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

Abstract

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 in field and greenhouse hemp research 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 mycelium and conidia on the adaxial surface of leaves that gradually spread to entire leaves and stems. Lesions were excised using a scalpel from ‘Bay Mist’ leaves and the isolate was labelled PMH2. Hyphae were septate. Conidiophores were hyaline, unbranched, measuring 130-240 µm in length and produced 1-4 conidia in chains. Conidia were catenescent, hyaline, ellipsoid to ovoid in shape and measured 20-36 x 10-18 µm (n=30). Oil-like drops were present within conidia, although no distinct fibrosin bodies were observed. Chasmothecia were also not observed in both years. Morphological observations are consistent with those of Golovinomyces spp. on Heliantheae hosts (Golovinomyces spadaceus and Golovinomyces ambrosiae) as described by Braun and Cook 2012. A multilocus phylogenetic analysis of the internal transcribed spacer (ITS), large ribosomal subunit 28S, intergenic spacer (IGS), beta- tubulin (TUB2) and chitin synthase 1 (CHS1) genes as shown by Qiu et al. 2020, confirmed the grouping of the PMH2 isolate within the G. ambrosiae accessions. Sequence data were deposited in GenBank under the accessions OK626453 (ITS), OK626454 (28S), OL456201 (IGS), OL415512 (TUB2) and OL415513 (CHS1). Pathogenicity was confirmed by gently pressing symptomatic hemp leaves onto healthy hemp leaves, which were incubated indoors at 23°C. Non-inoculated healthy plants served as control. Inoculated plants developed powdery mildew symptoms within 10 to 12 days while all controls remained disease free. The powdery mildew on inoculated plants were found to be morphologically similar to the original. Although the current acreage of hemp production in New Jersey is fairly small, it is expected to significantly increase, particularly for greenhouse production, with the upcoming opening (15 December 2021) of cultivation licensing and retailing of recreational cannabis. This is the first known report of Golovinomyces ambrosiae causing powdery mildew on hemp in New Jersey.

The co‐occurrence of two powdery mildew species on Cannabis sativa in the Czech Republic

“…Complex views on Cannabis sativa diseases, including powdery mildews, and their management are treated in some recent papers and books (Gauthier & Thiessen, 2022; Punja, 2021; Vujanovic et al., 2020). Following the announcement of the release of restrictions on the cultivation of hemp in USA, the vast majority of articles describing the first occurrence of powdery mildew on hemp came from the USA and Canada (Akinrinlola & Hansen, 2023; Bates et al., 2021; Brochu et al., 2022; Farinas & Peduto Hand, 2020; Pépin et al., 2018; Rajmohan et al., 2022; Rivedal et al., 2024; Szarka et al., 2019; Vujanovic et al., 2020; Wiseman et al., 2021) and Israel (Maymon et al., 2020). Surprisingly, there is no official scientific report about the occurrence of powdery mildew on Cannabis sativa in Europe (Braun, 1995; Braun & Cook, 2012).…”

“…lat.) has been reported from USA, including Kentucky (Szarka et al, 2019), Ohio (Farinas & Peduto Hand, 2020), Oregon (Rivedal et al, 2024;Wiseman et al, 2021), New Jersey (Rajmohan et al, 2022), Tennessee (Akinrinlola & Hansen, 2023), and from Canada (Brochu et al, 2022;Pépin et al, 2018), and Israel (Maymon et al, 2020). Similarly, as for P. macularis, these reports date mainly from the last decade, probably reflecting lifting of restrictions on the cultivation of this crop in some states of USA.…”