliage quickly after rain or irrigation.
- Clean up thoroughly. When pruning in late winter, remove fallen leaves and diseased stems.
- Water wisely. Use soaker hoses when possible. If using overhead sprinklers, irrigate so foliage dries before evening.
- If desired, spray early. Use an organic or chemical fungicide or pesticide early in the season. Always follow label instructions.
Previously titled Fight ‘big four’ rose problems with prevention strategies
