First report of stem rot of Scrophularia ningpoensis caused by Fusarium commune in China.

Abstract

Chinese figwort (Scrophularia ningpoensis Hemsl.) is an important annual herb and its dried root tubers are used as a traditional Chinese herbal medicine. In May 2021, a disease with stem rot symptoms on S. ningpoensis was observed at three randomly selected fields (~0.67 ha per field) in Nanchuan district (28.93°N, 107.27°E) of Chongqing, China. Disease incidence was estimated between 10% and 17% based on calculating the proportion of symptomatic plants. Initially, watery dark brown spots appeared on the epidermis of the stem. Then the spots expanded into spindle or strip shape, and the center of lesions were sunken, constricted and rotted finally (Figure 1A and Figure 1B). Leaves turned yellow and the plants wilted (Figure 1C). The infected parts of the stem broke easily and became brittle. The number of daughter buds used for reproduction was reduced by more than 24% and the production of root tubers decreased by more than 3%. Twelve stems with typical rot symptoms were sampled from the three fields for further investigation. Infested tissue fragments (4×4 mm) were surface sterilized with 75% ethanol for 30s and 2% sodium hypochlorite for 2 minutes in turn, finally, were rinsed 4 times with sterilized water. The disinfected tissue were air-dried and transferred onto potato dextrose agar (PDA) in the dark for 6 days at 25℃. The resulting fungal colonies were isolated by the single-spore isolation technique (Fang. 1998). Six different fungal colonies were isolated (X1-X6) and Koch's postulates were conducted to verify the pathogenicity of individual isolates. The stem surfaces of 8 months old plants were sterilized with 75% ethanol for 30 s, rinsed three times with sterilized water, and stabbed with a sterilized needle. Conidial from the fungal colonies grown on PDA plate were harvested by filtration through five layers of sterilized absorbent gauze. Conidial concentration was then adjusted to 106 conidia per mL. 10 μL of conidial suspension was sprayed on stems injured with a sterile syringe. For each isolate, 6 plants were inoculated. Stems inoculated with sterilized water were used as a blank control. All plants were all put in a growth chamber at 28℃ with 75 to 80% relative humidity under a 12 h photoperiod for 15 days. The pathogenicity test was repeated once. After 13 days, the stems inoculated with X3 showed the same rot symptoms as we observed in the fields (Figure 1D) whereas the control stems remained symptomless (Figure 1E). The fungus re-isolated from the plants showing 100% symptoms had a similar morphology than X3 as described below. At the same time, the stems inoculated with X1, X2, X4, X5 and X6 showed no sign of rot. After culturing on PDA for 9 days under 25℃ in dark, isolate X3 grew all over the dish with white or pale pink pigmentation in the center (Figure 1F). Macroconidia were produced on synthetic low nutrient agar (SNA) plates, which showed sickle or spindle, 3 septate, straight to slightly curved with a foot-shaped basal cell, ranging from 17.595~44.88 × 2.04~3.315 μm (n=30). Microconidia were oval, elliptical or reniform, 0 to 1 septate, 3.06~12.75 ×1.785~2.805 μm (n=30) in size (Figure 1G). Phialides of conidiophores were cylindrical, short and monophialides or polyphialides (Figure 1H). Chlamydospores were found terminal or cluster with round or oblong (Figure 1I). These morphological characteristics described as Fusarium commone (Skovgaard et al. 2003). For molecular identification, the ribosomal internal transcribed spacer (ITS), translation elongation factor 1-alpha (EF-1α), RNA polymerase II subunit 1 (RPB1), the largest subunit of RNA polymerase Ⅱ gene sequences (RPB2) and the mitochondrial small subunit rDNA (mtSSU) genes were amplified with primers V9G /ITS4 (Hoog et al. 1998; White et al. 1990), EF1-668F /EF1-1251R (Alves et al. 2008), Fa/G2R (O'Donnell et al. 2010), 5f2/7cr (Liu et al. 1999; O'Donnell et al. 2010) and NMS1/NMS2 (Li et al. 1994). The sequences of isolate X3 were deposited in GenBank (MZ571935 (ITS), MZ576201 (EF-1α), MZ882396 (RPB1), MZ882397 (RPB2) and MZ867716 (mtSSU)). All sequences were revealed more than 99.8% sequence identity with reported sequences of Fusarium commune (GenBank accession No: KY630717, JF740838, KU171680, KU171700 and MK439851). Based on the optimal nucleotide replacement model SYM of multi-gene series sequence matrix, the system development tree was constructed. Results showed the strain X3 and those of F. commune (Isolates numbers were NRRL 28387, MRC 2566, MRC 2564 and CZ3-5-6) were clustered into the same evolutionary branch with a post-mortem probability of 0.996 (Figure 2). According to the morphology, molecular identification and phylogenetic analysis based on the concatenated of EF-1α and RPB2 genes sequences, the isolated X3 was identified as F. commune. The ITS sequences of X1, X2, X4, X5 and X6 showed homology exceeding 97.1% to Fusarium tricinctum (MH931273), Plectosphaerella cucumerina (MH858371), Sordariomycetes sp. (JX179237), Whalleya microplace (EF026129) and Pestalotiopsis maculiformans (EU552147), respectively, suggested the five strains to be these species possibly. GeneBank accession number of X1, X2, X4, X5 and X6 was OM074010, OM074011, OM074013, OM074015 and OM074018, respectively. To our best knowledge, this is the first report of F. commune infecting S. ningpoensis in China. Stem rot caused by F. commune is a severe threat to Chinese figwort cultivation, and identification of this pathogen is important for effective disease management and control.