Symptoms: Black spot appears as circular spots up to 5 mm in diameter. The spots have fringed margins, a diagnostic characteristic that distinguishes this disease from other leaf spots on roses. Under favorable disease conditions, the spots enlarge, then coalesce, forming large, irregularly shaped spots. In susceptible rose varieties, it is common for the foliage to turn yellow and defoliation to occur. Excessive defoliation during the season weakens the plant, increasing its susceptibility to winter kill. Cane lesions are small, pin-head sized purple to reddish spots that are slightly raised with definite margins.
Conditions Favoring Disease: Iris leaf spot thrives in rainy conditions in cool to warm temperatures.
How Pathogen Survives/Disperses: Black spot is a serious problem in most areas where roses are grown. The fungus overwinters as mycelium in diseased leaf litter.
Photo and write up provided by Syngenta Professional Products