Models were developed to quantify the impact of Pratylenchus penetrans on the early season growth and yield of soybean in field and greenhouse environments and to estimate yield loss due to 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% to 2.8% for yield components at the mean field Pi. 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 two weeks after planting and continued through harvest. Estimates of the percent loss attributed to each nematode Pi 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 of 197 P. penetrans per 100 cm3 soil. Yield loss due to 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.

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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.