Rhg1 has been the most effective QTL deployed in soybeans (Glycine max) to control soybean cyst nematode (Heterodera glycines; SCN). However, the resistance mechanisms and specificity of Rhg1 towards SCN as opposed to other plant parasitic nematodes are not well known. In this study we report that Rhg1 can hinder the parasitism of the root lesion nematode Pratylenchus penetrans (Pp), which is commonly found along with SCN in northern U.S. soybean fields. Two elite public soybean varieties, carrying either rhg1-a + Rhg4 or rhg1-b, negatively impacted Pp populations measured at 30 dpi. This contribution of Rhg1 was more rigorously demonstrated using near isogenic lines for rhg1-b. Additionally, in three of the four soybean genetic backgrounds tested we observed significantly more Pp at 30 dpi when Pp were co-inoculated with SCN, consistent with a previous study. No significant changes in SCN cyst numbers upon co-inoculation with Pp were observed for any of the SCN-resistant rhg1 lines. No nematode-induced changes in Rhg1 transcript abundances were observed when sampling infected root sections in any of the plant genotypes, and no changes in a jasmonate response indicator were observed. However, the 3 dpi salicylate-mediated response to SCN or Pp became more robust during SCN + Pp co-infection. Multiple hypotheses for further dissection and manipulation of soybean resistance to Pratylenchus root lesion nematodes are suggested based on the present findings.

The prevalence of Heterodera glycines and other cyst and vermiform genera was determined from 8,009 soil samples over two decades. Prevalence of cyst nematodes for farms increased from 16% in 1998 to 1999, reaching a peak of 40%, with marked differences among Wisconsin’s nine agricultural districts in how much the odds of a positive test increased. Estimates at the sample scale also increased over time but peaked at 29%. Assay of all nematodes beginning in 2012 showed Pratylenchus, Helicotylenchus, and Xiphinema to be more prevalent in Wisconsin soybean fields than cyst nematodes. Prevalence estimates for Pratylenchus and Helicotylenchus for soybean and rotation crops ranged from 76 to 89% and 58 to 83%, respectively. Species identification of Pratylenchus from a subset of the samples revealed six species. The majority of cyst-positive samples were infested with Pratylenchus, and count data showed that the number of cyst eggs and juveniles per 100 cm3 soil was 60% lower in samples positive for Pratylenchus. The influence was reciprocal, as Pratylenchus population densities were 41% lower in samples positive for cyst nematodes, suggesting a competitive interaction. The Wisconsin soybean nematode testing program provides a useful model for estimating nematode prevalence using citizen-based surveys.

Models were developed to quantify the impact of Pratylenchus penetrans to the early season growth and yield of soybean in field and greenhouse environments and to estimate yield loss because of P. penetrans in Wisconsin. There was a negative linear relationship between initial nematode population densities (Pi) and shoot and total plant weight at V2 and yield, pod number, seed number, and seed mass at harvest in the field. Relative yield loss, modeled for the second year of the field experiment, suggested a loss of 4.5% for yield and between 2.4 and 2.8% for yield components at the mean field Pi value. Negative linear relationships were demonstrated for the relative loss in those variables as well as for harvest index and shoot, root, and total plant weight at harvest in the greenhouse. Stress imposed by P. penetrans began within 2 weeks after planting and continued through harvest. Estimates of the percent loss attributed to each nematode Pi value were 0.020% for yield, 0.015% for pod number, and 0.017% for seed number. Pratylenchus spp. was the most widely prevalent pest nematode among samples submitted to a statewide nematode testing program. Molecular identification of a subset of 63 samples suggested 15% were infested with P. penetrans at a mean Pi value of 197 P. penetrans per 100 cm3 soil. Yield loss because of P. penetrans, estimated from prevalence data and our empirical greenhouse model, ranged from 0.23 to 2.76% among Wisconsin’s agricultural districts. The cumulative impact for all Pratylenchus spp. is likely much greater, given this loss estimate does not account for the monoecious species present in 79% of the samples.

Pratylenchus smoliki is a new species of root-lesion nematode described from corn-soybean production fields in the Central Great Plains of North America. It is characterized by populations with relatively abundant males, two lip annuli, females with a round functional spermatheca and a conoid to subcylindrical tail with a non- crenate, smooth terminus. In host preference tests, corn and wheat produce the largest nematode populations, whereas sorghum and soybeans produce less than 20% the numbers observed on corn. Scanning electron microscopy reveals that the en face patterns compare to those seen in Pratylenchus pseudocoffeae, P. scribneri, P. hexincisus, and P. alleni. The pattern is described as rectangular to trapezoidal subdorsal and subventral lips adjoining oral disc, but with a clear demarcation between the oral disc and the subdorsal and subventral sectors. A Maximum Likelihood COI tree recognizes P. smoliki as a moderately-well-supported clade with several haplotype subgroups. A Maximum Likelihood partial 28S tree provides strong support for the P. smoliki clade and reinforces the close relationships between species with similar en face patterns. Topotype specimens of P. alleni were demonstrably different from P. smoliki using DNA markers. The geographic range of P. smoliki overlaps with the ranges of P. alleni, P. scribneri, P. neglectus, P. hexicisus, and P. dakotaensis. The observed host range (corn, rye, sunflower, and wheat) suggests that P. smoliki may be native to the tallgrass prairie region of the Great Plains.

Root lesion nematode virus 1 (RLNV1) was discovered in the migratory endoparasitic nematode species Pratylenchus penetrans. It was found in a P. penetrans population collected from soil samples in Beltsville, Maryland, USA. In this study, the distribution of the RLNV1 in 31 geographically distinct P. penetrans populations obtained from different crops was examined. The results demonstrate that RLNV1 is widespread in North American populations of P. penetrans and exhibits low genetic variability in the helicase and RNA-dependent RNA polymerase regions of the genome.

Pratylenchus penetrans is a common and important agricultural pest in Wisconsin, a USA state with a diverse agriculture. We compared populations from around the state to each other and to data published for populations around the world to gain insight on the variability of features important for identification of this cosmopolitan species. Thirteen isolates from samples collected in soybean fields in ten Wisconsin counties were established in monoxenic cultures. Analysis of morphological features revealed the least variable feature for all isolates collectively was vulva percentage. Features less variable within than among isolates were body width, lip region height, and stylet length. Some isolates showed only the smooth tail tip phenotype and others had a mix of smooth and annulated tail phenotypes. A suite of features provided sufficient pattern to group isolates into four clusters according to hierarchical agglomerative clustering and canonical discriminative analyses, but not with enough distinction to be useful for classification. Haplotype analysis based on the COI mitochondrial gene of the 13 cultured isolates, 39 Wisconsin field populations, and published sequences representing five additional USA states and six countries revealed 21 haplotypes, 15 of which occurred in Wisconsin. Ten haplotypes represented in Wisconsin were shared with populations from Europe, South America, Africa, or Asia. Five haplotypes were unique to Wisconsin, six were unique to The Netherlands, and one was unique to Japan suggesting that even more COI diversity will be revealed when more COI sequences for P. penetrans become available. The maximum pairwise sequence variation was 6% and the SNPs did not alter amino acids, indicating cryptic biodiversity within the species worldwide. The cosmopolitan to localized scale of distribution of COI haplotypes could be due to frequent and ongoing dispersal events, facilitated by life history traits and the broad host range of P. penetrans. Regions of diverse agriculture, like Wisconsin, show promise for studying this important pest and our study confirms the utility of the COI mtDNA gene for studying variation within a species.

Pratylenchus penetrans induce necrotic lesions, the hallmark symptom for the genus, soon after infection. The objective of our study was to characterize and quantify gender differences in lesion development. Independent experiments were conducted in vitro for three hosts; pea (Pisum sativum L. cv. Early Alaskan), dill (Anethum graveolens cv. Long Island Mammoth), and alfalfa (Medicago sativa cv. Vernal). Each experimental unit was an excised radical placed on water agar in a Petri dish and inoculated with either 40 adult males or 40 fourth-stage juvenile females. Length, size, and number of lesions were recorded during the experiment and the radicals were harvested 14 days after introducing nematodes. Lesions were first observed on pea after two days for female-inoculated roots, and 24 hr after introducing both genders to dill and alfalfa. Lesions expanded either by multiple lesions coalescing or individual lesions expanding over time. Males made fewer, smaller lesions with less discoloration for all three hosts. There was no difference among genders for the total number of nematodes recovered per Petri dish or the number of endoparasitic nematodes after 14 days. The survival rate of males and females at harvest was not different, indicating that the difference in lesion formation was not related to nematode population densities. This study verified and quantified the observation that lesions induced by males are less extensive and in smaller numbers than lesions by females.

HomePlant DiseaseVol. 103, No. 8First Report of the Root-Lesion Nematode, Pratylenchus fallax, on Soybean in Wisconsin, U.S.A. PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of the Root-Lesion Nematode, Pratylenchus fallax, on Soybean in Wisconsin, U.S.A.K. Saikai, Z. A. Handoo, and A. E. MacGuidwinK. SaikaiDepartment of Plant Pathology, University of Wisconsin–Madison, Madison, WI 53706Search for more papers by this author, Z. A. HandooMycology and Nematology Genetic Diversity and Biology Laboratory, USDA-ARS, Beltsville, MD 20705Search for more papers by this author, and A. E. MacGuidwin†Corresponding author: A. E. MacGuidwin; E-mail Address: [email protected]http://orcid.org/0000-0002-4807-8953Department of Plant Pathology, University of Wisconsin–Madison, Madison, WI 53706Search for more papers by this authorAffiliationsAuthors and Affiliations K. Saikai1 Z. A. Handoo2 A. E. MacGuidwin1 † 1Department of Plant Pathology, University of Wisconsin–Madison, Madison, WI 53706 2Mycology and Nematology Genetic Diversity and Biology Laboratory, USDA-ARS, Beltsville, MD 20705 Published Online:13 Jun 2019https://doi.org/10.1094/PDIS-02-19-0288-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat 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 of soil were detected from a sample received for nematode diagnostics from a soybean field in Outagamie County, WI, in October 2013. Corn was planted in the previous year. Monoxenic 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 3 months there were many adult females and males. Nematodes were extracted from the culture by incubating roots for 48 h on a Baermann funnel. Morphometrics were determined for 25 females and 25 males: measurements are presented as means (range). The body length of females was 529 μm (438 to 615 μm), with a maximum body width of 20.1 μm (15.3 to 23.6 μm). The a, b, and c ratios were 26.4 (24.1 to 30.1), 5.4 (4.3 to 7.5), and 21.6 (16.3 to 28.3), respectively. The head region was 3.0 μm high (2.1 to 3.9 μm), with two to three lip annules that were obscure in most specimens. The stylet had rounded knobs with a length of 15.0 μm (13.8 to 16.4 μm). The excretory pore was at or posterior to the nerve ring. There were four lateral lines at midbody. The vulva opening was located at 79% (77 to 81%) of the body length from the anterior end. The postuterine sac was smaller than the body width at the vulva, with a length of 15.0 μm (10.6 to 20.0 μm). The female tail shape was conical or slightly irregular with a round or striated tail tip, and the tail length was 29 μm (23.5 to 36.7 μm) with 23 annules (20 to 30). The body length of males was 454 μm (381 to 543 μm), and the stylet length was 13.4 μm (11.8 to 14.8 μm). The 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 Pratylenchus fallax based on species keys (Castillo and Vovlas 2007; Handoo and Golden 1989). Molecular characterization based on 18S rDNA (Chizhov et al. 2006), the D2–D3 expansion region of 28S rDNA (Subbotin et al. 2008), and cytochrome c oxidase subunit I (COI) (Derycke et al. 2010) was conducted for five individual nematodes collected from the monoxenic cultures. DNA was extracted using the DNeasy Blood and Tissue Kit (Qiagen). The 18S sequence (GenBank accession no. MK217948) was 98% identical to P. fallax and 97% identical to P. penetrans, P. convallariae, and P. pinguicaudatus according to the Basic Local Alignment Search Tool (BLAST). The COI sequence (GenBank accession no. MK041095) was 99% identical to P. fallax (87% identical to P. pinguicaudatus; 86% identical to P. penetrans and P. convallariae). The 28S sequence (GenBank accession no. MK217949) was 99% identical to P. fallax (97% identical to P. penetrans and P. pinguicaudatus; 95% identical to P. convallariae). The species-specific primer Ppen for P. penetrans based on the D3 region of 28S rDNA (Al-banna et al. 2004) was negative for our P. fallax isolate. This species was reported beneath oak in North Dakota (Donald and Hosford 1980), but this is the first molecular information for a U.S. isolate of P. fallax. Specimens were sent to the Mycology and Nematology Generic Diversity and Biology Laboratory, USDA, ARS, Northeast Area (Beltsville, MD) for species confirmation and was validated as P. fallax. Three soybean plants (cv. Corsoy) were inoculated with 1,000 mixed stages of the isolate and grown at 24°C in pasteurized loamy sand soil. The reproductive factor 3 months after inoculation was 2.94 (0.185 standard deviation), thus confirming soybean as a host of P. fallax. Lesions were noted only on plants that were inoculated. 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. Because P. fallax is recognized as an economically important Pratylenchus species in Europe, further study of its distribution and economic impact on soybean is warranted.The author(s) declare no conflict of interest.References:Al-banna, L., et al. 2004. J. Nematol. 36:142. ISI, Google ScholarCastillo, P., and Vovlas, N. 2007. Page 289 in: Pratylenchus (Nematoda: Pratylenchidae): Diagnosis, Biology, Pathogenicity and Management. Koninklijke Brill NV, Leiden, the Netherlands. https://doi.org/10.1163/ej.9789004155640.i-523 Crossref, Google ScholarChizhov, V. N., et al. 2006. Russ. J. Nematol. 14:179. ISI, Google ScholarDerycke, S., et al. 2010. PLoS One 5:e13716. https://doi.org/10.1371/journal.pone.0013716 Crossref, ISI, Google ScholarDonald, P. A., and Hosford, R. M., Jr. 1980. Plant Dis. 64:45. https://doi.org/10.1094/PD-64-45 Crossref, ISI, Google ScholarHandoo, Z. A., and Golden, A. M. 1989. J. Nematol. 21:202. ISI, Google ScholarSubbotin, S. A., et al. 2008. Mol. Phylogenet. Evol. 48:491. https://doi.org/10.1016/j.ympev.2008.04.028 Crossref, ISI, Google ScholarThe author(s) declare no conflict of interest.Funding: This work is supported by Hatch Act Multistate Funding (WIS01993/MSN201561) from the USDA National Institute of Food and Agriculture.DetailsFiguresLiterature CitedRelated Vol. 103, No. 8 August 2019SubscribeISSN:0191-2917e-ISSN:1943-7692 DownloadCaptionPistachio fruit infected by Neofusicoccum mediterraneum (Moral et al.). Photo credit: T. J. Michailides. Leaf blight on Hosta ventricosa caused by Pseudomonas syringae pv. syringae (Liu et al.). Photo credit: Z. X. Liu. Metrics Article History Issue Date: 1 Aug 2019Published: 13 Jun 2019First Look: 12 Apr 2019Accepted: 10 Apr 2019 Pages: 2141-2141 Information© 2019 The American Phytopathological SocietyKeywordsnematodesfield cropsoilseeds and legumespathogen detectionThe author(s) declare no conflict of interest.Cited byTop Ten Most Important U.S.-Regulated and Emerging Plant-Parasitic Nematodes26 February 2022 | Horticulturae, Vol. 8, No. 3

In a search for an entomopathogenic nematode to control cranberry insect pests, three Oscheius populations (Rhabditidae) were recovered through the Galleria-bait method from one sample taken in a wild cranberry marsh in Jackson County, Wisconsin, USA. Morphological studies with light microscopy and scanning electron microscopy, as well as molecular analyses of the near-full-length small subunit rDNA gene, D2/D3 expansion segments of the large subunit rDNA gene, internal transcribed spacer, and mitochondrial cytochrome oxidase subunit 1 (CoxI) genes revealed this as Oscheius onirici, a species recently described from a karst cave soil of central Italy. The species belongs to the dolichura-group and is characterized by its DNA sequences; hermaphroditic reproduction; and males not found. A Bacillus-like bacterium appears to be associated with this nematode based on our microscopic and SEM observations; however its identity and persistent association with the nematode has not been confirmed. Nonetheless, this nematode is capable of infecting and killing the sparganothis fruitworm Sparganothis sulfureana Clemens (Lepidoptera: Tortricidae), the brown-banded cockroach Supella longipalpa Fabricius (Blattodea: Ectobiidae), and the cranberry fruitworm Acrobasis vaccinii Riley (Lepidoptera: Pyralidae), under laboratory conditions, and each in less than 72 hr. The mealworm Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae) and the greater wax moth Galleria mellonella Linnaeus (Lepidoptera: Pyralidae), are also susceptible, but take 3.5 and 5.2 days to die, respectively. This species is a new potential bio-control agent on insects.

The lesion nematode Pratylenchus penetrans is a common pest of corn in the north-central United States. There are relatively few studies documenting the impact of Pratylenchus spp. on grain yield even though they are recognized as pests of corn and the target of commercial seed treatments. We adapted a component error modeling approach to develop a damage function for P. penetrans that included the influence of year and site in the yield loss relationship. Field data from six site-years was used to derive panel data consisting of all pairwise comparisons of the difference in nematode population densities and the associated proportional yield difference. Fourteen regression models of the relationship between proportional yield loss and the difference in nematode density were developed from soil and root assays at different corn growth stages. Seven models were significant: four models based on nematode population densities in soil (initial and final samples) and three based on nematode densities in seminal roots (corn growth stages V1 to V2 and V6) and adventitious roots (corn growth stage R1 to R2). The model we consider to be the most important, that based on the initial soil assay, estimated the yield loss caused by each nematode to be 0.0142%. The grand mean of the 118 plots we sampled implied a yield loss of 3.79%. The random effects of year and field did not contribute significantly to any of the models but were close to significance for some, suggesting a benefit from larger data sets. Experimental error was the largest component of the variance for all of the models; therefore, the damage function is more useful for demonstrating impact of P. penetrans rather than for accurately predicting yield loss at the field level. All of the fields in our study were an irrigated loamy sand soil, with grain yields above the county average; therefore, it is possible that our damage function is conservative. The value of soil sampling has been questioned for P. penetrans and this study shows it to be equal to if not better than root assays for predicting yield.

Fusarium virguliforme (syn. Fusarium solani f. sp. glycines), the causal agent of sudden death syndrome, and Heterodera glycines, soybean cyst nematode (SCN), are economically important pathogens of soybean (Glycine max) in the Midwestern United States, including Wisconsin. In 2011 and 2012, samples submitted to a SCN detection program were assayed for SCN using a sieving/centrifugation method and for F. virguliforme using a real-time quantitative polymerase chain reaction (RT-qPCR) protocol. In 2011, 135 soil samples were submitted and H. glycines was detected in 56 samples, while 10 samples were positive for F. virguliforme. In 2012, 64 of 318 samples tested positive for H. glycines and 13 tested positive for F. virguliforme. The relationship between the occurrence of H. glycines and F. virguliforme was examined further for samples that were positive for H. glycines and/or F. virguliforme. Kendall's tau rank correlation coefficient was −0.59 (P < 0.01), indicating a negative association. Furthermore, the best-fitting logistic regression model that described the probability of detecting H. glycines in a soil sample based on detecting F. virguliforme confirmed the negative correlation. This result suggests that SCN and F. virguliforme do not rely on each other to colonize fields, indicating that fields heavily infested with SCN are not necessarily at greater risk of F. virguliforme colonization. Accepted for publication 24 October 2013. Published 28 January 2014.

Phosphorus (P) fertilization is essential for societal sustainability. However, plant P uptake is inefficient due to poor soil P solubility, especially for the potato (Solanum tuberosum L.) plant due to its relatively poor rooting efficiency. Phosphorus use efficiency (PUE) can be increased with rhizosphere modifications such as: (1) raising the bulk pH of acid soils, (2) placement of fertilizer in the rhizosphere by broadcast incorporation and/or, especially, as a concentrated band, (3) use of slow and controlled release P fertilizers, (4) use of organic acids to increase P solubility in alkaline soils, (5) use of an acid polymer, and (6) extending the effective rhizosphere zone via any practice that fosters root growth, such as through promotion of mycorrhizal development. Alone or in combination, these techniques offer an opportunity to increase PUE.

Root lesion nematodes are versatile parasites that move freely between root and soil habitats. Most laboratories conduct separate assays for soil or root tissue, using time-of-year as the selecting factor. We used a dual assay that simultaneously extracts nematodes from soil and root fragments in soil samples to identify the value of soil versus root tests using 920 research samples collected 1 April to 15 May, and 853 clinic samples collected year round. Nematodes were recovered from both soil and root fragments regardless of the time of year or origin of the sample. When the data were summarized by cohort, the mean percentage of nematodes recovered from root fragments was 65% for the research samples, 59% for clinic samples submitted 1 March to 15 June, 56% for clinic samples submitted 16 June to 31 July, and 49% for clinic samples submitted after 1 August. Both the incidence and population density of root lesion nematodes was underestimated if only the soil or only the root fraction was considered, indicating the need for testing methods that consider both habitats. The variability among samples for the distribution of nematodes between root and soil habitats was high, negating the option of running one assay and using a constant scaling factor to account for the other. Accepted for publication 30 October 2012. Published 20 November 2012.

We used cover crops with demonstrated efficacy against Verticillium dahliae and Pratylenchus penetrans in combination with the biocidal practice of solarization to determine the importance of targeting both organisms for managing potato early dying, an issue relevant to the search for alternatives to soil fumigation. Two experiments were conducted in commercial fields using a split-plot design with cover crop treatments of rapeseed, marigold, forage pearl millet, sorghum-sudangrass, and corn as the main plot factor and solarization as the subplot factor. Cover crops were grown and solarization applied in year one, followed by potato in year two. The main effect of solarization was significant for reduced inoculum levels of both organisms in year two and increased tuber yields. The main effect of cover crop was also significant with lower population densities of P. penetrans following the marigold and millet treatments and of V. dahliae following rape and sorghum-sudangrass. The cover crop treatments influenced yield in only one of the experiments in the absence of solarization. The combinatorial effect of cover crops and solarization resulted in a wide range of pathogen population densities. Mean soil inoculum levels were negatively related to yield for V. dahliae in experiment 1, and for P. penetrans and the P. penetrans × V. dahliae interaction in both experiments.