Blackspot, “Diplocarpon rosae”, is a nasty fungus that manifests itself on rose bushes as black spots on leaves progressing to black spots fringed with yellow rings on both sides of the leaves. As they develop the spots enlarge. Eventually, as the disease spreads, the entire leaves will go from green to yellow and then drop to the ground. With time the entire rose bush may become defoliated. Leaves less than two weeks old are the most susceptible to this disease.
Defoliation brought on by blackspot is worst during wet weather, especially humid weather. The fungus becomes active in a wet environment with a temperature of about 24 degrees Celsius (approximately 75 degrees Fahrenheit). It needs about 7 hours of these conditions to germinate and then symptoms will begin to appear on rose foliage within three to ten days. From then on spores are produced every three weeks. If unchecked, blackspot can affect the entire rose garden leaving an unattractive appearance of many ‘bare-naked’ plants. Spores can over winter in the garden so autumn cleanup is crucial otherwise the entire cycle can repeat itself the following spring and summer.
The worst case scenario can be avoided with some preventative measures, a keen eye and diligence. While plants are dormant in spring, spray thoroughly with fungicidal soap and wettable sulphur (both readily available at the local plant nursery). Sulphur is actually a historical remedy used or hundreds of years by farmers for their crops. It definitely has a place in the chemical-free garden. Fungal spores cannot germinate in the sulphur film and thus cannot get a chance to attack the plant. To be effective the sulphur must be on the plant and leaves before the spores land on them. Sulphur washes off in rain and so must be reapplied repeatedly. The product is sold in powder or liquid form and also works well against mildew and rust. Other preventative measures include keeping the leaves dry when watering (try soaker hoses or drip irrigation methods), water in the morning so that foliage has a chance to dry off throughout the day and pruning plants to improve air circulation. Also spacing the plants well when planting will insure good air circulation. It is recommended that hybrid teas and smaller rose bushes be spaced 3 feet apart and larger rose bushes be spaced 4 feet apart from one another.
Once you have discovered that your rose bushes are infected it is best to prune off the damaged parts of the plant and gather the diseased foliage. Dispose of this diseased material in bags or burn it. Do not add to the composter, as the fungus shall only return to haunt you when you recycle the soil back into the garden. It is vital to do an end of season cleanup so the spores will have no where to hide over winter.
After having removed the diseased parts from your rose bushes it is necessary to apply a preventative formula to minimize further attack. Using fungicidal soap or sulphur several times over the course of summer is one solution (especially after rain as these products tend to wash off). There are a few home remedies that have met with some success and are worth trying, especially for those that really do prefer organic garden methods. One is a solution made with baking soda: dissolve 1 teaspoon baking soda in a quart of water, add a few drops of liquid soap to the mix to help it cling better to the foliage, spray infected plants thoroughly. Another unusual remedy for fighting fungal diseases is manure tea. This formulation fights blackspot, as well as mildew and rust, while providing foliar nutrition. Place one gallon of well-composted manure in a 5-gallon bucket and fill with water. Stir the mixture well and let sit in a warm place for three days. Strain the mixture through a cheesecloth or mesh and use the resulting tea to spray disease affected plants (the solids left behind can be applied around the base of the plants as added fertilizer).