First Report of Taraxacum kok-saghyz Rodin Root Rot Caused by Alternaria tenuissima in Heilongjiang Province, China.

Abstract

Taraxacum kok-saghyz Rodin (TKS) is a perennial herb of the Asteraceae family, which is an important natural rubber supplement crop (Yang et al., 2023). In May 2023, a severe root rot disease was observed in TKS during cultivation in Harbin (45°75'N, 126°66'E), Heilongjiang Province, China. Field surveys indicated a disease incidence of approximately 10% within the 450 m² cultivation area (500 TKS seedlings), with symptoms including leaf wilting, stunted growth, root necrosis, and plant mortality. To isolate its pathogen, nine symptomatic root segments (about 1 cm) were randomly sampled from three diseased plants and surface sterilized in 2% NaClO for 30 s, rinsed three times with sterile distilled water, and then incubated on potato dextrose agar (PDA) supplemented with 30 mg/L streptomycin at 28 °C in darkness for 3 days. Nine isolates with similar colony morphology were obtained from nine diseased root segments by mycelial tip separation. All isolates exhibited cottony colonies on the PDA medium, initially gray-white and then turned into dark gray, with the reverse side becoming black. On potato carrot agar medium, the conidia of these isolates are oval, brown in color, simple or form unbranched chains, and have 1-4 transverse septa and 0-2 longitudinal septa; they measured 10.23 to 30.57 μm in length and 5.48 to 9.96 μm in width (n = 50), exhibiting morphological characteristics consistent with members of the genus Alternaria (Simmons et al., 2007). Three isolates (GURF-01, GURF-02, GURF-03) were randomly selected from different plants for further study. These isolates were molecularly characterized by amplifying the internal transcribed spacer (ITS), RNA polymerase II second largest subunit (RPB2), translation elongation factor 1-α (TEF), 18S small subunit rDNA (SSU), 28S large subunit rDNA (LSU), Alternaria major allergen (Alt-a1) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) regions by ITS1/ITS4, fRPB2-5f/fRPB2-7cR, EF1-728F/EF1-986R, NS1/NS4, LR0R/LR5, Alt-for/Alt-rev and gpd1/gpd2 primers (Lawrence et al., 2013; Woudenberg et al., 2015; Li et al., 2023), respectively. BLASTn search revealed that ITS (PQ182566, PQ180345, PQ182567), RPB2 (PQ740537 to PQ740539), TEF (PQ196801 to PQ196803), SSU (PQ180346 to PQ180348), LSU (PQ180349 to PQ180350), Alt-a1 (PQ196798 to PQ196800) and GAPDH (PQ287275 to PQ287277) sequences of these isolates showed 99% to 100% identity with A. tenuissima (ON514229, KC584435, KC584693, KC584567, KC584311, ON548915 and ON528099). A phylogenetic tree was constructed showing that GURF-01, GURF-02 and GURF-03 clustered with A. tenuissima. The above three isolates were used for test pathogenicity in the 3-month-old TKS plants with three repeats. The test groups were inoculated with 50 mL spore suspension (10⁶ spores/mL) while distilled water was supplied for the control. Plants were maintained at 25 ℃. After 20 days, all the tested plants exhibited leaf chlorosis and root browning, whereas controls remained healthy. The same fungus was re-isolated from symptomatic roots and identified as A. tenuissima through morphological and molecular analyses, fulfilled Koch's postulates. A. tenuissima was considered to cause tuber rot in potato (Liu et al., 2019). To our knowledge, this is the first report for TKS root rot caused by A. tenuissima, which may help to develop more effective field management strategy to prevent this disease in future TKS production.