Black spot is the most serious disease on roses in Florida. Various genotypes of M. rosae, i.e isolates that infect a specific cultivar or group of cultivars, are currently present.
The black spot symptoms start as small black spots on the upper surface of the leaves. Lesions can vary in size on the leaf surfaces.
These spots may have unique feathery borders. The leaves subsequently turns yellow around the black spot lesions finally leading to severe defoliation.
The defoliation usually starts on the lower parts of stems and gradually moves higher. Spots can also be found on peduncles, fruits, and sepals if the infection is severe.
Black acervuli can be seen on lesion.
Mycelial growth from acervuli. Black spot is promoted by warm, wet weather and is common in summer.
The fungal spores of the pathogen are easily spread by splashing rain or overhead irrigation water. The fungal conidia must be wet for several hours to infect plant tissues.