Berkeleyomyces rouxiae is responsible for black root rot on multiple milkweed species in South Carolina, USA

Abstract

Several outbreaks of black root rot on four milkweed species occurred at a South Carolina plant nursery specialized in growing host plants of butterflies. The diseased plants included Asclepias asperula and A. incarnata in 2022 and A. speciosa and A. verticillata in 2023. Symptoms included wilting and stunting of foliage and distinct black lesions and decay on roots. Berkeleyomyces rouxiae was isolated from the diseased roots and identified based on morphological features, DNA sequences, and phylogenetic characteristics. Koch’s postulates were completed to confirm its pathogenicity on A. incarnata and A. verticillata. Milkweed plants are an important food source for the monarch butterfly, an iconic North American treasure, as well as a nectar source for other pollinators. Outbreaks have caused significant plant losses at the plant nursery; however, it is unclear whether this disease could become an issue for milkweed plants in gardens and landscapes. Effective management of Berkeleyomyces species can be a challenge.