Septoria malagutii

Septoria malagutii is a fungal plant pathogen infecting potatoes. The casual fungal pathogen is a deuteromycete and therefore has no true sexual stage. As a result, Septoria produces pycnidia, an asexual flask shaped fruiting body, on the leaves of potato and other tuber-bearing spp. causing small black to brown necrotic lesions ranging in size from 1-5mm. The necrotic lesions can fuse together forming large necrotic areas susceptible to leaf drop, early senescence, dieback, and dwarfing. Septoria malagutii has been found only in the Andean countries of Bolivia, Ecuador, Peru, and Venezuela at altitudes of near 3000 meters. Consequently, the fungi grows and disperses best under relatively low temperatures with high humidities, with optimal growth occurring at 20 C (68 F). The disease has caused devastation on potato yields in South America and in areas where this disease is common, potato yields have been seen to drop by 60%.

from Wikipedia