Black spot is our most prevalent, troublesome, rose disease we experience in the humid Southeast. It is a fungal disease that is best prevented, because once your entire rose bush is covered with the little black spots it is harder to control. It also causes your bushes to drop most of their leaves which weakens the health of your beautiful roses. Go to www.rose.org and click on the Rose Education link, then the Diseases link. Click on this document and you will see excellent information on what this disease looks like and other important rose disease problems.
There are no proven organic controls in our region, we have the perfect environment for this leaf spot disease to spread, especially on hybrid teas and floribunda roses. The fungal spores need 7 hours of wet leaves with temperatures in the mid 60’s to upper 80’s which allow it to spread quickly, within 7 days, if no fungicides are applied. To help prevent this disease from spreading even faster, please try to plant your roses where they will receive morning sun and in a location that has good air circulation. The faster your rose leaves dry off the less susceptible they will be to black spot (Diplocarpon rosea).
So what should you do about it? Spraying with fungicides is the best way to control fungal leaf spot diseases and powdery mildew too. I’m going to recommend some fungicides by their active ingredients, the brand names are unimportant. Therefore, go to your favorite garden center with the chemical names and you should be able to find a product with the active ingredient. To prevent new black spot infections use a systemic fungicide that will last within the rose leaves from 7 to 14 days, for example, propiconazole, myclobutanil or tebuconazole; and then add to the same spray tank a contact fungicide, these type of fungicides only lasts a few days on your rose leaves but are very effective in killing the fungal spores, for example, chlorothalonil (for cool weather application only, it will damage rose leaves in the heat), or mancozeb. Always read the label directions on the product you purchase, more of a chemical is not better. Also please be sure to wear nitrile type gloves to protect your hands from any concentrated chemicals.
Spraying with both systemic and contact fungicides will help you get control over black spot. If your rose leaves are totally infected with black spot then you should spray mancozeb three times, three days apart. For example, spray Saturday then spray again on Tuesday, then again on Friday. Then, within the next few days, spray with your systemic fungicide. Continue with a spraying schedule, i.e., spray every 14 days with your systemic and contact fungicide.
If you would like to know more about specific brands and where to purchase them, please contact SW at email@example.com. The Greenville Rose Society has many ARS Consulting Rosarians in your area who will be happy to visit your garden and answer your questions.