First Report of <i>Neocosmospora parceramosa</i> Causing Root Rot on <i>Cannabis sativa</i> (Industrial Hemp) in Thailand

In December 2023, stems of Cannabis sativa L. (hemp) showing rot symptoms were collected from an outdoor plantation in Mae Sa Noi Village, Mae Rim District, Chiang Mai Province, Northern Thailand (18°51′50.06″ N, 98°50′45.57″ E; elevation 1,285 m / 4,216 ft), with ~30% disease incidence among 100 plants in a 500 m² plantation area. The stems exhibited dry symptoms and discolored lesions and were covered with abundant white, cottony spore growth. Ten symptomatic stems were randomly collected for pathogen isolation through a spore isolation method. The fungal strain (SDBR-CMU 781 = KUNCC25-20162) was obtained with an isolation frequency of 20%. Colonies grew rapidly on PDA, SNA, YEA, V8 juice agar and OA, reaching 80 to 82 mm in diameter after 7 days at 30 °C. On PDA, colonies were abundantly mycelial, pinkish white with an orange-pink reverse. On SNA, colonies were flat, white, with a white reverse. On YEA, colonies were white, and yellowish white in reverse. On V8 juice agar, colonies were pale orange, flat, with an orangish white reverse. On OA, colonies were white, with dense mycelium and flat margins, and a yellowish orange reverse. On PDA, conidiophores developed on aerial mycelium. Phialides monophialidic, subulate to subcylindrical or flask-shaped, hyaline, smooth, thin-walled, 5 to 54 × 1 to 4 µm. Conidia on aerial mycelium, cylindrical to slightly falcate, nearly parallel-sided, tapering at both ends, aseptate, hyaline, smooth, thin-walled, 6 to 11.8 × 2.3 to 4.8 µm (n = 30). Nucleotide sequences were obtained by amplification of three gene regions and deposited in GenBank: ribosomal internal transcribed spacer region (ITS, PX285924), the translation elongation factor-1 alpha (tef1-α, PX380328) and RNA polymerase second largest subunit (rpb2, PX380326), respectively (White et al. 1990; O'Donnell et al. 1998; Carbone and Kohn 1999; Liu et al. 1999). Multigene phylogenetic analyses demonstrated that strain SDBR-CMU 781 = KUNCC25-20162 clustered together with type strain Neocosmospora parceramosa CBS 115695 and thus the new strain is identified as N. parceramosa. Pathogenicity test performed by a detached seedling assay (Ei et al. 2026); hemp seeds were surface sterilized with 2.5% NaOCl for 5 min, rinsed three times with distilled water, and germinated on moist filter paper in sterile plastic boxes at 25 ± 2 °C for 3–5 days. Seedlings were transplanted into sterilized vermiculite: perlite: peat moss (1:1:2) and grown for 2 weeks. Fifteen two-week-old plants were inoculated with a spore suspension (1 × 10⁶ spores/mL), transferred to sterilized soil and maintained for 30 days at 25 to 34 °C; 60 to 80% moisture. After 4 weeks, inoculated plants developed wilt symptoms, including slow growth and yellowing. Inoculated roots developed brown lesions with white, cottony growth while controls remained symptomless. Disease severity was classified into four categories: mild (1–25%), moderate (26–50%), severe (51–75%), and very severe (76–100%). The disease affected 70–75%, indicating severe disease severity for this plant. To fulfill Koch’s postulates, fungi was re-isolated from seedlings and confirmed by morphological characteristics and DNA sequencing. The results confirm N. parceramosa as pathogenic to hemp in Thailand. This study highlights the potential threat of N. parceramosa to hemp production in Thailand, as soil-borne pathogen may spread through contaminated soil and infected seedlings, posing a risk of disease outbreaks in commercial cultivation.