Host effect on genetic variation of Marssonina brunnea pathogenic to poplars
A broad collection was made for 42 isolates of Marssonina brunnea affecting poplar trees from three different sections (Leuce, Aigeiros and Tacamahaca) within the same Populus genus in China. Genetic diversity among these isolates was analyzed for morphological traits, cultural features, pathogenicity, hyphal anastomosis and randomly amplified polymorphic DNA markers (RAPDs). No significant difference was found in conidial morphological features, such as size, shape and septum location. Yet, considerable differences occur in other characteristics, which leads to the classification of the 42 isolates into two distinct groups, M. brunnea f.sp. monogermtubi and M. brunnea f.sp. multigermtubi. Isolates of M. brunnea f.sp. monogermtubi, derived from section Leuce, germinate only one germ tube, grow fast, produce dark-reddish conidiosorus clusters on the PDA medium, and are highly pathogenic to Populus tomentosa of section Leuce. By contrast, isolates of M. brunnea f.sp. multigermtubi, derived from sections Aigeiros and Tacamahaca, germinate 1–5 germ tubes, grow slowly, produce yellow-greenish conidiosorus clusters on PDA medium, and are pathogenic to Populus ×euramericana cv I-45 and Populus canadensis of section Aigeiros. DNA amplification using 11 RAPD primers generate 78 polymorphic bands among isolates. Cluster analyses based on RAPD markers broadly support such a classification by phenotypes, but provide a new insight into the possible origins of M. brunnea. It is proposed that the pathogen co-evolves with the poplars of section Leuce and has been subsequently distributed to the poplars of sections Aigeiros and Tacamahaca. An isolate from Populus adenopoda of section Leuce is placed in the third group, which is most likely a transmission type from M. brunnea f.sp. monogermtubi to M. brunnea f.sp. multigermtubi.