First report of Powdery Mildew Caused by Golovinomyces ambrosiae on Industrial Hemp in New Jersey

In December 2019, New Jersey became one of the first states to have its industrial hemp (Cannabis sativa L.) plan approved by the U.S. Department of Agriculture (USDA) following enactment of the 2018 Farm Bill that authorized the production of hemp. Following this approval, hemp was legally grown for the first time in 2020. During the growing seasons of 2020 and 2021, powdery mildew-like symptoms were observed during the summer months (Jun to Aug) in greenhouse hemp research and fall months (Aug to Oct) in field production plots on Rutgers Agricultural Experiment Station farms in southern and northern New Jersey. Symptoms were observed on leaves and stems of hemp cultivars 'CB Genius', 'Cherry Wine' and 'Bay Mist'. Symptoms initially appeared as small white patches of mycelia and conidia on the adaxial surface of leaves that gradually spread to entire leaves and stems. Leaf discoloration (e.g., chlorosis) and premature leaf drop were observed. More severe symptoms and damage were observed in the greenhouse than outdoor cultivation. A voucher specimen was deposited in the U.S National Fungus Collections, USDA-ARS, Beltsville, MD (accession number 929187). Morphological examination of the white colonies from the cultivar 'Baymist' was carried out using light microscopy and further characterized by sequencing. This isolate was labelled PMH2. Hyphae were septate, conidiophores were hyaline, unbranched, measuring 130 to 240 μm in length and produced 1 to 4 conidia in chains. Conidia were hyaline, ellipsoid to ovoid in shape and measured 20 to 36 ×10 to 18 μm (n=30). Oil-like drops were present within conidia, although no distinct fibrosin bodies were observed. Chasmothecia were not observed. Morphological observations were consistent with those of Golovinomyces spp. as described by Braun and Cook (2012). Morphological observations (conidiophore and conidial measurements) were also similar to the description of G. ambrosiae on Hemp, as described in Wiseman et al, 2021. Sequencing of internal transcribed spacer (ITS), large ribosomal subunit (28S), intergenic spacer (IGS), beta- tubulin (TUB2) and chitin synthase 1(CHS1) region, were carried out with the primer sets ITS5/ITS4, LSU1/LSU2, IGS-12a/NS1R, TubF1/TubR1 and gCS1a1/gCS1b respectively, as shown by Qiu et al. (2020). Maximum-likelihood phylogenetic analysis confirmed the grouping of the PMH2 isolate within the G. ambrosiae accessions. Each individual gene alignment was treated as a separate partition. Sequ