First report of root rot caused by Fusarium solani on roselle in Nanning, Guangxi, China.

Abstract

Roselle (Hibiscus sabdariffa L.) is an annual herbaceous plant in the Malvaceae family with high anthocyanin and is widely cultivated in Nanning, Guangxi of China due to its economic and nutritional importance. In August 2021, a severe root rot disease with incidence of 42.4% (860 plants in the field) was observed in roselle plants in an open-field crop in Nanning (108°33"E, 22°84"N), Guangxi, China. The roots of the diseased plants were discolored and rotten, and the xylem became black, extending along the main root to the junction of the rhizomes. The above-ground symptoms were leaf yellowing, vascular tissue browning, wilting and death. Three diseased samples were rinsed thoroughly with sterile distilled water, cut with a sterile scalpel into approximately 0.5-cm pieces, surface disinfested with 75% ethanol for one minute, rinsed 3-4 times with sterile water, and finally incubated on potato dextrose agar (PDA) at 28 °C in the dark for 3 days. Emerging colonies were transferred to new PDA two-three times until a single colony was obtained. The aerial mycelium was initially white, turning pale yellow after 5 days of growth on PDA. Microscopic observations revealed that microconidia were hyaline and ovoid with sizes of 5.13 to 15.12 and 2.50 to 4.20 μm (average 9.02, 3.32 μm, n=30). Macroconidia were falciform with 3- to 4-septate, with sizes of 19.08 to 24.35 and 4.5 to 8.00 μm (average 24.35 and 5.23 μm, n=30). The morphological characteristics of the microscopic images were identical to those described for Fusarium solani (Leslie and Summerell 2006). A representative isolate (GXRST29) was selected for DNA extraction for further characterization. The internal transcribed spacer rRNA regions (ITSs), beta tubulin gene sequence and a fragment of the translation elongation factor 1-alpha (EF 1-α) gene sequence were amplified using the primer pairs ITS1/ITS4 (Chehri 2014), Bt-1/Bt-2 (Wang et al. 2014) and EF1-F/EF2-R (O'Donnell et al. 2010), respectively. PCR products were sequenced and deposited in GenBank (accession Nos. OL314654, ON157430 and ON157431, respectively). BLASTn analysis showed that the ITS sequence had 96.99% homology with sequence of F. solani (NR 163531), and 99.26% for Fusarium cf. solani (MG775565) obtained from Homo sapiens. The beta tubulin sequence had 97.96% similarity with BLAST sequence of F. solani (MN295052.1) and EF 1-α gene had 100% identity to published F. solani (MN977912.1). The fungus was identified as F. solani. Five roselle plants at the 5-leaf stage were artificially inoculated by root dipping into a 106-107-mL-1 spore suspension of the isolated GXRST29 for pathogenicity testing. The experiment was conducted three times, and the negative controls were replaced with sterile water. Compared to the control, the growth of plants was significantly inhibited, leaves turned yellow, plants dwarfed and wilted, and roots decayed three days post-inoculation. One week post-inoculation, all plants exhibited symptoms similar to those observed in the field, and F. solani was steadily reisolated from those diseased plants, while no positive isolations were obtained in the controls. F. solani has been reported to cause root rot on roselle in Upper Egypt (Hassan et al. 2014) and lisianthus in China (Xiao et al. 2018). To our knowledge, however, this is the first report of Fusarium wilt caused by F. solani in roselle plants in Nanning, Guangxi, China, and could result in severe crop losses.