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Title: Molecular and pathogenic variation within Melampsora on Salix in western North America reveals numerous cryptic species

Author
item BENNETT, CHANDALIN - University Of Idaho
item Aime, Mary
item NEWCOMBE, GEORGE - University Of Idaho

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2011
Publication Date: 10/10/2011
Citation: Bennett, C., Aime, M.C., Newcombe, G. 2011. Molecular and pathogenic variation within Melampsora on Salix in western North America reveals numerous cryptic species. Mycologia. 103(5):1004-1018.

Interpretive Summary: Plant pathogenic fungi especially those introduced into the United States cause billions of dollars damage each year to forest and crop plants. While some plants can resist diseases caused by these fungi, others are much more susceptible. Plants that have not previously been exposed to a disease-causing organism are considered “naïve”. The differing response of naïve and already exposed plants to fungal pathogens helps explain why some plants succumb to new diseases while others do not. In this research rust fungi that attack willows were used as a model system. It was determined that willow species belonging to large populations have greater resistance to disease caused by novel species of rust fungi than host species with smaller populations. Given that stands of trees are becoming increasingly fragmented, this research suggests that small populations of naïve hosts are more vulnerable than large populations to damage caused by pathogenic fungi. Endangered plant species that occur in small, isolated populations may be especially susceptible to introduced fungal diseases. This research will be used by plant pathologists and plant conservationists to predict the outcomes of naïve host/novel pathogen interactions in order to breed for resistance and preserve endangered plant species.

Technical Abstract: Novel pathogens have occasionally devastated naïve plants (e.g., chestnut blight), but genes for resistance can also condition harmless outcomes of such encounters. Both global and regional homogenization will increasingly expose plants for the first time to those ‘exotic’ or ‘emergent’ pathogens that are most likely to cause devastating epidemics: pathogens of closely related congeners of the naïve species. To predict the outcomes of specific encounters, we first need to understand the evolutionary process responsible for resistance or its absence in a ‘novel encounter series’ within a plant genus. Here we show that resistance to novel rusts prevailed among widely distributed species of Salix, or ‘large’ populations of willow, in the western U.S. On the other hand, susceptibility was common among small, ‘sky-island’ species and populations, known to be subject to genetic drift. Small populations are harbingers of much of the world’s plant diversity, since habitat conversion is reducing the population size and increasing the fragmentation of plant species generally. Epidemic outcomes of novel encounters will become more common, if dwindling populations generally lose unselected, resistance genes through drift.