Some roses need treatment for Blackspot, some don't. Climate and culture are also factors. First let's start with roses: Many hybrid teas and floribundas are succeptsble to Blackspot. Both new and older cultivars. Kordes has made some inroads recently in creating showy, and sometimes fragrant hybrid teas that are Blackspot resistant. Many old garden roses, particularly many (not all) Teas, China's, Hybrid Musks, Polyanthas, Rugosas and ramblers offer substantial resistance. Also some shrubs, such as Belinda's Dream and Knockout offer resistance. So if you decide you love a rose that is otherwise great but succeptable to BS, here are some factors to consider:
1. Blackspot occurs in wet humid climates more often than dry hot ones.
2. Keep your leaves dry. Plant your roses in areas that have good air circulation. Don't plant them too close together. Use only drip irrigation so leaves don't stay wet. Mulch so that spores don't spray from bare soil onto your plants. Pick up and remove diseased leaves.
3. Keep your roses healthy and well watered and fed. Don't skimp. Keep the pH correct. A weak rose is more disease prone.
4. Several fungicides exist to prevent Blackspot. Nothing is a perfect control. In low pressure areas Neem Oil might be sufficient. In high pressure areas such as the Gulf South (where I live) I have to spray to grow roses I love (such as Peace, Tiffany and Papa Meilland). I don't spray my Old Garden roses or shrubs, only my HTs and Floribundas. I prevent. Prevention is fairly easy and not intensive. After pruning in February I use a preventative called Lime-Sulfur. If I have canker I follow with copper 2 weeks later. In practice I try to spray (HTs only) twice a month from late March to Late October. I use a tank mix of chlorothalonil / propaconizole in cooler months and a tank mix of Mancozeb / propaconizole in warmer months. If I don't see disease in the summer I stretch the time between applications as much as possible. I rotate FRAC groups (modes of action) to keep the disease from developing resistance. If spraying any fungicides, organic or synthetic, use the protective clothing as stated on the label. Sorry if this is too technical but I have a minor in Chemestry and a Masters Degree in Horticulture .
