First report of the root-lesion Nematode, Pratylenchus fallax, on soybean in Wisconsin, USA

Authors: SAIKAI, KANAN - University Of Wisconsin; Handoo, Zafar; MACGUIDWIN, ANN - University Of Wisconsin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2019
Publication Date: 6/13/2020
Publication URL: http://handle.nal.usda.gov/10113/6471229
Citation: Saikai, K.K., Handoo, Z.A., MacGuidwin, A.E. 2019. First report of the root-lesion Nematode, Pratylenchus fallax, on soybean in Wisconsin, USA. Plant Disease. 103(8):1555. https://doi.org/10.1094/PDIS-02-19-0288-PDN.
DOI: https://doi.org/10.1094/PDIS-02-19-0288-PDN

Interpretive Summary: Plant-parasitic nematodes are microscopic worms that cause an estimated ten billion dollars of crop losses each year in the United States and 100 billion dollars globally. Lesion nematodes are the most economically important groups of plant-parasitic nematodes worldwide. One problem with lesion nematodes is that growers have no idea of how many kinds of nematodes exist and the host ranges of nematodes on specific crop cultivars. One problem with determining the extent of damage to crop plants is that the nematodes present in many areas are not known, such as in Wisconsin. This brief report describes how a team of Wisconsin State University and ARS scientists identified from a soybean field in Outagamie County, WI, a species of lesion nematode called Pratylenchus fallax by both morphological and molecular means. Anatomical features and measurements of females and males together with molecular analysis by sequencing were consistent with those of the lesion nematode, Pratylenchus fallax. The results are significant because this is the first report of soybean as a host of P. fallax and the molecular and morphological information obtained will allow this nematode to be more easily distinguished from closely related species. Therefore, this research will be used by scientists, growers, action agencies, and extension agencies involved in nematode research and control.

Technical Abstract: The root-lesion nematode (Pratylenchus spp.) is the most common nematode pest on soybean in Wisconsin. More than 100 root-lesion nematodes per 100 cm3 soil were detected from a sample received for nematode diagnostics from a soybean field in Outagamie County WI in October of 2013. Corn was planted in the previous year. Monoaxenic in vitro cultures were established from single females grown on “IOchief” sweet corn explants on Gamborg's B5 medium without auxins or cytokinin. Reproduction of the nematode was observed, and after three months there were many adult females and males. Nematodes were extracted from the culture by incubating roots for 48 hours on a Baermann funnel. Morphometrics were determined for 25 females and 25 males. The mean body length of females was 529 µm,(range = 438 to 615 µm), with a mean maximum body width of 20.1 µm, 15.3 to 23.3 µm). Mean values of a, b, and c ratios were 26.4, 5.4 and 21.6, respectively. The head region was 3.0 µm high, with two to three lip annules that were obscure in most specimens. The stylet had anteriorly flattened knobs with a mean length of 15.0 µm (13.1 to 16.0 µm). The excretory pore was at or posterior to the nerve ring. There were four lateral lines at mid-body. The vulva opening was located at 79% (77 to 81%) of the body length from anterior end. The post-uterine sac was smaller than the body width at the vulva with a mean length of 15.0 µm (10.6 to 20.0 µm). The female tail shape was conical or slightly irregular with round or striated tail tip and the tail length was 25.3 µm (23.6 to 30.1 µm). The mean body length and range of males was 454 µm (381 to 543 µm) and the stylet length was 13.4 µm (11.8 to 14.8 µm). The mean length and range of the spicule and gubernaculum, respectively were 16.6 µm (14.0 to 19.7 µm) and 4.2 µm (3.2 to 6.2 µm). The isolate was morphologically identified as P. fallax based on latest species keys. Molecular characterization based on 18S rDNA, the D2-D3 expansion region of 28S rDNA, and cytochrome c oxidase subunit I (COI) were conducted for five female nematodes collected from the monoaxenic cultures. The 18S sequence did not separate P. fallax from P. penetrans as the query sequence (GenBank Accession No. MK217948) for the two species were 99% similar to the Basic Local Alignment Search Tool (BLAST). The consensus sequences of COI (GenBank Accession No. MK041095) and 28S (GenBank Accession No. MK217949) were most closely related to P. fallax in the database (greater or less than 99% identical). This species has been reported on multiple crops in North Dakota but this is the first molecular information for a U.S. isolate of P. fallax. Specimens were sent to the Mycology & Nematology Generic Diversity & Biology Laboratory, USDA, ARS, Northeast Area (Beltsville, MD) for species confirmation, and validated as P. fallax. Soybean cv. Corsoy was inoculated with 1000 mixed stages of the isolate and grown at 24C in pasteurized loamy sandy soil. The reproductive factor 3 months after inoculation was 2.9 and many lesions were noted on roots, thus confirming soybean as a host of P. fallax. To our knowledge, this is the first report of soybean as a host of P. fallax as well as the first report of this species in Wisconsin. As P. fallax is recognized as an economically important Pratylenchus species in Europe, further study of its distribution and economic impact on soybean is warranted.