Botrytis blight is a disease caused by the fungus Botrytis cinerea. Common in all parts of the world, this fungus generally infects young flower buds, causing the failure of them to open as they normally would. The buds tend to droop and sit on the bush, often becoming covered with a fuzzy gray mold. Infected petals may show brownish or pinkish spots on them. It is more noticeable on the lighter colored flowers but affects all roses. Optimal conditions are high humidity & moisture; the spores are transferred by wind. The best way to deal with this fungus is to remove offending buds early. Photo courtesy of Bob Anderson, Severna Park, M.D.
Also called leaf spot (and a few other names), Blackspot is another fungal disease that affects roses all over the world. As it’s name suggests, it can be identified as black spots on the leaves. Leaves eventually turn yellow, then brown and die. Where blackspot is severe, plants may become completely defoliated. Growth and distribution of the fungus are dependant upon water—the spores need water to germinate and it is via splashing water that blackspot easily spreads from plant to plant. In humid conditions, wet leaves don’t dry as easily, thereby promoting the spread of the disease. Prevention of blackspot includes a clean, well-aerated garden which will help keep leaves dry. Also, avoid overhead watering. Watering plants early in the morning will give any wet leaves a chance to dry quickly. Control of this disease includes removing infected leaves and cleaning up old leaves under the bushes. Fungicides such as Immunox and Funginex may be used. As always, refer to product labels for direction when using chemical products. Photo provided by S.Muragin
Canker, also known as dieback, is a fungal disease that causes canes to turn dark brown or black. This dieback often progresses down the infected cane, leading to the death of the cane. The fungus enters the cane through pruning cuts or wounds on stems. To help minimize dieback, angular pruning cuts should be made about ¼” above a healthy leaf node. Cuts form calluses and heal faster when made in this fashion. Treat infected canes by cutting them back to good wood (green bark outside, white pith inside) or when necessary, remove cane entirely. Photo provided by S.Muragin
Crown gall is an ailment caused by a soil-borne bacteria. It affects roses worldwide. On infected plants, round or irregular growths (galls) form at the crown, bud union or at the roots. Plants afflicted with this disease are usually stunted in growth, have weak foliage and fewer blooms. Infected plants may survive in the garden for several years but will harbor the bacteria the whole while. It is therefore recommended that the plant be removed, along with the surrounding soil. Photo courtesy of Myrna Cariaga.
A white, powdery growth covering young leaves and buds is characteristic of powdery mildew, another fungal disease of roses. New growth can often be stunted and distorted when infected. Older rose leaves are less likely to be affected. The disease is fostered by warm humid days followed by cool damp nights. The spores of this fungus are spread around the garden through air movement. Cleaning up old leaves is a good first line of defense against powdery mildew. Full sun and good air circulation help to minimize disease as well. Various fungicides may be used though they work best when applied in the early stages of the disease. Photo provided by S.Muragin
Rose Mosaic Virus
Rose mosaic virus (RMV) is the most common of many viruses that can affect roses. Yellow line patterns, ring spots, or mottled spots on rose leaves are characteristic traits of RMV. These infected plants often experience a decrease in bloom production and are frequently weak in growth. RMV is transmitted by propagation; there is no cure. It is not contagious in the garden so infected specimens in Hawai’i can generally be retained as long as flower production is good. Photo courtesy of Barbara Adair, the Utah Rose Society.