Yam is an important medicinal and edible dual-purpose plant with high economic value. However, nematode damage severely affects its yield and quality. One of the major effects of nematode infestations is the secondary infection of pathogenic bacteria or fungi through entry wounds made by the nematodes. Understanding the response of the symbiotic microbial community of yam plants to nematodes is crucial for controlling such a disease. In this study, we investigated the rhizosphere and how endophytic microbiomes shift after nematode infection during the tuber expansion stage in the Dioscorea opposita Thunb. cultivar Tiegun. Our results revealed that soil depth affected the abundance of nematodes, and the relative number of Meloidogyne incognita was higher in the diseased soil at a depth of 16 to 40 cm than those at a depth of 0 to 15 and 41 to 70 cm. The abundance of and interactions among soil microbiota members were significantly correlated with root-knot nematode (RKN) parasitism at various soil depths. However, the comparison of the microbial α-diversity and composition between healthy and diseased rhizosphere soil showed no difference. Compared with healthy soils, the co-occurrence networks of M. incognita-infested soils included a higher ratio of positive correlations linked to plant health. In addition, we detected a higher abundance of certain taxonomic groups belonging to Chitinophagaceae and Xanthobacteraceae in the rhizosphere of RKN-infested plants. The nematodes, besides causing direct damage to plants, also possess the ability to act synergistically with other pathogens, especially Ramicandelaber and Fusarium, leading to the development of disease complexes. In contrast to soil samples, RKN parasitism specifically had a significant effect on the composition and assembly of the root endophytic microbiota. The RKN colonization impacted a wide variety of endophytic microbiomes, including Pseudomonas, Sphingomonas, Rhizobium, Neocosmospora, and Fusarium. This study revealed the relationship between RKN disease and changes in the rhizosphere and endophytic microbial community, which may provide novel insights that help improve biological management of yam RKNs.

Common lambsquarters (Chenopodium album Linn.) is one of the most problematic weeds associated with crops worldwide due to its fast-growing, high fecundity, and wide tolerance to various conditions. Meanwhile, C. album is also an herbaceous vegetable plant, and the leaves and young shoots of this plant are considered nutritious in the human diet (Aman et al. 2016). In September 2023, C. album plants exhibiting yellowing, stunted growth, and extensive galled root symptoms were collected from a yam field in Fengqiu (34°54'24"N; 114°34'57"E), Henan Province, China. At the selected sampling site, we randomly selected 100 C. album plants, and the disease incidence was 73% on a 0.67-ha field. A RKN species belonging to the genus Meloidogyne was found, comprising an average of 550 second-stage juveniles (J2s) from 100 g of the 10 to 30 cm soil layer. The J2s were isolated from fresh soil with a Baermann funnel. C. album roots were thoroughly washed with tap water and dissected. Nematodes at different stages were collected and morphologically identified. Females and egg masses were obtained by dissecting galls. Females were white with a protruding neck, globular to pear-shaped. The perineal patterns of females predominantly exhibited a pronounced dorsal arch, characterized by either a square or trapezoid shape, lacking obvious lateral lines. Males isolated from root galls were vermiform, annulated, and showed a trapezoidal labial region, including a high head cap that was concaved at the center of the top end in lateral view. J2s were distinguished by the conspicuous, round stylet knobs, and they had wrinkled tails with a hyaline region and an obtuse tip. Morphological measurements are described in the supplementary material. All features were consistent with the morphological characteristics of Meloidogyne incognita (Eisenback and Hirschmann 1981). Identification was accomplished with subsequent species-specific PCR and sequencing analysis. The genomic DNA of 10 individual females was extracted, and the molecular identification was carried out with M. incognita-specific primers Mi-F/Mi-R, and Inc-K14-F/Inc-K14-R (Meng et al. 2004; Randig et al. 2002). PCR amplification generated 955 and 399 bp fragments for the analyzed samples, respectively, and the amplicons were confirmed by sequence analyses. The sequences were deposited in GenBank under accession number PP836070 and PP836071. BLASTn searches showed 100% identity with available GenBank M. incognita sequences (accession no. MK410954, OQ427638). To verify reproduction on C. album, 10 healthy plants (30 days old) grown in pots with sterilized soil were inoculated with 1,000 M. incognita J2s under greenhouse conditions (light/dark: 16 h/8 h, temperature: 25-28°C). Five uninoculated plants were used as negative control. Two months after inoculation, stunted growth and root-galling symptoms were observed similar to those in field, whereas control plants remained symptomless. Many root galls and egg masses were observed in all inoculated plants. The root galling index (scale of 0 to 10; Poudyal et al. 2005) was ~7 and nematode reproduction factor (final population density/initial population density) was 5.3. The morphological features of the nematodes reisolated from root tissue closely match the description of M. incognita, fulfilling Koch's postulates. The pathogenicity test was carried out twice with similar results. M. incognita is an emerging disease of economic importance in many crop plants worldwide, and may cause serious economic losses (Phani et al. 2021). This widely distributed C. album plant is likely a reservoir for the pathogen and serves as an alternate host for nematodes. The findings are significant for the integrated management practices of RKNs, particularly for crops that are infested with C. album. To our knowledge, this is the first report of the nematode parasitizing C. album in China. The development of effective short- and long-term control procedures is urgently needed for managing M. incognita.

Dioscorea opposita is an annual twining plant in China that is used for consumption and medicinal purposes. The planting area of D. opposita is near 500,000 hectares in China, mainly in Shangdong, Hebei, Henan, Jiangxi and Yunnan provinces. In August 2021, we observed that some D. opposita plants grew poorly with smaller and chlorotic leaves in Changyuan (35°8'12"N; 114°43'52"E), Henan Province, China. Galls with hook-shaped roots and tuber damage were also observed, typical of root-knot nematode. Thirty tubers were randomly collected and 60% were infested with root-knot nematodes. During a disease survey in Changyuan, the incidences of root-knot nematode damage were 31.5%, 21%, and 18% in three fields (0.33, 0.67, and 4 ha, respectively) at harvest. The average tuber length of infected plants was decreased by 65.8%, and the average weight was decreased by 70.1% compared to the healthy plants. Males, females, second-stage juveniles (J2s), and eggs were extracted from individual diseased tubers from the three fields for morphological identification. Females were white, pear-shaped with a projecting neck. Males showed a trapezoidal labial region with prominent stylet knobs, including a high head cap which had a stepped outline and was centrally concave in lateral view. Morphological measurements are described in the supplementary material. All data and descriptions conformed to the morphological characteristics of Meloidogyne incognita. Genomic DNA was extracted from J2s (n=9) using PCR lysis buffer, and used for PCR amplification of the sequence characterized amplified region (SCAR) markers specific for M. incognita. Two pairs of the SCAR primers, Mi-F/Mi-R, and Inc-K14-F/Inc-K14-R, were used to diagnose whether these nematodes from D. opposita were M. incognita (Meng et al. 2004; Randig et al. 2002). The PCR produced expected amplification products of 955 and 399 bp, confirming the nematode to be M. incognita. Primers specific for M. arenaria (Far/Rar) and M. javanica (Fjav/Rjav) were used but failed to amplify fragments (Randig et al. 2002; Zijlstra et al. 2000). The obtained PCR fragments were sequenced and deposited in GenBank (accession no. OQ420602.1, OQ427638.1). They showed 99.9 and 100% identity to the available GenBank M. incognita sequence (accession no. MK410954.1, ON861825.1), respectively. A pathogenicity test was conducted in greenhouse conditions. Bulbils of D. opposita were sown in the pots filled with 2,000 ml of autoclaved soil mixture (loamy soil/sand, 1:1). One month later, 15 seedlings (five to six leaf stage) were inoculated with 1,000 M. incognita J2s individually. Five plants without nematode inoculation were used as the control. Two months after inoculation, all of the inoculated roots had galling symptoms similar to those observed in the field, and 100% of root system tissues had galls. The root gall index was ~6 according to a 0 to 10 RKN damage rating scale (Poudyal et al. 2005). No symptoms were found on the control plants. The nematodes were reisolated from root tissue and identified. M. incognita has a broad host range in many species of economic importance including Salvia miltiorrhiza (Wen et al. 2023), Ipomoea batatas (Maleita et al. 2022), and Zea mays (López-Robles et al. 2013). So far, M. incognita has been reported in D. alata and D. rotundata in Africa (Onkendi et al. 2014). To our best knowledge, this is the first record of M. incognita on D. opposita in Henan Province, China. With the increased planting area of D. opposita in China, root-knot nematodes are becoming more serious and reducing tuber production, with yield losses more than 60%. This identification is a preliminary step in developing effective disease management schemes. Declaration of interest The authors declare no conflict of interest. Funding This work was financially supported by the Key Scientific Research Projects of Higher Education Institutions of Henan Province (21A180013), China Agriculture Research System (CARS-21), The Zhongyuan high level talents special support plan-Science and Technology Innovation Leading Talents (224200510011) and Science and Technology Research Project of Henan Province (222102310211). References López-Robles, J., et al. 2013. Plant Dis. 97:694. https://doi.org/10.1094/PDIS-07-12-0674-PDN. Maleita, C., et al. 2022. Plant Dis. 106:2536. https://doi.org/10.1094/PDIS-12-21-2680-PDN. Meng, Q. P., et al. 2004. Acta Phytopathol. Sinica 34:204. https://doi.org/10.13926/j.cnki.apps.2004.03.003. Onkendi, E. M., et al. 2014. Plant Pathol. 63:727. https://doi.org/10.1111/ppa.12202. Poudyal, D. S., et al. 2005. Australas. Plant Pathol. 34:181. https://doi.org/10.1071/AP05011. Randig, O., et al. 2002. Genome 45:862. https://doi.org/10.1139/g02-054. Wen, Y., et al. 2023. Plant Dis. Accepted. https://doi.org/10.1094/PDIS-05-22-0997-PDN. Zijlstra, C., et al. 2000. Nematology 2:847. https://doi.org/10.1163/156854100750112798.