Black Spot is a bothersome problem for many rose gardeners. Understanding how to recognize it and what causes it will help reduce your chances of getting Black Spot. If your roses do get it, there are both chemical and organic treatments.
According to the Virginia Cooperative Extension, Black Spot is caused by the fungus Diplocarpon rosae. Without treatment, it can cause rose bushes to lose all their leaves.
The spores of this fungus thrive in warm, humid conditions. Another condition for Black Spot arises when the rose is sprayed during watering and the plant's leaves do not dry out before nighttime.
As its name suggests, this fungus leaves circular black spots on the rose's leaves. The leaf will turn yellow and die. Reddish-purple spots can appear on the stems and canes of the rose bush.
If Black Spot is left untreated, it can cause the rose bush to lose all its leaves and can continue to weaken the stems and canes of the bush; eventually it will die.
Fallen rose leaves and petals provide an ideal aspect for the fungus to grow and live, even during the winter. Although it cannot be completely prevented, good sanitation will reduce chances of the fungus infecting a rose. Clean up all rose debris on the ground underneath a rose bush; discard or destroy this debris, and do not add to a compost. Avoid getting the rose's leaves wet when watering by watering the base of the bush or watering early in the morning.
Remove leaves and canes infected with Black Spot. Use fungicide designed for Black Spot to treat the rose and as a preventative measure. Bayer Advanced All in One Rose and Flower can treat and help prevent Black Spot from occurring on roses.