Black spot and the rose rosette virus are two problems that plague roses in Missouri, especially in periods of wet weather, a University of Missouri horticulturist said.
This website brought to you in part by the following sponsor:
“Rose rosette virus, also known as Witch’s Broom, is a rapidly emerging problem on roses in Missouri,” said Mary Kroening, MU extension horticulturist. “It has been a serious problem on wild multiflora roses in pastures, woods, and field edges, but in the past few years we have seen a rapid emergence on domestic roses.”
Early symptoms of this disease include rapid stem elongation followed by some branches developing thickened, abnormally thorny stems, she said. Many short, deformed shoots will display a red pigmentation and have small, misshapen leaves, and the plant usually dies within a year or two.
“The exact cause of this disease is unknown, but the disease is moved from infected plants to healthy plants by insects and mites,” Kroening said. “At this point, there is no effective control for this disease. Infected plants should be removed and destroyed.”
Black spot, the most destructive disease on roses, is a fungal disease that is the most severe during wet and humid weather, she said. “A wet leaf surface is essential for the spread of the disease.” The disease appears as black spots or rings on the leaf surface, often surrounded by a yellowish halo. The affected leaves then turn completely yellow and drop from the plant.
“If severe, this defoliation will weaken the plant, reduce flowering, and make the plant more susceptible to winter damage,” Kroening said. “Black spot is an annual problem on roses and can be difficult to control as sprays are mostly preventative. The chemical must be on the leaves before the disease gets started.”
Once black spot spores germinate on the leaf, the infection cannot be reversed. Preventing the onset of black spot requires a regular spraying regimen, she said. “It can be quite a chore. Many rose gardeners just give up.”
Hybrid tea roses are most susceptible to black spot, but rose breeders in recent years have directed their efforts toward black-spot resistant roses, and many are now available. Modern shrub roses can tolerate black spot and still bloom, she said, and some hybrid tea roses have improved resistance.
“Keeping roses healthy will help them resist black spot,” Kroening said. “It is important to provide roses with at least six hours of sun daily and space them far enough apart to provide good air circulation, which enables leaves to dry and not have moisture sitting on the leaf surface.”
If possible, she said, avoid wetting the leaves when watering, remove and destroy any diseased and fallen leaves and pick off infected leaves. “The fungus is carried over-winter by dead leaves on the ground and infections on the stem. Rain and splashing water can spread the spores from old leaves or stems.”
For more information, refer to MU Guide G6601 – “Roses: Care After Planting,” at http://muextension.missouri.edu/xplor/agguides/hort/g06601.htm.