Summary Meloidogyne floridensis is one of the emerging species of root-knot nematodes in vegetable production in the Southern USA. It can reproduce on commercial vegetable cultivars that have resistance to the common tropical root-knot nematode species ( i.e. , M. incognita , M. arenaria and M. javanica ). The objective of this study was to evaluate the Solanum sisymbriifolium line Sis Syn II as rootstock, in combination with drench application of a non-fumigant nematicide, fluazaindolizine, for managing M. floridensis on tomato. This study was conducted in artificially infested microplots under field conditions at two different locations in Georgia, USA in 2021. Treatments were arranged in a randomised complete block design with eight replications at one location and four replications at the other. The tomato cultivar ‘Roadster’ was used as the scion. Control treatments included non-grafted, and ‘Roadster’ grafted onto ‘Roadster’ rootstock (self-grafted). Our results showed that, in both experiments, Sis Syn II (reproduction factor: 0.07) significantly reduced the population of M. floridensis compared to the non-grafted (4.37) and self-grafted (6.86) controls. Similarly, the galling index was lower for Sis Syn II (0.61) compared to the non-grafted (4.13) and self-grafted (3.90) controls. Moreover, grafting onto Sis Syn II increased the average biomass and yield of the tomato in both experiments. However, fluazaindolizine drenching did not exhibit any effect on M. floridensis populations or the yield increment of tomato. Our study demonstrates that Sis Syn II is effective in management of M. floridensis . Grafting the desirable tomato cultivar onto this rootstock can significantly suppress M. floridensis population and increase crop yield.
The stem nematode Ditylenchus weischeri was recently reported on creeping thistle (Cirsium arvense) in Canada. Two greenhouse studies examined host suitability of crops commonly grown in the Canadian Prairies for D. weischeri and the closely related parasite of many crops, D. dipsaci. In the first study, common pulse crops (yellow pea, chickpea, common bean, and lentil), spring wheat, canola, creeping thistle, and garlic were evaluated. Plant biomass and reproductive factor (Rf = nematode recovered/inoculated) 8 weeks postinoculation were used to determine host suitability. Creeping thistle biomass was reduced by D. weischeri whereas D. dipsaci reduced biomass of four of five pea and two of three bean varieties. Two pea varieties were weak hosts for D. weischeri, with Rf slightly >1. D. weischeri aggressively reproduced on creeping thistle (Rf = 5.4). D. dipsaci reproduced aggressively on garlic (Rf = 6.4; a known host), moderately on pea varieties (Rf > 2), and weakly on chickpea and bean (Rf > 1). In the second study, using creeping thistle and yellow pea, D. weischeri was recovered from aboveground parts of the plants and seed of the former and D. dipsaci from the later. The results show that D. weischeri parasitizes creeping thistle but not other crops and that D. weischeri host preference is different from that of D. dipsaci.
Soil and root samples were collected from two plots (5.5 x 6 m) cultivated with bamboo Phyllostachys reticulata (syn. P. bambusoides) at the USDA Southeastern Fruit and Tree Nut Research Station in Byron, Georgia, USA, in November 2021, December 2022, and December 2023. Each soil sample consisted of five randomly collected subsamples, which were analyzed for the presence of plant-parasitic nematodes as part of regular monitoring. No aboveground symptoms were seen in the field plots, but roots showed discoloration and stunted growth (Fig. 1A). An average population density of 160, 36, and 188 second-stage juveniles (J2) of cyst nematodes (Heterodera spp.) per 100 cm3 of soil was detected in 2021, 2022, and 2023, respectively. To confirm the cyst nematode parasitism, three healthy P. reticulata plants were transplanted into 10 cm diameter pots filled with nematode-infested soil and grown in a greenhouse at 25-28°C. After three months, young cysts containing eggs protruding from the bamboo roots were seen, confirming nematode parasitism (Fig. 1B and C). Morphological and molecular analyses were conducted to identify the nematode species. The J2 body shape was cylindrical with three incisures in lateral field and well-developed rounded stylet knobs. Measurements of J2 (n = 10) included the length (range = 406.0-518.1 μm, mean = 458.2 ± 16.1 μm) and width (16.0-17.2 μm, 16.6 ± 0.5 μm) of body, labial region height (3.0-3.6 μm, 3.2 ± 0.2 μm), stylet (17.2-18.9 μm, 18.02 ± 0.6 μm), tail (61.1-65.9 μm, 63.1 ± 5.8 μm), body width at anus (9.7-12.4 μm, 10.8 ± 0.3 μm), and hyaline tail terminus (35.4-53.0 μm, 44.0 ± 3.8 μm). The cysts (n = 5) were relatively lemon-shaped, light to dark brown in color with projected neck and vulval cone that lacked fenestration, bullae, and underbridge. Morphometrics were body length including neck (420.5-745.0 μm, 550.4 ± 72 μm), body width (345.0-544.1 μm, 430.1 ± 59 μm), and L/W ratio (1.0-1.4 μm, 1.1 ± 0.1 μm). The morphometric of vulval cone included fenestral length (48 to 69 µm, 60.2 ± 4.0 µm), fenestral width (43 to 51 µm, 48.9 ± 4.0 µm) and vulval slit (47 to 58 µm, 49.1 ± 2.6 µm). Morphology and morphometric measurements of the cysts and J2 (Fig. 2A-C) were comparable to previous reports of Korean cyst nematode, H. koreana from other Asian countries (Mundo-Ocampo et al., 2008; Wang et al., 2012; Tanha Maafi and Taheri, 2015; Sekimoto et al., 2017). For molecular diagnosis, DNA was extracted from single cysts (n= 3) using DNeasy Blood and Tissue Kit, and 28S rRNA and partial cytochrome oxidase I (COI) gene fragments were amplified using D2A/D3B and Het-coxiF/Het-coxiR primer sets, respectively according to Subbotin et al. (2015) and Hajihassani et al. (2018). PCR products were then purified by a DNA Clean & Concentrator kit (Zymo Research, CA, USA) and sequenced at Genewiz (South Plainfield, NJ, USA). The resulting sequences were deposited into the GenBank database and subjected to BLASTn searches. Sequence analysis of the 28S rRNA gene (GenBank accession number OL812726) showed that the H. koreana isolate from Georgia was 100% identical (692/692 bp) with H. koreana populations from Japan and South Korea. Analysis of the H. koreana isolate from Georgia based on the COI sequence (OL813218 and OL813219) revealed 100% similarity (421/422 bp) with a population of H. koreana from Japan and 98% similarity (409/409 bp) with a population from South Korea. To our knowledge, this is the first report of H. koreana parasitizing P. reticulata in Georgia as the second state in the USA. This nematode was first reported in Florida infecting fish pole bamboo (P. aurea) propagated from plant material from the Far East (Inserra et al. 1999). This report expands the information on the geographical distribution of H. koreana. Given the significant potential for damage to bamboo, precautions should be taken to prevent the nematode's spread to unaffected regions.
Meloidogyne partityla is the dominant root-knot nematode (RKN) species parasitizing pecan in Georgia. This species is known to cause a reduction in root growth and a decline in the yields of mature pecan trees. Rapid and accurate diagnosis of this RKN is required to control this nematode disease and reduce losses in pecan production. In this study, a loop-mediated isothermal amplification (LAMP) method was developed for simple, rapid, and on-site detection of M. partityla in infested plant roots and validated to detect the nematode in laboratory and field conditions. Specific primers were designed based on the sequence distinction of the internal transcribed spacer (ITS)-18S/5.8S ribosomal RNA gene between M. partityla and other Meloidogyne spp. The LAMP detection technique could detect the presence of M. partityla genomic DNA at a concentration as low as 1 pg, and no cross reactivity was found with DNA from other major RKN species such as M. javanica, M. incognita and M. arenaria, and M. hapla. We also conducted a traditional morphology-based diagnostic assay and conventional polymerase chain reaction (PCR) assay to determine which of these techniques was less time consuming, more sensitive, and convenient to use in the field. The LAMP assay provided more rapid results, amplifying the target nematode species in less than 60 min at 70°C, with results 100 times more sensitive than conventional PCR (~2-3 hrs). Morphology-based, traditional diagnosis was highly time-consuming (2 days) and more laborious than conventional PCR and LAMP assays. These features greatly simplified the operating procedure and made the assay a powerful tool for rapid, on-site detection of pecan RKN, M. partityla. The developed LAMP assay will facilitate accurate pecan nematode diagnosis in the field and contribute to the management of the pathogen.
Meloidogyne floridensis is of particular concern because it reproduces on tomato, pepper, corn, and tobacco cultivars that have resistance to the common tropical root-knot nematode (RKN) species (i.e., Meloidogyne incognita, M. arenaria, and M. javanica). During a survey of 436 randomly selected vegetable fields in Georgia in 2018, 6 M. floridensis-infested fields were found and cultured from single egg-mass isolates on a susceptible tomato (cultivar Rutgers), and speciated using molecular analyses. Five isolates of M. floridensis were identified from collard, cowpea, cucumber, watermelon, and tomato fields by DNA sequence-based identification targeting mitochondrial genes (cytochrome c oxidase subunit II, transfer RNAHis, large subunit ribosomal RNA, and NADH dehydrogenase subunit 5). Two greenhouse trials determined the host preference and reproduction level for each M. floridensis isolate. Evaluations were conducted on Rutgers tomato, a resistant tomato (cultivar Skyway, carrying the Mi-1.2 gene), and vegetable crops associated with the origin of M. floridensis populations. This study confirmed that most associated vegetables, except collards, were good hosts to M. floridensis, having a reproduction factor >1. All isolates were able to reproduce aggressively on the resistant tomato. We found variations among M. floridensis isolates in pathogenicity and reproduction levels on the vegetable crops tested which should be considered when using or developing host resistance.
Multi-cropping of vegetables on the same plastic mulch builds up the population of root-knot nematodes (RKN; Meloidogyne spp.), which can severely reduce crop growth and yield. Vegetable growers in the southeastern US usually fumigate soil while laying the plastic mulch in the spring. They then apply non-fumigant nematicides via drip irrigation systems for subsequent crops grown on the mulch. With the advent of new and emerging nematicides, this research was aimed to investigate the best chemical control practice for M. incognita in a pepper and squash plasticulture system. Field trials were conducted in the spring (pepper) and summer (squash) of 2019 and 2020. The spring treatments were soil fumigants of 1,3-dichloropropene, allyl isothiocyanate, and 1,3-dichloropropene plus chloropicrin (Pic60), a RKN-resistant cultivar (Carolina Wonder), and an untreated check. Summer treatments were the non-fumigant nematicides fluopyram, fluensulfone, fluazaindolizine, oxamyl, and Burkholderia spp. strain A396. All spring treatments, except allyl isothiocyanate, reduced (P < 0.05) root galling compared to the untreated check at pepper harvest. At the end of the season, the population density of M. incognita in the soil was only lower (P < 0.05) for the RKN-resistant cultivar treatment than that of the untreated check. Though the RKN-resistant cultivar treatment had the lowest (P < 0.05) soil RKN population and significantly reduced root galling, it had the lowest pepper fruit yield. In contrast, pepper associated with Pic60 treatment had the highest fruit yield. For the summer trial, squash plots treated with fluensulfone had the lowest root gall index and oxamyl had the highest (P < 0.05); however, no difference was observed between fluensulfone and oxamyl with other treatments. In 2019, plots treated with Burkholderia spp. and fluensulfone had higher fruit yield (P < 0.05) than fluazaindolizine; however, squash yield was similar among the treatments in 2020. This study suggests that Pic60 (a mix of nematicide and fungicide active ingredients) is likely an ideal fumigant to apply when laying the plastic mulch because of its broad-spectrum effect, and any of the non-fumigant nematicides may be used in RKN control on squash.
The ability of the recently described stem nematode of creeping thistle (Cirsium arvense L.), Ditylenchus weischeri, to develop on and parasitize yellow pea (Pisum sativum L.) is uncertain. The current study examined nematode life-stage progression and generation time on yellow pea as affected by temperature with the related pest, D. dipsaci, used as a positive control. Relationships for body length of the two nematode species and life stage were unaffected by rearing on plant hosts compared with carrot disks. Then plant-reared J4 individuals of both nematode species were used to determine the effect of temperature (17, 22, and 27°C) on life-stage progression and minimum generation time with yellow pea. At 17 and 22°C, D. weischeri J4 individuals progressed to only the adult stage whereas, at 27°C, the minimum generation time from J4 to J4 was 30 days or 720 growing degree-days. The minimum generation time for D. dipsaci was 24, 18, and 22 days or 336, 342, and 528 growing degree-days at 17, 22, and 27°C, respectively. The results indicate that development of D. weischeri is temperature dependent and reproduction is unlikely on yellow pea in the Canadian Prairies, where mean daily air temperatures of 27°C are rare and not sustained.
