Why Are There Black Spots on My Roses?
I recently planted two rose gardens in my yard. One of the gardens has six bushes and is producing many blooms. But they aren't blooming in the other garden, which contains five bushes. I have a Rio Samba bush that gave me one beautiful bloom, but now the leaves have brown spots all over them and so does the bush right next to it. My brother accidentally sprayed mosquito insect repellent on them. Could that be what caused the brown spots, and what can I do? I know that the bushes were fine before the repellent got on them.
It's hard to tell if the repellent 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. 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 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.