Symptoms

Blackspot is a plant disease caused by a fungus (Diplocarpon rosae) that is generally damaging and usually a source of major problems. Blackspot looks like circular black spots with irregular edges on the top side of the leaves. The tissue around the spots or the entire leaf may turn yellow and the infected leaf may drop off. Plants with a severe case may lose all of their leaves if not treated. Flower production is often at a minimum and the quality of bloom suffers badly.

Biology

High humidity is one factor that helps the spores to germinate. The spores germinate in 9-18 days on a moist leaf at 70-80°F temperatures. The spores can be spread by splashing water and by the Rosarians themselves. The spores are wind-borne only in water drops. The spores can be spread on clothing, tools or even your hands, but the way it is spread most often is by infected leaves that have wintered over in the rose bed.

Control

Blackspot can be satisfactorily controlled by spraying with a good fungicide every seven to ten days (read the label and follow the directions). There are also a number of measures that should be taken to keep from getting and/or controlling the disease. Avoid watering in a way that splashes water up on the leaves and avoid watering late in the evening with a hose or sprayer. Make sure to clean up the beds completely of all leaves or stems to help keep the disease from wintering over. Always have good ventilation through the plant and good soil drainage. Apply fungicides after a rain to keep down spores. Put the plants on a spray schedule and spray with a fungicide that gives good control, such as, Manzate®, Maneb®, Daconil® and Lime-Sulfur compounds.

There are also organic methods of controlling Blackspot. Baking soda has been tried as a cure and as a preventative measure. It was found that using baking soda and spray oil mixed with water as a spray can damage roses if it is not mixed in the proper proportions. It was also found that baking soda gave only moderate control of Blackspot, but appeared to be effective as a preventative. There is a new product coming on the market that has been used by our local Rose Society that does show promise. This product is derived from the Neem tree. It is called "Rose Defense" by The Green Light Co.

One other way to prevent Blackspot is to plant roses that are disease resistant. There are some roses that have some resistance built into their genes. But remember, they are Resistant not Immune. They still need to be sprayed on a regular schedule.

Roses should be kept on a regular spray schedule regardless of which method is used. Remember, prevention is the key to controlling Blackspot.