First report of root and crown rot caused by Armillaria gallica on Cannabis sativa in California, U.S.A.

Authors: FENG, XUEWEN - Al & L Crop Solutions, Inc; Baumgartner, Kendra; DUBROVSY, LANA - Al & L Crop Solutions, Inc; FABRITIUS, ANNA-LIISA - Al & L Crop Solutions, Inc

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2022
Publication Date: 12/15/2022
Citation: Feng, X., Baumgartner, K., Dubrovsy, L., Fabritius, A.L. 2022. First report of root and crown rot caused by Armillaria gallica on Cannabis sativa in California, U.S.A. Plant Disease. 106(12):3215. https://doi.org/10.1094/PDIS-03-22-0483-PDN.
DOI: https://doi.org/10.1094/PDIS-03-22-0483-PDN

Interpretive Summary: In October 2018, symptoms of Armillaria root disease (wilting leaves and shoots, stunted growth) were observed on the upper portion of 7-month-old cannabis (Cannabis sativa L.) plants in Mendocino County, California, U.S.A. Symptomatic plants died within 24 hours. Of 200 affected plants, 80% were symptomatic. All affected plants were grown in non-woven polypropylene containers (Smart pots, Oklahoma, USA) set directly on the ground approximately 3 m apart outdoors, surrounded by native forest of Quercus spp. And Pseudotsuga menzeisii. Closer examination revealed diagnostic signs of Armillaria root disease: white mycelial fans at the base of the woody stem (root collar) and abundant rhizomorphs on the roots and root collar. Also, woody roots and the root collar were severely rotted. Rotted wood, mycelial fans, and rhizomorphs (n=20) were plated on PDA amended with tetracycline (1 mg/L). Species identity was confirmed by sequencing the internal transcribed spacer (ITS) region of rDNA and the translation elongation factor subunit 1-alpha (TEF1a) loci. Pathogenicity tests were conducted twice using two A. gallica isolates by inoculating rooted tissue-cultured cannabis plants (8 inoculated plants and 2 non-inoculated controls per duplicate experiment) with A. gallica liquid inoculum, added aseptically to the surface of the vermiculite, near the plant stem. At eight weeks post inoculation, all eight inoculated plants showed foliar symptoms and A. gallica was reisolated. This is the first report of A. gallica causing root and crown rot of C. sativa. Considering the expanding cultivation of Cannabis crops due to legalization of the industry in many U.S. states, A. gallica root and crown rot may become a serious issue affecting the industry, even for plants maintained in non-woven polypropylene containers in direct contact with soil.

Technical Abstract: In October 2018, symptoms of Armillaria root disease (leaf necrosis, wilted shoots, and stunted growth) were observed on the upper portion of the 7-month-old cannabis (Cannabis sativa L.) plants in Mendocino County, California, U.S.A. Foliar symptoms were followed by rapid death within 24 hours. Of 200 affected plants, 80% were symptomatic. All affected plants were grown in non-woven polypropylene containers (Smart pots, Oklahoma, USA) set directly on the ground approximately 3 m apart outdoors, surrounded by native forest of Quercus spp. And Pseudotsuga menzeisii. Closer examination revealed diagnostic signs of Armillaria root disease: white mycelial fans at the base of the woody stem (root collar) and abundant rhizomorphs on the roots and root collar. Also, woody roots and the root collar were severely rotted. Rotted wood, mycelial fans, and rhizomorphs (n=20) were plated on PDA amended with tetracycline (1 mg/L). Species identity was confirmed by sequencing the internal transcribed spacer (ITS) region of rDNA and the translation elongation factor subunit 1-alpha (TEF1a) loci. Pathogenicity tests were conducted twice using two A. gallica isolates by inoculating rooted tissue-cultured cannabis plants (8 inoculated plants and 2 non-inoculated controls per duplicate experiment) with A. gallica liquid inoculum, added aseptically to the surface of the vermiculite, near the plant stem. At eight weeks post inoculation, all eight inoculated plants showed foliar symptoms and A. gallica was reisolated. This is the first report of A. gallica causing root and crown rot of C. sativa. Considering the expanding cultivation of Cannabis crops due to legalization of the industry in many U.S. states, A. gallica root and crown rot may become a serious issue affecting the industry, even for plants maintained in non-woven polypropylene containers in direct contact with soil.