A foliar disease of invasive black swallow-wort (<i>Vincetoxicum nigrum</i>) caused by <i>Boeremia exigua</i>

Black swallow-wort (<i>Vincetoxicum nigrum</i>) is an aggressive invasive vine infesting pastures and fields in the northeastern United States. An unknown fungal pathogen was recovered from foliar lesions occurring on black swallow-wort at two locations in Rhode Island in 2022 and was identified as <i>Boeremia exigua</i> based on morphological and molecular descriptions of eight isolates. The potential weed biological control value of a single isolate, FDWSRU 22-216, was evaluated through colonized agar block and conidial spray inoculations of black swallow-wort. Minor leaf-spotting and tip die-back symptoms were reproduced under controlled conditions but did not appear to substantially reduce the health of infected plants. Since broad host range pathogen species found on weeds may pose a disease threat to crops, cultivated plants including tomato, barley, sunflower, and flax were evaluated for susceptibility to the pathogen. None of these crops developed disease symptoms following inoculation with <i>B. exigua</i>, suggesting that the pathogen population associated with black swallow-wort poses little direct risk to crop production. However, using taxon-specific markers, two <i>B. exigua</i> genotypes, including one that does not correspond to existing reference material, were described. While both genotypes grew most rapidly at 25°C in vitro, the dominant genotype was capable of higher growth rates at 30°C, suggesting potential phenotypic differentiation. This study reports the first instance of <i>B. exigua</i> on a swallow-wort species and highlights the potential for widespread invasive plants to harbor previously undescribed pathogen genotypic diversity. Although <i>B. exigua</i> has limited potential as a living biological control agent for black swallow-wort, the isolates from this study could be screened for production of phytotoxic secondary metabolites with weed suppressive activity.