A New Species of Didymella from the Eastern United States Is a Pathogen of Invasive Reynoutria japonica.

Reynoutria japonica (Polygonaceae) is widely considered one of the most persistent and problematic nonnative invasive weeds introduced to North America. Conventional methods to manage infestations are labor-intensive and often require annual monitoring and retreatment. Biological control agents (BCAs) present an alternative management strategy. Currently, few organisms are effective for managing Re. japonica, presenting a need to discover additional candidate BCAs. Recently, leaf spots were observed on Re. japonica in two locations in the eastern United States, and the causal agent was investigated as a potential BCA. In this study, fungi isolated from symptomatic Re. japonica were identified as belonging to a previously undescribed species of Didymella based on DNA sequence homology, phylogenetic analysis, and morphological characteristics. The host range of related Didymella spp. includes other polygonaceous plants, notably cultivated Rheum rhabarbarum. Therefore, the pathogenicity of isolates representing this new fungal species was tested on seedlings of Re. japonica and Rh. rhabarbarum 'Victoria'. The novel species was capable of infecting both Re. japonica and Rh. rhabarbarum under artificial conditions. An isolate of D. rhei (Ellis & Everh.) Qian Chen & L. Cai, a close relative and known pathogen of Rh. rhabarbarum, was included in pathogenicity tests but failed to cause disease on Re. japonica. The pathogen from Re. japonica was described as D. polygonacearum sp. nov. This study suggests D. polygonacearum is a poor candidate for biological control use and that Re. japonica may harbor crop pathogenic microbes.