Black Spot fungus ( Diplocarpon rosae )
Diplocarpon rosae is a black spot fungus disease which occurs on rose. This disease active in spring during moist and humidity and its infection causes defoliation and black spots on leaves, canes and stems.
It spreads very quickly. This disease is contaminated. Water and wind help this disease to spread. Also, if the infected parts of the plant are not removed and the fallen dead leaves also not collected from the ground it will spread to other leaves and infect other nearby plants.
Therefore, this disease must be treated as soon as spotted. The treatment will not cure the infected parts but will slow the spread of infection to other plants.