Association of Globodera rostochiensis (Nematoda) with Stunted and Chlorotic Potato Plants in Yunnan and Sichuan Provinces in China

On a global basis, potato cyst nematodes (Globodera spp. Skarbilovich 1959 [Behrens 1975]) are one of the most serious soilborne pathogens in potato (Solanum tuberosum L.) production. In 2019 to 2020, 188 soil samples were taken from rhizosphere soil associated with the roots of stunted and chlorotic potato plants in the main potato-growing areas of Yunnan and Sichuan Provinces of China. Globodera rostochiensis Wollenweber 1923 (Skarbilovich 1959) was recovered from 112 of the samples. Nematode identification was as confirmed by morphometric, light microscopy, electron microscopy, and molecular methodologies. Population densities of G. rostochiensis ranged from 47.0 to 69.0 eggs/g of soil. A BLASTn homology search program was used to compare the sequences of populations of G. rostrochienses from Yunnan and Sichuan Provinces with populations of other Heteroderinae spp. and populations of G. rostochiensis from other nations. Although potato has been grown in China for at least 400 years and the nation produces more potato than any other country, potato cyst nematodes were not reported in China until 2022.

Developing a One-Step Multiplex PCR Assay for Rapid Detection of Four Stubby-Root Nematode Species, Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus

Four trichodorid species, Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus, were found in multiple states in the United States. Traditional diagnosis based on morphology and morphometrics is laborious and requires an experienced taxonomist. Additionally, end-point diagnosis using PCR was only available for P. allius. To increase diagnostic efficiency and reduce costs, a one-step multiplex PCR assay was developed to simultaneously identify these four species using one PCR reaction. Available sequences of 18S ribosomal DNA and internal transcribed spacer 1 (ITS1) region of these species were aligned and five primers were designed. The conserved forward primer located in the 18S region, in combination with the species-specific antisense primer in the ITS1 region, amplified a single distinctive PCR fragment for each species (421/425 bp for P. allius, 190 bp for P. minor, 513 bp for P. porosus, and 353 bp for T. obtusus). In silico analysis with 10 other trichodorid species and experimental analysis using samples with these four species, 20 other plant-parasitic and three non-plant-parasitic nematodes demonstrated high specificity with the primers designed. The multiplex PCR amplified desirable fragments using a set of artificially mixed templates containing one, two, three, or four targeted species. The reliability of multiplex PCR results was demonstrated by using nematode populations isolated from infested fields from diverse geographic regions in eight states. The multiplex PCR-based tool developed in this study for the first time provides a simple, rapid, and cost-friendly assay for accurate diagnosis of the four major trichodorid nematodes in the United States.

Molecular Characterization and Identification of Stubby Root Nematode Species From Multiple States in the United States

Stubby root nematodes (SRN) are important plant parasites infecting many crops and widely distributed in many regions of the United States. SRN transmit Tobacco rattle virus, which causes potato corky ringspot disease, thereby having a significant economic impact on the potato industry. In 2015 to 2017, 184 soil samples and 16 nematode suspensions from North Dakota, Minnesota, Idaho, Oregon, Washington, South Carolina, North Carolina, and Florida were assayed for the presence of SRN. SRN were found in 106 soil samples with population densities of 10 to 320 SRN per 200 g of soil and in eight of the nematode suspensions. Sequencing of ribosomal DNA (rDNA) or species-specific polymerase chain reaction assays revealed the presence of four SRN species, including Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus. Accordingly, their rDNA sequences were characterized by analyzing D2-D3 of 28S rDNA, 18S rDNA, and internal transcribed spacer (ITS) rDNA obtained in this study and retrieved from GenBank. Both intra- and interspecies variations were higher in ITS rDNA than 18S rDNA and D2-D3 of 28S rDNA. Based on phylogenetic analysis, the four SRN species formed a monophyletic group, with P. allius more closely related to P. porosus than P. minor and T. obtusus. Indel variation of ITS2 rDNA was present in P. allius populations from the same geographic regions. This study documented the occurrence of SRN species across multiple states. The intra- and interspecies genetic diversity of rDNA in this study will provide more information for understanding the evolutionary relationships of SRN and will be valuable for future studies of SRN species identification and management.

First Report of Stubby-Root Nematode, Paratrichodorus minor, on Onion in Georgia, U.S.A

Onions ( Allium cepa L.) are the leading vegetable crop in Georgia accounting for 13.7% of total state vegetable production ( Wolfe and Stubbs, 2017 ). In November 2017, two samples each of onion (var. Candy Ann) seedlings and soil were received from the University of Georgia Cooperative Extension office in Tattnall County, GA. The samples were collected from a nursery fumigated with metam sodium and used for sweet onion transplant production. Symptoms of the damaged plants included stunted growth both in the root system and foliage, tip die-back of the leaves ( Fig. 1A,B ), and slight swelling at the tip of roots. Vermiform life stages from the soil samples were extracted using centrifugal-flotation technique ( Jenkins, 1964 ). On an average, 67 stubby-root nematodes per 100 cm 3 of soil were obtained. Additional two soil samples were collected from the nursery in December 2017 to confirm the presence of the nematode. On an average, 1 and 75 nematodes per 100 cm 3 of soil were recovered from areas with healthy and infested plants, respectively. Because the male individuals were not found in the soil samples, females were used for species identification. Morphological and molecular analyses of females ( Fig. 2A-C ) identified the species as Paratrichodorus minor (Colbran) Siddiqi; ( Decraemer, 1995 ). Nematode body shape was "cigar-shaped" with dorsally curved "onchiostyle" stylet Females had an oval-shaped vagina, vulva a transverse slit, and lateral body pores were absent. The measurements of females ( n = 20) included: body length 671.1 (570.1-785.3) µm; body width 32.5 (27.8-37.0) µm; onchiostyle 32.5 (31.1-34.8) µm; anterior end to esophagus-intestinal valve 117.6 (101.2-128.5) µm; a 21.5 (15.3-28.1) µm; b 5.2 (4.9-6.3) µm; V 52.9% (48.1-55.4%) µm; and vagina length 8.7 (7.8-10.7) µm. To confirm the identity of P. minor, DNA was extracted from single females ( n = 3) using Extract-N-Amp ™ Tissue PCR Kit (Sigma-Alredich Inc., St. Louis, MO). The partial 18S rRNA, the D2-D3 expansion segments of 28S rRNA, and ITS1 rDNA were amplified using primer pairs 360F (5' CTACCACATCCAAGGAAGGC 3')/932R (5' TATCTGATCGCTGTCGAACC 3'), D2A (5' ACAAGTACCGTGAGGGAAAGTTG 3')/D3B (5' TCGGAAGGAACCAGCTACTA 3'), and BL18 (5' CCCGTCGCTACTACCGATT 3')/5818 (5' ACGARCCGAGTGATCCAC 3'), respectively ( Riga et al., 2007 ; Duarte et al., 2010 ; Ye et al., 2015 ; Shaver et al., 2016 ). The obtained PCR fragments were purified using QIAquick Gel Extraction Kit (Qiagen Inc., Santa Clara, CA, USA), sequenced and deposited in the GenBank databases (18S rRNA: MG856931; 28S rRNA: MG856933; ITS1 rDNA: MH464152). The 18S rRNA, 28S D2-D3, and ITS1 rDNA sequences shared 99% similarity (100% coverage) with GenBank accessions of P. minor from California, Arkansas, and China (18S rRNA: JN123365; 28S D2-D3: JN123395; ITS1 rDNA: GU645811). In a pathogenicity test, five sweet onion seeds var. Pirate were planted (one per pot) in 11.5-cm-diameter polyethylene pots containing 1,000 cm 3 of equal parts of pasteurized field soil and sand, and then inoculated with 1,000 fresh P. minor . Plants were grown for 9 wk in a greenhouse at 25 ± 2°C prior to extraction of nematodes from soil. Plant roots were abbreviated and final population density of P. minor was 2,856 ± 104 per pot (285 nematodes/100 cm 3 of soil) confirming the nematode parasitism on onion. To our knowledge, this is the first report of P. minor parasitizing onion in Georgia. Stubby-root nematode ( Paratrichodorus sp.) has already been reported on corn, St. Augustine grass, and switchgrass in Georgia ( Heald and Perry, 1969 ; Davis and Timper, 2000 ; Mekete et al., 2011 ). In the U.S.A, P. minor is known to occur on diverse crops in most of the states ( Decraemer, 1995 ; CABI/EPPO, 2002 ). A survey of vegetable-producing areas in Georgia is currently under investigation to determine the distribution of this economically important nematode species. Figure 1Damage symptoms caused by stubby-root nematode Paratrichodorus minor on sweet onion in Georgia. A large area of stunted and chlorotic plant foliage (A); Infested seedlings with abbreviated roots and necrotic leaf tips (B). Figure 2Light microscopy micrographs showing morphological characters of stubby-root nematode, Paratrichodorus minor. Entire body (A), anterior end (B), and posterior region (C) of female nematode.