Cooler temperatures, moisture and weakened plant tissue create conditions that invite Botrytis blight. Roses under stress are highly susceptible to this disease. Black spot disease in Colorado is more common on some of the old-fashioned rose varieties. Hybrid teas often don't show symptoms in this climate. Black spot is favored by wet weather and 65 to 75 degree temperatures.
Severe infections cause defoliation.
Coniothyrium canker is not a common problem in this area. However, it can be found in greenhouses. Occurrences were reported on outdoor roses in the spring of 1991. The fungal cankers eventually can girdle the plant. Crown gall is caused by a bacterium that survives in the soil.