First report of Fusarium tricinctum Species complex causing root rot of tobacco in China.

Abstract

Tobacco (Nicotiana tabacum L.) is an economically important crop in China. In June 2021, a root rot disease was observed on tobacco (cv. Yunyan99) in Lushi, Mianchi, and Luoning counties of western Henan, China. Diseased tobacco plants exhibited wilting with leaf chlorosis and root rot accompanied by purplish to brown vascular discoloration. The symptoms were observed in four surveyed fields, 57 ha in total, and disease incidence ranged from 21 to 56%. Five symptomatic plants with leaf chlorosis and root rot were randomly collected from each field for pathogen isolation. Tissue pieces from diseased roots were surface sterilized in 75% ethanol for 30 s then rinsed with sterile distilled water three times, air dried, and placed onto potato dextrose agar (PDA) medium. Five isolates, SL1, SL2, SL3, LN and KC, were purified by single-spore culturing. On PDA, colonies grew at a rate of 2-5 mm/day and produced abundant cottony, white to pink aerial mycelia and rose pigment on the reverse side of the culture plate. From 7-day-old cultures grown on carnation leaf agar (CLA), macroconidia were straight to subarcuate, with blunt and slightly hooked apical and basal cells, had three to four septa, measured 23.4 to 44.6×3.5 to 4.2 μm (n=30). Cylindrical, napiform or oval microconidia were one to two-celled, measuring 6.3 to 22.9×2.2 to 4.9 μm (n=30). Spherical chlamydospores were intercalary or terminal, in chains. Such characteristics resembled those of the Fusarium tricinctum species comples (FTSC; Batra and Lichtwardt 1962; Leslie and Summerell 2006). To confirm the morphological diagnosis, the genomic DNA of the isolates were extracted, the translation elongation factor 1-alpha (EF-1α), RNA polymerase I largest subunit (RPB1) and second largest subunit (RPB2) genes were amplified with primers EF1/EF2, F5/G2R and 5f2/7cr respectively (O'Donnell et al. 2010), and sequenced. Maximum likelihood analysis was carried out using MEGA 7. Sequences were 97.55% to 100% identical to corresponding DNA sequences of FTSC based on GenBank and Fusarium MLST BLASTn analysis, and deposited in GenBank (ON637268.1-ON637272.1, ON637275.1-ON637279.1, ON637282.1-ON637286.1). Based on the morphological characteristics and phylogenetic analysis, the isolates were identified as F. acuminatum (SL1, SL2, SL3 and LN; Senatore et al. 2021) and F. reticulatum (KC; Moreira et al. 2019). Koch's postulates were conducted to verify the pathogenicity of individual isolates. The four-leaf stage healthy tobacco seedlings (Yunyan99, n=30) were inoculated by pouring 20 mL conidial suspension (1×107 conidia/mL) around the rhizosphere. Control seedlings were inoculated with sterilized water (n=30). All the treatments were maintained under greenhouse conditions with a 12-h light/dark photoperiod at 25±0.5℃ and 70% relative humidity for 30 days. The assay was conducted three times. Foliage chlorosis and root rot were observed on the inoculated tobacco seedlings, whereas the control seedlings remained asymptomatic after 30 days. The pathogens were reisolated from the necrotic tissue from all inoculated seedlings and were identified by sequencing partial EF-1α and RPB2 genes. Fusarium tricinctum species complex are known as an important causal of cereals Fusarium Head Blight (FHB; Laraba, et al. 2022). In China, F. acuminatum can also infect herb plants and fruits, such as Angelica sinensis, Schisandra chinensis (Ma et al. 2022; Shen et al. 2021). To our knowledge, this is the first report of root rot on tobacco caused by FTSC members in China as well as the world. This finding expands the host range known for FTSC and will be helpful for developing effective control strategies of tobacco root rot.