Experts and BHG readers answer.
Leaves with brown spots on roses
Well it's hard to tell if the Off caused this problem or if it's a natural problem called Black Spot disease. If the spots have dark centers with a touch of yellow around them, it's likely these two plants have Black Spot. This is a fungal disease that's a common problem, especially for roses such as hybrid teas and floribundas during humid periods. Or, if the spots are small and reddish it could be Spot Anthracnose which is also a fungal disease. To prevent both of these problems you should space your plants out to allow good air circulation around them, not water the plants from above which gets the foliage wet (water from below only), and to plant disease-resistant varieties (read the plant tag before you buy to see if the variety you are buying is labeled as disease-resistant). Once your plants show signs of either of these diseases you'll need to use a commercial fungicide spray every 7 to 10 days to keep the disease from spreading. You can buy rose fungicides at most garden centers. Plants can contaminate each other by water droplets from rainstorms or irrigation spreading the fungus. In the end, if you have either of these problems, you might want to consider removing the plants (rake up all fallen leaves) and replanting with disease resistant types. Most shrub or landscape roses are very resistant to this problem."