Roses have been cultivated for 5,000 years for their beauty and fragrance. They are also attractive to diseases, especially the fungus blackspot. Spores that land in wet spots on the leaves germinate and penetrate the plant tissue in three to four hours. Infected leaves fall, weakening the plant.
The best treatment is prevention, say rose experts at the Nova Scotia Department of Agriculture and Fisheries. Failing that, gardeners must recognize and treat the disease quickly.
Identifiy the Disease
Blackspot, Diplocarpon rosae, is a fungus spread by spores that float from infected foliage, are carried by insects, on garden tools, even transmitted from one plant to another on gardeners' fingers.
Check new rose leaves for circular black spots with fringed edges. They appear on bottom leaves first, then upper ones. The black spots grow and become surrounded by yellow tissue, which is unique to the blackspot fungus. Infected canes show reddish purple spots. Infected leaves turn yellow and drop off, according to the University of Illinois.
Blackspot becomes active in a wet environment at a temperature around 24 degrees C or 75 degrees F, says "Rose Magazine." Characteristic spots appear within three to 10 days; new spores are produced every three weeks.
Blackspot infection can only occur when leaves are wet, according to the Nova Scotia Department of Agriculture and Fisheries. Spores in wet spots can germinate and infect a rose within hours. Blackspot is widespread in damp coastal areas west of the Rocky Mountains. To keep leaves dry, use drip or soaker hoses and water in the mornings to allow the sun to dry any splashes.
Plant Resistant Plants
Choose roses that are resistant to blackspot. Select cultivars that have genotypic resistance, meaning they are resistant anywhere. Phenotypic plants are resistant depending on climate, location or horticultural practices.
If you love a non-resistant rose for its color or fragrance, plant it in a separate location to prevent cross contamination.
Plan Healthy Beds
Rose beds should have well-drained soil. Roses need lots of room to allow for good air circulation that keeps leaves dry. Space roses by planting hybrid teas 3 feet apart and large bushes 4 feet from each other.
Prune roses in the middle so there is room around the trunk for air to circulate.
To keep roses free of blackspot, spray them in the spring when plants are dormant. Fungicidal soap or wettable sulphur create a hostile pH so spores can't germinate. The spray must be on the leaves before the spores become active. Reapply it after every rain or heavy dew or fog. Garden centers carry it in liquid or powder form.
For infected plants, "Rose Magazine" recommends a spray made by dissolving 1 teaspoon baking soda in a quart of water. Add a few drops of liquid soap to help it cling to the foliage and spray infected plants thoroughly, repeating after rains.