First Report of the Root-Lesion Nematode, Pratylenchus alleni, on Soybean in Wisconsin, U.S.A

Root-lesion nematodes (Pratylenchus spp.) are economically important nematode pathogens on a wide range of crops. In soybean fields in Wisconsin, it is the most common nematode pest present. In 2013, root-lesion nematodes were detected during routine diagnostics from a soil sample collected in a soybean field in Grant Co., WI. Numerous males were observed, and all life stages appeared smaller than what is typical for Pratylenchus penetrans in Wisconsin. Cultures were established by placing a single adult female in a Petri dish containing ‘IOChief’ sweet corn root explants growing on Gamborg’s B5 medium without auxins or cytokinin. After 3 months, infected root pieces in agar were transferred to new in vitro root cultures on Gamborg’s B5 medium to continue to increase the population. Nematodes were collected for morphological and molecular identification by incubating roots on a Baermann funnel for 48 h. Morphometric data were collected for 25 females and 25 males; measurements are presented as means (range). The body length of females was 482.1 μm (427.8 to 540.6 μm), with a maximum body width of 20.3 μm (18.4 to 22.6 μm). Values of a, b, and c ratios were 23.8 (21.8 to 26.3), 5.1 (4.5 to 6.0), and 20.1 (16.0 to 23.6), respectively. The height of the head was 2.3 μm (1.9 to 2.7 μm), with two lip annules. The stylet length was 14.1 μm (13.4 to 15.1 μm) with anteriorly flattened knobs. The lateral field was marked with four lines at midbody. The vulva percentage was 79% (77 to 81%). Females had a round spermatheca filled with sperm and a postuterine sac with a length of 13.3 μm (8.7 to 19.9 μm) that was slightly shorter than the body width at the vulva. The female tail was subcylindrical or slightly conical with a smooth tail tip and a tail length of 24.2 μm (20.8 to 31.7 μm) with 20 annules (16 to 25). Males were shorter with a body length of 418.0 μm (373.2 to 518.4 μm) and stylet length of 12.6 μm (11.5 to 14.0 μm). The spicules were arched, 16.5 μm long (12.8 to 19.4 μm), and the length of the gubernaculum was 4.0 μm (3.0 to 5.3 μm). The isolate was identified as Pratylenchus alleni based on morphological characters (Castillo and Vovlas 2007; Handoo and Golden 1989). Molecular characterization was based on 18S rDNA (Chizhov et al. 2006), the D2 to D3 expansion region of 28S rDNA (Subbotin et al. 2008), and cytochrome c oxidase subunit I (Derycke et al. 2010) from five specimens. The 28S sequence (GenBank accession no. MK037385) was 99% identical to P. alleni reported in Canada (Dauphinais et al. 2018) with a difference of two nucleotides. The 18S sequence (GenBank accession no. MK035844) was 98% similar to P. scribneri, but the COI sequence (GenBank accession no. MK045330) was only 84% similar to P. scribneri. There were no sequences for either loci for P. alleni available in GenBank. DNA samples of a Canadian isolate of P. alleni obtained from Agriculture and Agri-Food Canada, Saint-Jean-sur-Richelieu Research and Development Centre (Saint-Jean-sur-Richelieu, Canada) were 99 and 100% identical to the 18S and COI sequences of the Wisconsin isolate, respectively. Therefore, we identified the Wisconsin isolate as P. alleni based on the morphological and molecular data. Three soybean plants of cultivar ‘Corsoy’ were inoculated with 1,000 mixed-stage nematodes of the isolate and grown at 24°C with a 14-h photoperiod in pasteurized loamy sand soil. The reproductive factor (final population density/initial population density) 3 months after inoculation was 3.8, and many lesions were observed on roots. To our knowledge, this is the first report of P. alleni on soybean in Wisconsin. Further investigation about the economic impact of this species to soybean production and its distribution in Wisconsin are planned.