The invasive plant Solidago canadensis L. suppresses local soil pathogens through allelopathy

Recent studies suggest that invasive plants pose a significant effect on local soil pathogens, which in turn affects on the plant invasion. However, the mechanisms by which invasive plants affect soil pathogens were less well known. We conducted four experiments to test the hypothesis that the invasive plant species Solidago canadensis L. may affect soilborne pathogens through exudation of allelochemicals. Two common soilborne pathogens Pythium ultimum and Rhizoctonia solani were used in the study. Tomato (Lycopersicon esculentum Mill) variety Qianhong No.1 which is sensitive to soil pathogens P. ultimum and R. solani was used to indicate pathogenic activity (in terms of seedling mortality and damping-off). Extracts from root and rhizome of S. canadensis significantly suppressed the growth and pathogenic activity of both pathogens under Petri dish culture and sand culture (experiments 1 and 2), providing direct evidence that S. canadensis exerts allelopathic effects on these pathogens. Subsequently, a pathogen inoculation experiment under sand culture showed that pathogenic activity of both P. ultimum and R. solani was lower under the soil with S. canadensis compared to that under the soil with a common native plant Kummerowia striata (Thunb.) Schindl (experiment 3), implying that invasive S. canadensis had but native K. striata did not have allelopathic effects on soil pathogens through root and rhizome exudation. Finally, results from field soil tests showed that mortality and damping-off rate of tomato seedlings were significantly lower under the soils collected from the fields dominated by S. canadensis than that dominated by native plants at both sampling sites, suggesting that suppression of pathogens also occurs in the field. From the present experimental results we suggest that invasive S. canadensis may acquire spreading advantage in non-native habitat by using “novel weapons” to inhibit not only local plants but also soilborne pathogens.