RESUMO
Hemp (Cannabis sativa L.) is grown for medicinal and industrial uses. Symptomatic hemp (Mountain Mango) seedlings were received from a grower's greenhouse in Towner County (48.7486° N, 99.2761° W), North Dakota (ND), USA in July 2020. Seedlings had brown to blackish root tips, thread-like hypocotyl rot and seedling collapse, with about 8 to 10% disease incidence. Roots were surface disinfested in 1% sodium hypochlorite for 1 min, rinsed thrice with sterile distilled water, and blotted dry. About 1-cm sectioned root tips were plated on water agar (WA) and acidified potato dextrose agar (PDA, pH: 4.8) media and incubated under fluorescent light with a 12-h photoperiod at 25° C. After 7 days, single spores were isolated and sub-cultured on PDA and carnation leaf agar for morphological observations (Dhingra and Sinclair, 1995). Colonies had uniform appearances and produced white, thick and floccose mycelium. Conidiophores produced from lateral hyphae were simple to branched. Phialides were slender, smooth, hyaline and septate. Macro-conidia were 12.5 to 30.2 x 2.2 to 3.6 µm, septate (3-5), thick walled, hyaline and moderately curved shaped. Micro-conidia were oval to ellipsoid, smooth walled, no septa and measured 3.4 to 8.8 µm and 1.3 to 4.3 µm. Chlamydospores were round shaped, thick-walled, and produced singly or in pairs. Based on morphological characteristics, isolates were identified as Fusarium solani (Mart.) (Carbone and Kohn 1999; Leslie and Summerell 2006). For molecular identification, genomic DNA of three representative fungal isolates were extracted using DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). PCR was done using both the primers (EF1/EF2) of the translation elongation factor (TEF-1α) and primers 5F2/7cR and 7cF/11aR of the RNA polymerase II (RPB 2) for Fusarium species (O'Donnell et al. 2009 and 2010). All isolates had identical PCR product sequences for the respective primer sets. The DNA sequences were deposited to NCBI GenBank with accession No. OK880264 (TEF-1α), OK880266 (RBP 5F2/7cR), and OK880265 (RBP 7cF/11aR). The NCBI Megablast search of the OK880264, OK880266, and OK880265 showed 100% similarity with respective homologue sequences from F. solani species complex (GU170628, KC808344, and EU329608). Similar results were obtained by BLASTN search in the FUSARIUM ID database (Geiser et al. 2004). For pathogenicity testing, 200 µl conidial suspension (1 x 106 conidia/ml) was pipetted, without wounding roots, onto the soil around the base of four plants individually potted in peat mix (Sunshine mix 1, Sun Gro Horticulture Ltd.; Alberta, Canada) and maintained in the greenhouse with 12 h photoperiod and temperature of 23 ± 2°C (Argus Control Systems Ltd.; British Columbia, Canada). Four plants inoculated with distilled water served as control. The test was conducted twice. At 10 days post inoculation (dpi), yellowing of leaves and damping off were observed in all inoculated plants. Re-isolated fungi from infected plant samples were morphologically identical to the isolate used for root inoculation. F. solani has been reported to cause damping off and root rot in several states in the U.S., Canada and Italy (Gauthier et al. 2019; Iriarte et al. 2021; Sorrentino et al. 2019). This is the first report of F. solani causing seedling damping off and root rot on hemp in ND. Hemp acreage has decreased in ND because of diseases (Buetow et al. 2020). Information on identification and management of diseases affecting hemp will be useful to producers.
