Discovery of a major QTL for resistance to the guava root-knot nematode (Meloidogyne enterolobii) in 'Tanzania', an African landrace sweetpotato (Ipomoea batatas).

Sweetpotato, Ipomoea batatas (L.) Lam. (2n = 6x = 90), is among the world's most important food crops and is North Carolina's most important vegetable crop. The recent introduction of Meloidogyne enterolobii poses a significant economic threat to North Carolina's sweetpotato industry and breeding resistance into new varieties has become a high priority for the US sweetpotato industry. Previous studies have shown that 'Tanzania', a released African landrace, is resistant to M. enterolobii. We screened the biparental sweetpotato mapping population, 'Tanzania' x 'Beauregard', for resistance to M. enterolobii by inoculating 246 full-sibs with 10,000 eggs each under greenhouse conditions. 'Tanzania', the female parent, was highly resistant, while 'Beauregard' was highly susceptible. Our bioassays exhibited strong skewing toward resistance for three measures of resistance: reproductive factor, eggs per gram of root tissue, and root gall severity ratings. A 1:1 segregation for resistance suggested a major gene conferred M. enterolobii resistance. Using a random-effect multiple interval mapping model, we identified a single major QTL, herein designated as qIbMe-4.1, on linkage group 4 that explained 70% of variation in resistance to M. enterolobii. This study provides a new understanding of the genetic basis of M. enterolobii resistance in sweetpotato and represents a major step towards the identification of selectable markers for nematode resistance breeding.

A Comparison of Three Automated Root-Knot Nematode Egg Counting Approaches Using Machine Learning, Image Analysis, and a Hybrid Model.

Meloidogyne spp. (root-knot nematodes [RKNs]) are a major threat to a wide range of agricultural crops worldwide. Breeding crops for RKN resistance is an effective management strategy, yet assaying large numbers of breeding lines requires laborious bioassays that are time-consuming and require experienced researchers. In these bioassays, quantifying nematode eggs through manual counting is considered the current standard for quantifying establishing resistance in plant genotypes. Counting RKN eggs is highly laborious, and even experienced researchers are subject to fatigue or misclassification, leading to potential errors in phenotyping. Here, we present three automated egg counting models that rely on machine learning and image analysis to quantify RKN eggs extracted from tobacco and sweet potato plants. The first method relied on convolutional neural networks trained using annotated images to identify eggs (M. enterolobii R2 = 0.899, M. incognita R2 = 0.927, M. javanica R2 = 0.886), whereas a second contour-based approach used image analysis to identify eggs from their morphological characteristics and did not rely on neural networks (M. enterolobii R2 = 0.977, M. incognita R2 = 0.990, M. javanica R2 = 0.924). A third hybrid model combined these approaches and was able to detect and count eggs nearly as well as human raters (M. enterolobii R2 = 0.985, M. incognita R2 = 0.992, M. javanica R2 = 0.983). These automated counting protocols have the potential to provide significant time and resource savings annually for breeders and nematologists and may be broadly applicable to other nematode species.

A Comparison Between Meloidogyne floridensis and M. incognita from California on Susceptible and Resistant Sweetpotato Cultivars.

The reproduction and ability to cause root-galling of a California isolate of the peach root-knot nematode Meloidogyne floridensis was evaluated on seven sweetpotato (Ipomea batatas) cultivars and compared with an M. incognita race 3 and an M. incognita Mi-gene resistance-breaking isolate. The susceptible tomato (Solanum lycopersicum) cultivar Daniela and the Mi-gene-carrying resistant cultivar Celebrity were included as controls. Repeated trials were done in pots in a nematode-quarantine greenhouse at the University of California, Riverside. The three Meloidogyne isolates reproduced equally well on susceptible tomato. On Mi-gene resistant tomato, the reproduction and root-galling by M. floridensis was intermediate between the avirulent M. incognita race 3 and the resistance-breaking M. incognita isolate. The sweetpotato cultivars 'Beauregard' and 'Diane' were excellent hosts for all three Meloidogyne isolates. Cultivars Bellevue, Burgundy, and Covington were resistant to these isolates. The cultivars Bonita and Murasaki-29 were hosts for the M. floridensis and the resistance-breaking M. incognita isolate, which allowed an increase in nematode levels, but they were poor hosts, resulting in a decrease in nematode levels for the M. incognita race 3 isolate. The study showed that M. floridensis can reproduce on tomato and some sweetpotato cultivars that are considered resistant to M. incognita.

Occurrence and Distribution of Meloidogyne spp. in Fields Rotated with Sweetpotato and Host Range of a North Carolina Population of Meloidogyne enterolobii.

Root-knot nematodes (RKNs, Meloidogyne spp.) are some of the most economically important and common plant parasitic nematodes in North Carolina (NC) cropping systems. Soil samples collected from fields planted with crops rotated with sweetpotato (Ipomoea batatas [L.] Lam.) in 39 NC counties in 2015 to 2018 were processed at the NC Nematode Assay Laboratory. The occurrence of second-stage juvenile (J2) RKN populations was examined based on collection year, month, county, and previous planted crop. The highest number of RKN-positive samples originated from Cumberland (53%), Sampson (48%), and Johnston (48%) counties. The highest average RKN population density was detected in Sampson (147 J2/500 cm3 of soil) and Nash (135 J2/500 cm3 of soil) counties, while Wayne (7 J2/500 cm3 of soil) and Greene (11 J2/500 cm3 of soil) counties had the lowest average RKN population density. Meloidogyne enterolobii is a new invasive species that is impacting sweetpotato growers of NC. The host status of an NC population of M. enterolobii, the guava RKN, was determined by examining eggs per gram of fresh root (ER) and the final nematode egg population divided by the initial population egg count (reproductive factor, RF) in greenhouse experiments. This included 18 vegetable, field, and cover crops and weed species. The tomato 'Rutgers' was used as a susceptible control. Cabbage 'Stonehead', pepper 'Red Bull', and watermelon 'Charleston Gray' and 'Fascination' were hosts and had similar mean ER values to the positive control, ranging from 64 to 18,717. Among field crops, cotton, soybean 'P5018RX', and tobacco were hosts with ER values that ranged from 185 to 706. Members of the Poaceae family such as sweet corn (Zea mays) and sudangrass (Sorghum × drummondii) were nonhosts to M. enterolobii, and the mean ER values ranged from 1.85 to 7. The peanut 'Tifguard' and winter wheat (Triticum aestivum) also had lower ER values than the vegetable hosts. Growers should consider planting less susceptible hosts or nonhosts such as peanut, sudangrass, sweet corn, and winter wheat in 2- to 3-year crop rotations to lower populations of this invasive nematode.

Enhancing Reniform Nematode Management in Sweetpotato by Complementing Host-Plant Resistance with Nonfumigant Nematicides.

The reniform nematode (Rotylenchulus reniformis Linford and Oliveira) adversely impacts the quality and quantity of sweetpotato storage roots. Management of R. reniformis in sweetpotato remains a challenge because host plant resistance is not available, fumigants are detrimental to the environment and health, and crop rotation is not effective. We screened a core set of 24 sweetpotato plant introductions (PIs) against R. reniformis. Four PIs were resistant, and 10 were moderately resistant to R. reniformis, suggesting these PIs can serve as sources of resistance for sweetpotato resistance breeding programs. PI 595869, PI 153907, and PI 599386 suppressed 83 to 89% egg production relative to the susceptible control 'Beauregard', and these PIs were employed in subsequent experiments to determine if their efficacy against R. reniformis can be further increased by applying nonfumigant nematicides oxamyl, fluopyram, and fluensulfone. A 34 to 93% suppression of nematode reproduction was achieved by the application of nonfumigant nematicides, with oxamyl providing the best suppression followed by fluopyram and fluensulfone. Although sweetpotato cultivars resistant to R. reniformis are currently not available and there is a need for the development of safer yet highly effective nonfumigant nematicides, results from the current study suggest that complementing host plant resistance with nonfumigant nematicides can serve as an important tool for effective and sustainable nematode management.

Fumigation Using 1,3-Dichloropropene Manages Meloidogyne enterolobii in Sweetpotato More Effectively than Fluorinated Nematicides.

Meloidogyne enterolobii is an emerging global threat and is damaging to sweetpotato (Ipomoea batatas) production in the southeast United States. Nematicide application is one of the few management strategies currently available against this nematode, and field testing is urgently needed. The objective of this study was to assess common nematicides for management of M. enterolobii and nontarget effects on free-living nematodes in sweetpotato field production. Treatments were (i) untreated control, (ii) fumigation using 1,3-dichloropropene, or at-transplant drench of fluorinated nematicides (iii) fluazaindolizine, (iv) fluopyram, or (v, vi) fluensulfone at 2 or 4 kg a.i./ha. In 2022, a field trial was conducted under severe M. enterolobii pressure and was repeated in 2023 in the same location without treatment rerandomization. Fumigation using 1,3-dichloropropene (1,3-D) was the only consistently effective nematicide at improving marketable yield relative to control and also consistently reduced most storage root galling measurements and midseason Meloidogyne soil abundances. Fluensulfone at 4 kg a.i./ha consistently improved total yield but not marketable yield, whereas fluensulfone at 2 kg a.i./ha, fluazaindolizine, and fluopyram did not improve yield. Each fluorinated nematicide treatment reduced at least one nematode symptom or nematode soil abundances relative to control, but none provided consistent benefits across years. Even with 1,3-D fumigation, yield was poor, and none of the nematicide treatments provided a significant return on investment relative to forgoing nematicide application. There were minimal effects on free-living nematodes. In summary, 1,3-D is an effective nematicide for M. enterolobii management, but additional management will be needed under severe M. enterolobii pressure.

Characterization of Nucleotide Binding -Site-Encoding Genes in Sweetpotato, Ipomoea batatas(L.) Lam., and Their Response to Biotic and Abiotic Stresses.

Sweetpotato, Ipomoea batatas (L.) Lam., is an important and widely grown crop, yet its production is affected severely by biotic and abiotic stresses. The nucleotide binding site (NBS)-encoding genes have been shown to improve stress tolerance in several plant species. However, the characterization of NBS-encoding genes in sweetpotato is not well-documented to date. In this study, a comprehensive analysis of NBS-encoding genes has been conducted on this species by using bioinformatics and molecular biology methods. A total of 315 NBS-encoding genes were identified, and 260 of them contained all essential conserved domains while 55 genes were truncated. Based on domain architectures, the 260 NBS-encoding genes were grouped into 6 distinct categories. Phylogenetic analysis grouped these genes into 3 classes: TIR, CC (I), and CC (II). Chromosome location analysis revealed that the distribution of NBS-encoding genes in chromosomes was uneven, with a number ranging from 1 to 34. Multiple stress-related regulatory elements were detected in the promoters, and the NBS-encoding genes' expression profiles under biotic and abiotic stresses were obtained. According to the bioinformatics analysis, 9 genes were selected for RT-qPCR analysis. The results revealed that IbNBS75, IbNBS219, and IbNBS256 respond to stem nematode infection; Ib-NBS240, IbNBS90, and IbNBS80 respond to cold stress, while IbNBS208, IbNBS71, and IbNBS159 respond to 30% PEG treatment. We hope these results will provide new insights into the evolution of NBS-encoding genes in the sweetpotato genome and contribute to the molecular breeding of sweetpotato in the future.

Discovery of a major QTL for root-knot nematode (Meloidogyne incognita) resistance in cultivated sweetpotato (Ipomoea batatas).

KEY MESSAGE: Utilizing a high-density integrated genetic linkage map of hexaploid sweetpotato, we discovered a major dominant QTL for root-knot nematode (RKN) resistance and modeled its effects. This discovery is useful for development of a modern sweetpotato breeding program that utilizes marker-assisted selection and genomic selection approaches for faster genetic gain of RKN resistance. The root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] (RKN) causes significant storage root quality reduction and yields losses in cultivated sweetpotato [Ipomoea batatas (L.) Lam.]. In this study, resistance to RKN was examined in a mapping population consisting of 244 progenies derived from a cross (TB) between 'Tanzania,' a predominant African landrace cultivar with resistance to RKN, and 'Beauregard,' an RKN susceptible major cultivar in the USA. We performed quantitative trait loci (QTL) analysis using a random-effect QTL mapping model on the TB genetic map. An RKN bioassay incorporating potted cuttings of each genotype was conducted in the greenhouse and replicated five times over a period of 10 weeks. For each replication, each genotype was inoculated with ca. 20,000 RKN eggs, and root-knot galls were counted ~62 days after inoculation. Resistance to RKN in the progeny was highly skewed toward the resistant parent, exhibiting medium to high levels of resistance. We identified one major QTL on linkage group 7, dominant in nature, which explained 58.3% of the phenotypic variation in RKN counts. This work represents a significant step forward in our understanding of the genetic architecture of RKN resistance and sets the stage for future utilization of genomics-assisted breeding in sweetpotato breeding programs.

Identification of Sweet Potato Germplasm Resistant to Pathotypically Distinct Isolates of Meloidogyne enterolobii from the Carolinas.

Meloidogyne enterolobii (syn. mayaguensis) is an emergent species of root-knot nematode that has become a serious threat to sweet potato (Ipomoea batatas) production in the southeastern United States. The most popular sweet potato cultivars grown in this region are highly susceptible to M. enterolobii. As a result, this pest has spread across most of the sweet potato growing counties in the Carolinas, threatening the industry as well as other crops in the region. The development and release of new sweet potato cultivars with resistance to M. enterolobii would help to manage and slow the spread of this pest. To support sweet potato resistance breeding efforts, 93 accessions selected from the U.S. Department of Agriculture germplasm collection and breeding programs in the United States were screened to identify 19 lines with strong resistance to M. enterolobii. The resistance in these accessions was tested against two M. enterolobii isolates that were collected from sweet potato production fields in the Carolinas. These isolates were found to have distinct pathotypes, with galling and nematode reproduction differences observed on cotton as well as sweet potato. This study is the first report of intraspecific pathotypic variation in M. enterolobii, and it identifies sweet potato germplasm with resistance against both pathogenic variants of this nematode.

Transcriptome analysis of root-knot nematode (Meloidogyne incognita)-resistant and susceptible sweetpotato cultivars.

MAIN CONCLUSION: Transcriptome analysis was performed on the roots of susceptible and resistant sweetpotato cultivars infected with the major root-knot nematode species Meloidogyne incognita. In addition, we identified a transcription factor-mediated defense signaling pathway that might function in sweetpotato-nematode interactions. Root-knot nematodes (RKNs, Meloidogyne spp.) are important sedentary endoparasites of many agricultural crop plants that significantly reduce production in field-grown sweetpotato. To date, no studies involving gene expression profiling in sweetpotato during RKN infection have been reported. Therefore, in the present study, transcriptome analysis was performed on the roots of susceptible (cv. Yulmi) and resistant (cv. Juhwangmi) sweetpotato cultivars infected with the widespread, major RKN species Meloidogyne incognita. Using the Illumina HiSeq 2000 platform, we generated 455,295,628 pair-end reads from the fibrous roots of both cultivars, which were assembled into 74,733 transcripts. A number of common and unique genes were differentially expressed in susceptible vs. resistant cultivars as a result of RKN infection. We assigned the differentially expressed genes into gene ontology categories and used MapMan annotation to predict their functional roles and associated biological processes. The candidate genes including hormonal signaling-related transcription factors and pathogenesis-related genes that could contribute to protection against RKN infection in sweetpotato roots were identified and sweetpotato-nematode interactions involved in resistance are discussed.

Limited mobility of target pests crucially lowers controllability when sterile insect releases are spatiotemporally biased.

The sterile insect technique (SIT) is a genetic pest control method wherein mass-reared sterile insects are periodically released into the wild, thereby impeding the successful reproduction of fertile pests. In Okinawa Prefecture, Japan, the SIT has been implemented to eradicate the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire), which is a flightless agricultural pest of sweet potatoes. It is known that E. postfasciatus is much less mobile than other insects to which the SIT has been applied. However, previous theoretical studies have rarely examined effects of low mobility of target pests and variation in the spatiotemporal evenness of sterile insect releases. To theoretically examine the effects of spatiotemporal evenness on the regional eradication of less mobile pests, we constructed a simple two-patch population model comprised of a pest and sterile insect moving between two habitats, and numerically simulated different release strategies (varying the number of released sterile insects and release intervals). We found that spatially biased releases allowed the pest to spatially escape from the sterile insect, and thus intensively lowered its controllability. However, we showed that the temporally counterbalancing spatially biased releases by swapping the number of released insects in the two habitats at every release (called temporal balancing) could greatly mitigate this negative effect and promote the controllability. We also showed that the negative effect of spatiotemporally biased releases was a result of the limited mobility of the target insect. Although directed dispersal of the insects in response to habitats of differing quality could lower the controllability in the more productive habitat, the temporal balancing could promote and eventually maximize the controllability as released insects increased.

Mitochondrial genome of the sweet potato hornworm, Agrius convolvuli (Lepidoptera: Sphingidae), and comparison with other Lepidoptera species.

In the present study, we sequenced the complete mitochondrial genome (mitogenome) of Agrius convolvuli (Lepidoptera: Sphingidae) and compared it with previously sequenced mitogenomes of lepidopteran species. The mitogenome was a circular molecule, 15 349 base pairs (bp) long, containing 37 genes. The order and orientation of genes in the A. convolvuli mitogenome were similar to those in sequenced mitogenomes of other lepidopterans. All 13 protein-coding genes (PCGs) were initiated by ATN codons, except for the cytochrome c oxidase subunit 1 (cox1) gene, which seemed to be initiated by the codon CGA, as observed in other lepidopterans. Three of the 13 PCGs had the incomplete termination codon T, while the remainder terminated with TAA. Additionally, the codon distributions of the 13 PCGs revealed that Asn, Ile, Leu2, Lys, Phe, and Tyr were the most frequently used codon families. All transfer RNAs were folded into the expected cloverleaf structure except for tRNASer(AGN), which lacked a stable dihydrouridine arm. The length of the adenine (A) + thymine (T)-rich region was 331 bp. This region included the motif ATAGA followed by a 19-bp poly-T stretch and a microsatellite-like (TA)8 element next to the motif ATTTA. Phylogenetic analyses (maximum likelihood and Bayesian methods) showed that A. convolvuli belongs to the family Sphingidae.

A myo-inositol-1-phosphate synthase gene, IbMIPS1, enhances salt and drought tolerance and stem nematode resistance in transgenic sweet potato.

Myo-inositol-1-phosphate synthase (MIPS) is a key rate limiting enzyme in myo-inositol biosynthesis. The MIPS gene has been shown to improve tolerance to abiotic stresses in several plant species. However, its role in resistance to biotic stresses has not been reported. In this study, we found that expression of the sweet potato IbMIPS1 gene was induced by NaCl, polyethylene glycol (PEG), abscisic acid (ABA) and stem nematodes. Its overexpression significantly enhanced stem nematode resistance as well as salt and drought tolerance in transgenic sweet potato under field conditions. Transcriptome and real-time quantitative PCR analyses showed that overexpression of IbMIPS1 up-regulated the genes involved in inositol biosynthesis, phosphatidylinositol (PI) and ABA signalling pathways, stress responses, photosynthesis and ROS-scavenging system under salt, drought and stem nematode stresses. Inositol, inositol-1,4,5-trisphosphate (IP3 ), phosphatidic acid (PA), Ca(2+) , ABA, K(+) , proline and trehalose content was significantly increased, whereas malonaldehyde (MDA), Na(+) and H2 O2 content was significantly decreased in the transgenic plants under salt and drought stresses. After stem nematode infection, the significant increase of inositol, IP3 , PA, Ca(2+) , ABA, callose and lignin content and significant reduction of MDA content were found, and a rapid increase of H2 O2 levels was observed, peaked at 1 to 2 days and thereafter declined in the transgenic plants. This study indicates that the IbMIPS1 gene has the potential to be used to improve the resistance to biotic and abiotic stresses in plants.

Assembling of Holotrichia parallela (dark black chafer) midgut tissue transcriptome and identification of midgut proteins that bind to Cry8Ea toxin from Bacillus thuringiensis.

Holotrichia parallela is one of the most severe crop pests in China, affecting peanut, soybean, and sweet potato crops. Previous work showed that Cry8Ea toxin is highly effective against this insect. In order to identify Cry8Ea-binding proteins in the midgut cells of H. parallela larvae, we assembled a midgut tissue transcriptome by high-throughput sequencing and used this assembled transcriptome to identify Cry8Ea-binding proteins by liquid chromatography-tandem mass spectrometry (LC-MS/MS). First, we obtained de novo sequences of cDNAs from midgut tissue of H. parallela larvae and used available cDNA data in the GenBank. In a parallel assay, we obtained 11 Cry8Ea-binding proteins by pull-down assays performed with midgut brush border membrane vesicles. Peptide sequences from these proteins were matched to the H. parallela newly assembled midgut transcriptome, and 10 proteins were identified. Some of the proteins were shown to be intracellular proteins forming part of the cell cytoskeleton and/or vesicle transport such as actin, myosin, clathrin, dynein, and tubulin among others. In addition, an apolipophorin, which is a protein involved in lipid metabolism, and a novel membrane-bound alanyl aminopeptidase were identified. Our results suggest that Cry8Ea-binding proteins could be different from those characterized for Cry1A toxins in lepidopteran insects.

Resistance to Ditylenchus destructor Infection in Sweet Potato by the Expression of Small Interfering RNAs Targeting unc-15, a Movement-Related Gene.

Stem nematode (Ditylenchus destructor) is one of most serious diseases that limit the productivity and quality of sweet potato (Ipomoea batatas), a root crop with worldwide importance for food security and nutrition improvement. Hence, there is a global demand for developing sweet potato varieties that are resistant to the disease. In this study, we have investigated the interference of stem nematode infectivity by the expression of small interfering RNAs (siRNAs) in transgenic sweet potato that are homologous to the unc-15 gene, which affects the muscle protein paramyosin of the pathogen. The production of double-stranded RNAs and siRNAs in transgenic lines with a single transgene integration event was verified by Northern blot analysis. The expression of unc-15 was reduced dramatically in stem nematodes collected from the inoculated storage roots of transgenic plants, and the infection areas of their storage roots were dramatically smaller than that of wild-type (WT). Compared with the WT, the transgenic plants showed increased yield in the stem nematode-infested field. Our results demonstrate that the expression of siRNAs targeting the unc-15 gene of D. destructor is an effective approach in improving stem nematode resistance in sweet potato, in adjunct with the global integrated pest management programs.

Shared binding sites for the Bacillus thuringiensis proteins Cry3Bb, Cry3Ca, and Cry7Aa in the African sweet potato pest Cylas puncticollis (Brentidae).

Bacillus thuringiensis Cry3Bb, Cry3Ca, and Cry7Aa have been reported to be toxic against larvae of the genus Cylas, which are important pests of sweet potato worldwide and particularly in sub-Saharan Africa. However, relatively little is known about the processing and binding interactions of these coleopteran-specific Cry proteins. The aim of the present study was to determine whether Cry3Bb, Cry3Ca, and Cry7Aa proteins have shared binding sites in Cylas puncticollis to orient the pest resistance strategy by genetic transformation. Interestingly, processing of the 129-kDa Cry7Aa protoxin using commercial trypsin or chymotrypsin rendered two fragments of about 70 kDa and 65 kDa. N-terminal sequencing of the trypsin-activated Cry7Aa fragments revealed that processing occurs at Glu(47) for the 70-kDa form or Ile(88) for the 65-kDa form. Homologous binding assays showed specific binding of the two Cry3 proteins and the 65-kDa Cry7Aa fragment to brush border membrane vesicles (BBMV) from C. puncticollis larvae. The 70-kDa fragment did not bind to BBMV. Heterologous-competition assays showed that Cry3Bb, Cry3Ca, and Cry7Aa (65-kDa fragment) competed for the same binding sites. Hence, our results suggest that pest resistance mediated by the alteration of a shared Cry receptor binding site might render all three Cry toxins ineffective.

Genetic variations underlying root-knot nematode resistance in sweetpotato.

Root-knot nematode (Meloidogyne incognita) causes severe crop damage and large economic losses worldwide. Several cultivars of sweetpotato [Ipomoea batatas (L.) Lam)] have been developed with root-knot nematode-resistant traits; however, many of these cultivars do not have favorable agronomic characteristics. To understand the genetic traits underlying M. incognita resistance in sweetpotato, whole genome resequencing was conducted on three RKN-susceptible (Dahomi, Shinhwangmi, and Yulmi) and three RKN-resistant (Danjami, Pungwonmi, and Juhwangmi) sweetpotato cultivars. Three SNPs (single nucleotide polymorphisms) in promotor sequences were shared in RKN-resistant cultivars and were correlated with disease resistance. One of these SNPs was located in G6617|TU10904, which encoded a homolog of RIBOSOMAL PROTEIN EL15Z, and was associated with reduced expression in RKN-resistant cultivars only. Alongside SNP analysis, mRNA-seq data were analyzed for the same cultivars with and without nematode infection, and 18 nematode-sensitive genes were identified that responded in a cultivar-specific manner. Of these genes, expression of G8735|TU14367 was lower in sensitive cultivars than in RKN-resistant cultivars. Overall, this study identified two genes that potentially have key roles in the regulation of nematode resistance and will be useful targets for nematode resistance breeding programs.

Effects of Paecilomyces lilacinus and Bacillus pumilus on stem nematode and rhizosphere bacterial communities of sweet potato.

Stem nematode (Ditylenchus destructor Thorne) is considered one of the most economically devastating species affecting sweet potato production. Biocontrol offers a sustainable strategy for nematode control. This study conducted a pot experiment to evaluate the biocontrol efficacy of Paecilomyces lilacinus CS-Z and Bacillus pumilus Y-26 against the stem nematode, as well as to examine their influence on the bacterial communities in the sweet potato rhizosphere. The findings indicated that B.pumilus Y-26 and P.lilacinus CS-Z exhibited respective suppression rates of 82.9% and 85.1% against the stem nematode, while also stimulating sweet potato plant growth. Both high-throughput sequencing and Biolog analysis revealed distinct impacts of the treatments on the bacterial communities. At the phylum level, B.pumilus Y-26 enhanced the abundance of Actinobacteria but reduced the abundance of Cyanobacteria, with P.lilacinus CS-Z exhibiting similar effects. Additionally, the treatment with B.pumilus Y-26 resulted in increased abundances of Crossiella, Gaiella, Bacillus, and Streptomyces at the genus level, while the treatment with P.lilacinus CS-Z showed increased abundances of Crossiella and Streptomyces. In contrast, the abundance of Pseudarthrobacter was reduced in the treatment with B.pumilus Y-26. Conversely, the application of the nematicide fosthiazate exhibited minor influence on the bacterial community. The findings indicated that the application of P.lilacinus CS-Z and B.pumilus Y-26 led to an increase in the relative abundances of beneficial microorganisms, including Gaiella, Bacillus, and Streptomyces, in the rhizosphere soil. In conclusion, P.lilacinus CS-Z and B.pumilus Y-26 demonstrated their potential as environmentally friendly biocontrol agents for managing stem nematode disease of sweet potato.

Relative susceptibility of two sweetpotato varieties to storage root damage by sweetpotato flea beetle (Coleoptera: Chrysomelidae) and wireworm (Coleoptera: Elateridae).

The feeding of soil dwelling insects on storage roots is one of the most serious management issues faced by sweetpotato, Ipomoea batatas (L.) Lam. (Convolvulaceae), growers in the southern United States. Field studies were conducted to evaluate the relative susceptibility of two commonly grown sweetpotato varieties to sweetpotato flea beetle, Chaetocnema confinis Crotch (Coleoptera: Chrysomelidae), and wireworms (Coleoptera: Elateridae, various species). The incidence and severity of sweetpotato flea beetle damage was significantly lower in the variety Covington than Beauregard in two small plot replicated studies. Surveys conducted in commercial sweetpotato fields also showed significantly less sweetpotato flea beetle damage in fields planted to Covington compared with those planted to Beauregard. There was no clear evidence of varietal effect on the incidence of wireworm damage in the study. Results indicate that the severity of wireworm damage as measured by the size of feeding scars may be less in Covington than Beauregard.

Use of universal primers for the 18S ribosomal RNA gene and whole soil DNAs to reveal the taxonomic structures of soil nematodes by high-throughput amplicon sequencing.

Nematodes are abundant metazoans that play crucial roles in nutrient recycle in the pedosphere. Although high-throughput amplicon sequencing is a powerful tool for the taxonomic profiling of soil nematodes, polymerase chain reaction (PCR) primers for amplification of the 18S ribosomal RNA (SSU) gene and preparation of template DNAs have not been sufficiently evaluated. We investigated nematode community structure in copse soil using four nematode-specific (regions 1-4) and two universal (regions U1 and U2) primer sets for the SSU gene regions with two DNAs prepared from copse-derived mixed nematodes and whole soil. The major nematode-derived sequence variants (SVs) identified in each region was detected in both template DNAs. Order level taxonomy and feeding type of identified nematode-derived SVs were distantly related between the two DNA preparations, and the region U2 was closely related to region 4 in the non-metric multidimensional scaling (NMDS) based on Bray-Curtis dissimilarity. Thus, the universal primers for region U2 could be used to analyze soil nematode communities. We further applied this method to analyze the nematodes living in two sampling sites of a sweet potato-cultivated field, where the plants were differently growing. The structure of nematode-derived SVs from the two sites was distantly related in the principal coordinate analysis (PCoA) with weighted unifrac distances, suggesting their distinct soil environments. The resultant ecophysiological status of the nematode communities in the copse and field on the basis of feeding behavior and maturity indices was fairly consistent with those of the copse- and the cultivated house garden-derived nematodes in prior studies. These findings will be useful for the DNA metabarcoding of soil eukaryotes, including nematodes, using soil DNAs.