It makes you want to cry when you see your prime roses covered with black spots. What on earth is happening to your prize roses you ask. Black spot disease is yet another evil villain of our beloved flowers and another common fungal disease. And roses are its biggest victim.
The disease typically begins as black spots on the foliage. They are most prevalent on upper leaf surfaces, and may be up to ½″ across. The leaves will eventually turn yellow around the spots, then become all yellow and fall off. The spots may also appear on rose canes, appearing first purple and then turning black.
Again, the culprit for this disease is too wet of conditions for too long of a time frame. It only takes seven hours for infection to set in, so when watering your roses be sure to water early in the morning to give ample sun time for drying and avoid watering on known cloudy days. Also, make sure to water from the ground avoiding all foliage and flowers. Use proper spacing so your roses can get good circulation. These things will help to minimize the disease and or prevent it totally.
Black spot fungus overwinters in fallen leaves and infected canes. Prune out infections and rake the fallen leaves at the end of the season.
Rose varieties vary greatly in their resistance to black spot, so choose resistant ones. If this information is not indicated on plant labels, you can research the variety online or check with experts at your local nursery. Several of the shrub roses show resistance to black spot—and powdery mildew. Many of the other shrub roses and old-fashioned roses are resistant as well.
To help control black spot disease, dust roses infected with black spot with sulfur powder. Sulfur will not kill the fungus spores, but it will prevent a new generation from germinating. You can also use a spray solution of 1 tsp. baking soda mixed in 1 qt. warm water in the early morning hours on your roses foliage. In addition, when your plants are in the dormant season, spray your roses with a dormant oil or Bordeaux mixture.