Diplocarpon rosae

Diplocarpon rosae causes the rose disease Black Spot. The brownish-black spots on the upper surface of the leaves are more or less circular, with a ragged edge, sometimes coalescing into a larger affected area.  Radiating strands of hyphae may occasionaly be seen in the spot.  The leaf often turns yellow and drops early.   Acervuli on the surface of the spot produce masses of one- or two-celled conidia which are spread in water to form new infections - hence the disease thrives in warm, wet weather.  Lesions can also appear on the stems, and the fungus overwinters on fallen leaves and in stems and dormant buds.

host

Photograph the fungus on the host plant and in close-up; note host species/genus in your comments

on Roses, chiefly cultivated varieties and hybrids

The fungus was almost unknown in the UK before the Clean Air Act of 1956.  The Act served to improve air quality and reduced the amount of sulphur dioxide pollution in the air.  As a result of the reduction in this fungicide, black spot disease increased rapidly to become a serious horticultural problem.