Recent and extensive die-back of Phragmites australis in the Mississippi River Delta (MRD), Louisiana, USA, is associated with large populations of nonnative Roseau Cane Scale (RCS), Nipponaclerda biwakoensis. The role of scale infestation on P. australis dieback is not known; however, initial observations suggested different P. australis phenotypes displayed different susceptibilities to scale infestation and die-back. Paired stands of contiguous Delta and European phenotypes (Delta- and EU-type, respectively) were monitored for stem heights, stem densities, scale infestation, and parasitism over two years. A mesocosm experiment was conducted to assess whether RCS abundance and P. australis growth was dependent on water salinity and phenotype. Phragmites australis phenotypes were obtained from eight source populations, grown in small pools under fresh or mesohaline conditions (10-15 ppt) and RCS infestated or non-infested treatments. Scale densities were up to 7 times greater for Delta- than EU-type. Scale infestation resulted in 22%-39% reductions in the proportion of stems with green leaf tissue for all phenotypes, and 12% reduction in stem heights for Delta-type. Salinity was detrimental to all phenotypes, reducing stem heights by 20% compared to freshwater. Our results suggest that the RCS can result in die-back symptoms similar to what is observed in the MRD.