Taxonomy and Identification of Principal Foliar Nematode Species (Aphelenchoides and Litylenchus)

Discovery and Characterization of Four Aphelenchid Species from Cultivated Regions of Southern Alberta, Canada

The nematode family Aphelenchoididiae is considered fungal-feeding, predatory, or root hair feeders. Some members of this family are universally present in agricultural landscapes and are an integral part of soil health and conservation studies. In the present soil nematode biodiversity survey, we detected four species of the genera Aphelenchus, Aphelenchoides, and Robustodorus. Because fungal-feeding nematodes from southern Alberta have not previously been reported, we conducted a detailed morphological and molecular investigation, identifying these species as Aphelenchus avenae, Aphelenchoides limberi, Aphelenchoides prairiensis n. sp. and Robustodorus paramegadorus n. sp. The first two species we document as new records from southern Alberta, whereas A. prairiensis n. sp. and R. paramegadorus n. sp. we describe in detail as new taxa. Briefly, A. prairiensis n. sp. is an amphimictic species having 4 lateral lines; hemispherical anteriorly flattened lip region; delicate stylet and swelling-like stylet knobs; excretory pore at the posterior edge of nerve ring. Female tail conical, gradually tapering towards a truncated end with single mucro. Spicule 23.0 (20.0-25.0) µm long having elongated rounded condylus, small, blunt conical rostrum, and lamina that gradually tapers towards the rounded distal end; three pairs of caudal papillae were present on the male tail. Robustodorus paramegadorus n. sp., is a parthenogenetic species with 3 lines in the lateral fields; lip region rounded, anteriorly flattened; stylet robust, with knobs rounded to bean-shaped; excretory pore located posterior to nerve ring; reproductive components were quite indiscernible with a short 24.0 (18.0-27.0) µm post-vulval uterine sac; tail conical, ending with pointed to wedge-shaped tip. We performed molecular characterizations for each species and constructed phylogenetic trees to study the phylogenetic relationship of these aphelenchid species. The discovery of A. prairiensis n. sp. and R. paramegadorus n. sp. indicates that soil nematode diversity is relatively unexplored in southern Alberta. The findings of this study will significantly enhance the identification processes and may contribute towards future soil health and biodiversity efforts.

Rapid Detection of the Strawberry Foliar Nematode Aphelenchoides fragariae Using Recombinase Polymerase Amplification Assay with Lateral Flow Dipsticks

Rapid and reliable diagnostic methods for plant-parasitic nematodes are critical for facilitating the selection of effective control measures. A diagnostic recombinase polymerase amplification (RPA) assay for Aphelenchoides fragariae using a TwistAmp® Basic Kit (TwistDx, Cambridge, UK) and AmplifyRP® Acceler8® Discovery Kit (Agdia, Elkhart, IN, USA) combined with lateral flow dipsticks (LF) has been developed. In this study, a LF-RPA assay was designed that targets the ITS rRNA gene of A. fragariae. This assay enables the specific detection of A. fragariae from crude nematode extracts without a DNA extraction step, and from DNA extracts of plant tissues infected with this nematode species. The LF-RPA assay showed reliable detection within 18-25 min with a sensitivity of 0.03 nematode per reaction tube for crude nematode extracts or 0.3 nematode per reaction tube using plant DNA extracts from 0.1 g of fresh leaves. The LF-RPA assay was developed and validated with a wide range of nematode and plant samples. Aphelenchoides fragariae was identified from seed samples in California. The LF-RPA assay has great potential for nematode diagnostics in the laboratory with minimal available equipment.

First Report of Aphelenchoides Bicaudatus (Imamura, 1931) Filipjev and Schuurmans Stekhoven, 1941 Associated with Grass in South Africa

Aphelenchoides bicaudatus associated with grass in South Africa was identified morphologically and molecularly. This population is characterized by a body length of 409 - 529 μm, a stylet length of 9.5 - 13 μm, a post-vulval uterine sac of 45 - 50 μm, and the characteristic tail bifurcated at the end with one prong longer than the other. Molecular analyses based on the 18S and ITS rDNA data confirmed the primary morphological identification of the A. bicaudatus species. The obtained phylogenetic trees revealed a close positioning of the South African population to other representatives of A. bicaudatus with the maximum (1.00) posterior probability value. Principal component analysis (PCA) also indicated a variation within the populations of A. bicaudatus. This is the first report of A. bicaudatus from South Africa.

Top Ten Most Important U.S.-Regulated and Emerging Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) are important pests that cause an estimated ten billion dollars of crop loss each year in the United States and over 100 billion dollars globally. The Animal and Plant Health Inspection Service (APHIS) within the U.S. Department of Agriculture maintains and updates the U.S. Regulated Plant Pest list. Currently, the number of PPNs regulated by APHIS includes more than 60 different species. This review focuses on the top ten most economically important regulated and emerging plant-parasitic nematodes and summarizes the diagnostics of morphological and some molecular features for distinguishing them. These ten major previously described nematode species are associated with various economically important crops from around the world. This review also includes their current distribution in the U.S. and a brief historical background and updated systematic position of these species. The species included in this review include three PPNs considered by the U.S. Department of Agriculture as invasive invertebrates Globodera pallida, Globodera rostochiensis, and Heterodera glycines; four regulated PPNs, namely Bursaphelenchus xylophilus, Meloidogyne fallax, Ditylenchus dipsaci, and Pratylenchus fallax; and the three emerging PPNs Meloidogyne chitwoodi, Meloidogyne enterolobii, and Litylenchus crenatae mccannii.

Morphological and Molecular Characterization of Pratylenchus dakotaensis n. sp. (Nematoda: Pratylenchidae), a New Root-Lesion Nematode Species on Soybean in North Dakota, USA

Root-lesion nematodes (Pratylenchus spp.) of the genus Pratylenchus Filipjev, 1936, are among the most important nematode pests on soybean (Glycine max (L.) Merr.), along with soybean cyst and root-knot nematodes. In May 2015 and 2016, a total of six soil samples were collected from a soybean field in Walcott, Richland County, ND and submitted to the Mycology and Nematology Genetic Diversity and Biology Laboratory (MNGDBL), USDA, ARS, MD for analysis. Later, in 2019, additional nematodes recovered from a greenhouse culture on soybean originally from the same field were submitted for further analysis. Males, females, and juveniles of Pratylenchus sp. were recovered from soil and root samples and were examined morphologically and molecularly. DNA from single nematodes were extracted, and the nucleotides feature of three genomic regions targeting on the D2–D3 region of 28S rDNA and ITS rDNA and mitochondrial cytochrome oxidase subunit I (COX1) gene were characterized. Phylogeny trees were constructed to ascertain the relationships with other Pratylenchus spp., and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was performed to provide a rapid and reliable differentiation from other common Pratylenchus spp. Molecular features indicated that it is a new, unnamed Pratylenchus sp. that is different from morphologically closely related Pratylenchus spp., including P. convallariae, P. pratensis, P. fallax, and P. flakkensis. In conclusion, both morphological and molecular observations indicate that the North Dakota isolate on soybean represents a new root-lesion nematode species which is named and described herein as Pratylenchus dakotaensis n. sp.