Abstract
The conventional definition of endophytes is that they do not cause disease, whereas pathogens do. Complicating this convention, however, is the poorly explored phenomenon that some microbes are endophytes in some plants but pathogens in others. Black cottonwood or poplar (Populus trichocarpa) and wheat (Triticum aestivum) are common wild and crop plants, respectively, in the Pacific Northwest USA. The former anchors wild, riparian communities, whereas the latter is an introduced domesticate of commercial importance in the region. We isolated Fusarium culmorum – a well-known pathogen of wheat causing both blight and rot – from the leaf of a black cottonwood tree in western Washington. The pathogenicity of this cottonwood isolate and of a wheat isolate of F. culmorum were compared by inoculating both cottonwood and wheat in a greenhouse experiment. We found that both the cottonwood and wheat isolates of F. culmorum significantly reduced the growth of wheat, whereas they had no impact on cottonwood growth. Our results demonstrate that the cottonwood isolate of F. culmorum is endophytic in one plant species but pathogenic in another. Using sequence-based methods, we found an additional 56 taxa in the foliar microbiome of cottonwood that matched the sequences of pathogens of other plants of the region. These sequence-based findings suggest, though they do not prove, that P. trichocarpa may host many additional pathogens of other plants.
Highlights
The plant microbiome is thought to aid plants under stressful conditions to enable them to adapt to new habitats
We summarize the pathogens of other plants that were found in the sequence-based study (Barge et al, 2019), and report on an experiment testing the pathogenicity of Fusarium culmorum isolated from P. trichocarpa on both wheat and cottonwood
An isolate of F. culmorum collected from asymptomatic leaves of P. trichocarpa in a previous study (SNO-11, Busby et al, 2016) produced a colony resembling a wheat isolate of Fusarium culmorum, a known pathogen of wheat
Summary
The plant microbiome is thought to aid plants under stressful conditions to enable them to adapt to new habitats. Further examples from other woody plants have been summarized in a study of white pine endophytes that often resembled pathogens of other plants (Ganley et al, 2004) In all of these studies, pathogenic function was suggested by the identification of isolates and/or sequences representative of pathogenic taxa of other plants; function was not, proven via inoculation assay in these studies. P. trichocarpa is the wild black cottonwood tree of riparian communities in the Pacific Northwest It is frequently found in close proximity to wheat and other crops of the region. We summarize the pathogens of other plants that were found in the sequence-based study (Barge et al, 2019), and report on an experiment testing the pathogenicity of Fusarium culmorum isolated from P. trichocarpa on both wheat and cottonwood. We expected that our assay with a cottonwood isolate could reveal positive, negative, or neutral effects on growth of cottonwood and wheat
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