Examination of samples received by the RHS has shown that two different fungi may be associated with the leaf spots (although they are never found together in the same spot). The first, and by far the most common, is a Septoria species. This produces the tiny black fruiting bodies described above – under wet conditions colourless tendrils of spores ‘ooze’ from these fruiting bodies. Occasionally, a Cercospora species can be found instead – this produces ‘tufts’ of spores rather than black fruiting bodies.
Many fungi produce asexual and sexual types of spore at different stages of their life-cycle. Unfortunately, the fungus is often given a completely different scientific name depending on what type of spore it is producing, which can be very confusing! Both Septoria and Cercospora species produce asexual spores. In each case, if the fungus were to produce sexual spores it would be known as a Mycosphaerella species. Although these sexual spores have yet to be found on affected leaves, the disease is therefore usually referred to as Mycosphaerella leaf spot.
As this is a relatively new problem the precise conditions for spread of the disease are unknown, but it is certainly favoured by wet weather conditions. Spores are likely to be splashed around by rain droplets, germinating to set up new infections if the leaf surface stays wet for an extended period. The fungus probably persists on fallen leaf debris.