Astragalus mongholicus is a medicinal plant that is known to decrease in quality in response to continuous cropping. However, the differences in the root-associated microbiome and root exudates in the rhizosphere soil that may lead to these decreases are barely under studies. We investigated the plant biomass production, root-associated microbiota, and root exudates of A. mongholicus grown in two different fields: virgin soil (Field I) and in a long-term continuous cropping field (Field II). Virgin soil is soil that has never been cultivated for A. mongholicus. Plant physiological measurements showed reduced fresh and dry weight of A. mongholicus under continuous cropping conditions (i.e. Field II). High-throughput sequencing of the fungal and bacterial communities revealed differences in fungal diversity between samples from the two fields, including enrichment of potentially pathogenic fungi in the roots of A. mongholicus grown in Field II. Metabolomic analysis yielded 20 compounds in A. mongholicus root exudates that differed in relative abundance between rhizosphere samples from the two fields. Four of these metabolites (2-aminophenol, quinic acid, tartaric acid, and maleamate) inhibited the growth of A. mongholicus, the soil-borne pathogen Fusarium oxysporum, or both. This comprehensive analysis enhances our understanding of the A. mongholicus microbiome, root exudates, and interactions between the two in response to continuous cropping. These results offer new information for future design of effective, economical approaches to achieving food security.

Root rot is a serious disease in plantations of Angelica sinensis, severely reducing yield and quality and threatening sustainable production. Fusarium isolates (n = 32) were obtained from field samples of root rot tissue, leaves, and infected soil. Isolates were identified by comparison of the sequences of their internal transcribed spacer region and translation elongation factor 1-α to sequences of known species in the National Center for Biotechnology Information database. These Fusarium isolates include Fusarium tricinctum (43.75%), F. equiseti (31.25%), F. solani (9.37%), F. oxysporum (6.25%), F. acuminatum (6.25%), and F. incarnatum (3.12%). For pathogenicity testing under greenhouse conditions, seven isolates were selected based on a phylogenetic analysis, including four strains of F. tricinctum and one strain each of F. solani, F. oxysporum, and F. acuminatum. The seven isolates were all pathogenic but differed in their ability to infect: The four F. tricinctum strains were capable of causing root rot in A. sinensis at 100% incidence and were highly aggressive. Furthermore, the symptoms of root rot induced by those seven isolates were consistent with typical root rot cases in the field, but their disease severity varied. Observed histopathological preparations of F. tricinctum-infected seedlings and tissue slide results showed that this fungal species can penetrate epidermal cells and colonize the cortical cells where it induces necrosis and severe plasmolysis. Plate confrontation experiments showed that isolated rhizosphere bacteria inhibited the Fusarium pathogens that cause root rot in A. sinensis. Our results provide timely information about the use of biocontrol agents for suppression of root rot disease.

Codonopsis pilosula Franch., also known as Dangshen, is an important medicinal plant in China. It is widely cultivated for a major income of local farmers in Dingxi, Gansu Province. Its dried roots have the effects of supplementing vital energy, nourishing spleen and lung, enhancing organic immunity, helping depressurization, and improving microcirculation, etc., for humans. In June to October, 2018-2020, root rot disease was observed on C. pilosula with incidences up to 20% in the Dingxi region. We collected ten diseased and healthy plants from Dingxi (35°06'N, 104°29'E, 2206 m a.s.l.) in October 2019. The rotting root tissues were sterilized with 70% ethanol for 30 s and 3% NaOCl for 5 min and placed on potato dextrose agar (PDA) plates incubated at 25℃to isolate the pathogen (Shang et al. 2014). From the similar fungal cultures isolated after 7 days on PGA, isolate B17 was purified for morphological and molecular characterization. Its colony appeared light purple and produced long aerial hyphae. Slightly curved macroconidia (12.3 to 31.7 × 3.1 to 5.1 μm, n=40) and oval-ellipsoid and cylindrical microconidia (6.1 to 9.9 × 2.8 to 4.5 μm, n=30) were observed. The internal transcribed spacer region (ITS) and the translation elongation factor-1 alpha (TEF-1α) gene were amplified using primers ITS1/ITS4 and EF-1/EF-2 (Uwaremwe et al. 2020), respectively. The 489 bp (ITS) and 631 bp (TEF-1α) sequences were deposited in GenBank (Accession No. MN744360 and MN786974, respectively). The ITS sequence had 100% homology to isolate JJF2 (No. MN626452, ITS) (Ma et al. 2020), and the TEF-1α sequence had 100% homology to isolate Fo353 (No. KM065860) (Koyyappurath et al. 2016) of Fusarium oxysporum Schlecht. emend. Snyder & Hansen, which caused root rot of Panax ginseng and Vanilla planifolia, respectively. A phylogenetic tree was generated using the unweighted pair-group method with arithmetic average in the MycoBank database (O'Donnell et al. 2015), which clustered isolate B17 in the F. oxysporum species complex. Twenty 1-year-old plants of C. pilosula were inoculated with were inoculated by dipping the washed roots in a conidial suspension (2 ×106 conidia/ml added with 0.2% Tween 20) for 20 min before transplanted into pots (16 × 16 × 23 cm) with four plants per pot filled with sterilized peat and soil mixture (2:1 v/v) and grown in a greenhouse at 26oC with >70% humidity and 16 h light. Sterilized water added with 0.2% Tween 20 was used as a control. One week after inoculation, the leaves of pathogen-inoculated plants became yellow, and wilting occurred at the leaf tips 18 days later. Some of the inoculated plants died 45 days after inoculation, and the low part of roots had dark brown to black lesions and became rotting. The control plants did not show symptoms. The pathogenicity test was repeated three times with the same fungus isolated from the infected root tissue. To the best of our knowledge, this is the first report that F. oxysporum causes root rot on C. pilosula in China. F. oxysporum is a serious threat to C. pilosula cultivation, and the finding of this pathogen provides a clear target for root rot control.

Cucumber mosaic virus (CMV) is damaging to the growth and quality of lettuce crops in Lanzhou, China. Recently, however, for the first time an isolate of lettuce necrotic yellows virus (LNYV) has been detected in lettuce crops in China, and there is concern that this virus may also pose a threat to lettuce production in China. Consequently, there is a need to develop a rapid and efficient detection method to accurately identify LNYV and CMV infections and help limit their spread. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) assays were developed to detect the nucleoprotein (N) and coat protein (CP) genes of LNYV and CMV, respectively. RT-LAMP amplification products were visually assessed in reaction tubes separately using green fluorescence and gel electrophoresis. The assays successfully detected both viruses in infected plants without cross reactivity recorded from either CMV or LNYV or four other related plant viruses. Optimum LAMP reactions were conducted in betaine-free media with 6 mM Mg²⁺ at 65°C for LNYV and 60°C for 60 min for CMV, respectively. The detection limit was 3.5 pg/ml and 20 fg/ml using RT-LAMP for LNYV and CMV plasmids, respectively. Detection sensitivity for both RT-LAMP assays was greater by a factor of 100 compared to the conventional reverse transcription polymerase chain reaction assays. This rapid, specific, and sensitive technique should be more widely applied due to its low cost and minimal equipment requirements.