Controlling Rose Diseases

Blackspot on roses. Photo by Mary Kroening.

Rose rosette virus is a rapidly emerging problem on roses in mid-Missouri. It has been a serious problem on multiflora (wild) roses in pastures, woods and field edges. In past years we have seen a rapid emergence on domestic roses. It is often referred to as “Witches Broom” disease, as the disease causes plant growth that resembles the appearance of a witches broom. Early symptoms of this disease are rapid stem elongation, followed by certain branches of the plant developing thickened, abnormally thorny stems. Then many short, deformed shoots will form that often display a red pigmentation and will have reduced, misshapen leaves. Usually plants die within a year or two as the disease spreads throughout the plant. The exact cause of this disease is unknown, but the disease is moved from infected plants to healthy plants by insects and mites. 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 as a wet leaf surface is essential for spread of the disease. Black spot appears as black spots or rings on the leaf surface and many times the black spots have a yellowish halo around them. 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. 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. To achieve this protection takes diligence with weekly to bi-weekly spraying starting

Rose wet bag. Photo by Mary Kroening.

soon after the new leaves appear, and can be quite a chore that has led many rose gardeners to give up. Hybrid tea roses are the most susceptible to black spot. In recent years, rose breeders have directed their efforts toward black spot resistant roses and there are now many available. Modern shrub roses can tolerate black spot and still bloom well despite it. There are now hybrid tea roses that have improved black spot resistance. Keeping roses healthy will help them resist black spot. It is important to provide roses with at least 6 hours of sun daily and space them far enough apart to provide good air circulation to enable leaves to dry and not have moisture sitting on the leaf surface. If possible, it is good to avoid wetting the leaves when watering. It is also important to remove and destroy any diseased and fallen leaves, and pick off infected leaves as soon as you see black spots developing. The fungus is carried over-winter by dead leaves on the ground and infections on the stem. Rain and splashing water then spread the spores from old leaves or stems.

For more information, refer to MU Guide G6601 -- ‘Roses: Care After Planting’ ( http://muextension.missouri.edu/xplor/agguides/hort/g06601.htm), or the following Website on rose diseases, http://www.oznet.ksu.edu/dp_hfrr/extensn/problems/roserost.htm

Mary Kroening
573-882-9633