Southern blight of hemp caused by Athelia rolfsii detected in Virginia.
Hemp (Cannabis sativa) is an annual herb from Cannabaceae family grown for its multitude of uses including fiber, seed and/or oil, and medicinal benefits. In August 2019, a hemp farm was visited in Dinwiddie County, Virginia, USA. At one of the locations where the CBD cultivar 'Boax' plants were grown in drip-irrigated and plastic-mulch-covered raised beds, three suspect plants with intermittent but conspicuous yellowing of foliage were clearly discernible. Disease incidence during the time of the visit, including the few symptomatic plants that were already removed after succumbing to the disease, was estimated between 0.5 and 2.0%. Brown to dark lesions, covered for the most part by a white fluffy and, at times, fan-shaped mycelium that extended from the lower stem to the crown area near the soil line, were visible upon opening up the canopy. The lesions were irregular shaped and extended along the stem perimeter with lesion length ranging between 5 and 62 mm (mean = 24.4 mm). Brownish mustard-seed-like sclerotia of 0.4 to 1.6 mm in diameter (mean = 0.8, n = 34) were seen on the white fluffy mycelium at the soil line. Based on morphological characteristics, sequence analysis of the DNA internal transcribed spacer region and pathogenicity tests, the causal agent was identified as Athelia rolfsii. In the USA, southern blight has been detected affecting hemp in Kentucky, North Carolina and Tennessee but also reported from Italy in Europe. In recent years, the disease has also expanded its horizon toward the north and Midwestern U.S. states such as Missouri on other specialty crops. This is thought to be the first report of the disease on hemp from Virginia.