Septoria musiva (sexual stage: Mycosphaerella populorum) causes leaf spots and cankers on poplars (Populus spp. and hybrids). On native North American poplars the pathogen mainly causes leaf spots that can lead to defoliation but generally do not kill the host. But S. musiva can also cause cankers on branches and primary stems. These can be lethal and are particularly severe on hybrid poplars in plantations. They often develop on the primary shoots of 2- to 3-year-old trees, leading to restrictions in the movement of water and nutrients and weakening the wood within a few feet of ground level. The weakened trunks collapse easily, greatly reducing the production of biomass. Cankers caused by S. musiva can greatly hamper the production of hybrid poplars in the eastern United States and Canada and threaten poplars in western North America.
A major concern with S. musiva is with migration to new areas. The pathogen is endemic and appears to have originated on poplars in eastern North America, where it occurs commonly on leaves of the eastern cottonwood, P. deltoides. During the past 20 years S. musiva has appeared in South America and western Canada, where it is spreading rapidly on native and hybrid poplars causing economic damage as well as threatening native poplars in important riparian zones. It is not yet known in Europe or Asia but has the potential to cause extensive damage if introduced to those areas. Global warming and trade may facilitate the spread of the disease by making northern popular-growing areas more favorable to growth of the fungus.
Availability of a genome sequence for S. musiva will help with designing strategies to effectively manage this destructive disease. The genome of the black cottonwood poplar P. trichocarpa, another host for S. musiva, has been sequenced and provides a rare opportunity to analyze host-pathogen interactions when both host and pathogen have been sequenced. Opportunities for comparative genomics also are available. Other members of the genus Mycosphaerella with sequenced genomes include the pine pathogen Dothistroma septosporum (aka Mycosphaerella pini), the banana pathogen M. fijiensis and the wheat pathogen M. graminicola. Comparative genomics of these sequences with that of S. musiva will help identify the differences involved in pathogenicity to woody vs. herbaceous hosts and could help in our understanding of why S. musiva causes cankers on hybrids but not native poplars. Understanding the mechanisms of fungal pathogenicity and of resistance in the host will be essential for protecting our forests and ensuring a stable supply of renewable biomass as the climate warms in the future.