Brassicaceous Seed Meals as Soil Amendments to Suppress the Plant-parasitic Nematodes Pratylenchus penetrans and Meloidogyne incognita

Brassicaceous seed meals are the residual materials remaining after the extraction of oil from seeds; these seed meals contain glucosinolates that potentially degrade to nematotoxic compounds upon incorporation into soil. This study compared the nematode-suppressive ability of four seed meals obtained from Brassica juncea 'Pacific Gold', B. napus 'Dwarf Essex' and 'Sunrise', and Sinapis alba 'IdaGold', against mixed stages of Pratylenchus penetrans and Meloidogyne incognita second-stage juveniles (J2). The brassicaceous seed meals were applied to soil in laboratory assays at rates ranging from 0.5 to 10.0% dry w/w with a nonamended control included. Nematode mortality was assessed after 3 days of exposure and calculated as percentage reduction compared to a nonamended control. Across seed meals, M. incognita J2 were more sensitive to the brassicaceous seed meals compared to mixed stages of P. penetrans. Brassica juncea was the most nematode-suppressive seed meal with rates as low as 0.06% resulting in > 90% suppression of both plant-parasitic nematodes. In general B. napus 'Sunrise' was the least nematode-suppressive seed meal. Intermediate were the seed meals of S. alba and B. napus 'Dwarf Essex'; 90% suppression was achieved at 1.0% and 5.0% S. alba and 0.25% and 2.5% B. napus 'Dwarf Essex', for M. incognita and P. penetrans, respectively. For B. juncea, seed meal glucosinolate-degradation products appeared to be responsible for nematode suppression; deactivated seed meal (wetted and heated at 70 °C for 48 hr) did not result in similar P. penetrans suppression compared to active seed meal. Sinapis alba seed meal particle size also played a role in nematode suppression with ground meal resulting in 93% suppression of P. penetrans compared with 37 to 46% suppression by pelletized S. alba seed meal. This study demonstrates that all seed meals are not equally suppressive to nematodes and that care should be taken when selecting a source of brassicaceous seed meal for plant-parasitic nematode management.     

Evaluation of an Antibiotic-Producing Strain of Pseudomonas fluorescens for Suppression of Plant-Parasitic Nematodes

The antibiotic 2,4-diacetylphloroglucinol (DAPG), produced by some strains of Pseudomonas spp., is involved in suppression of several fungal root pathogens as well as plant-parasitic nematodes. The primary objective of this study was to determine whether Wood1R, a D-genotype strain of DAPG-producing P. fluorescens, suppresses numbers of both sedentary and migratory plant-parasitic nematodes. An experiment was conducted in steam-heated soil and included two seed treatments (with Wood1R and a control without the bacterium) and six plant-nematode combinations which were Meloidogyne incognita on cotton, corn, and soybean; M. arenaria on peanut; Heterodera glycines on soybean; and Paratrichodorus minor on corn. Wood 1R had no effect on final numbers of M. arenaria, P. minor, or H. glycines; however, final numbers of M. incognita were lower when seeds were treated with Wood1R than left untreated, and this reduction was consistent among host plants. Population densities of Wood1R were greater on the roots of corn than on the other crops, and the bacterium was most effective in suppressing M. incognita on corn, with an average reduction of 41%. Despite high population densities of Wood1R on corn, the bacterium was not able to suppress numbers of P. minor. When comparing the suppression of M. incognita on corn in natural and steam-heated soil, egg production by the nematode was suppressed in natural compared to steamed soil, but the presence of Wood1R did not result in additional suppression of the nematodes in the natural soil. These data indicate that P. fluorescens strain Wood1R has the capacity to inhibit some populations of plant-parasitic nematodes. However, consistent suppression of nematodes in natural soils seems unlikely.     

Serological Differentiation of Plant-parasitic Nematode Species with Polyclonal and Monoclonal Antibodies

Although several attempts have been made to differentiate nematode species with polyclonal antisera, these efforts thus far have met with limited success because of extensive crossreactivities of the sera. Since the hybridoma technique offers the opportunity to develop more specific serological reagents, some research groups have recently started to apply this technology to the problem of species identification in nematology. Monoclonal antibodies (MA) that differentiate the potato-cyst nematodes Globodera rostochiensis and G. pallida, as well as MA specific for Meloidogyne species, have been developed. The possibilities of developing serodiagnostic tools for identification of nematodes recovered from soil samples and the implications of such monitoring of nematode infestations in view of integrated control of plant-parasitic nematodes are discussed.     

Review of Pasteuria penetrans: Biology,Ecology, and Biological Control Potential

Pasteuria penetrans is a mycelial, endospore-forming, bacterial parasite that has shown great potential as a biological control agent of root-knot nematodes. Considerable progress has been made during the last 10 years in understanding its biology and importance as an agent capable of effectively suppressing root-knot nematodes in field soil. The objective of this review is to summarize the current knowledge of the biology, ecology, and biological control potential of P. penetrans and other Pasteuria members. Pasteuria spp. are distributed worldwide and have been reported from 323 nematode species belonging to 116 genera of free-living, predatory, plant-parasitic, and entomopathogenic nematodes. Artificial cultivation of P. penetrans has met with limited success; large-scale production of endospores depends on in vivo cultivation. Temperature affects endospore attachment, germination, pathogenesis, and completion of the life cycle in the nematode pseudocoelom. The biological control potential of Pasteuria spp. have been demonstrated on 20 crops; host nematodes include Belonolaimus longicaudatus, Heterodera spp., Meloidogyne spp., and Xiphinema diversicaudatum. Pasteuria penetrans plays an important role in some suppressive soils. The efficacy of the bacterium as a biological control agent has been examined. Approximately 100,000 endospores/g of soil provided immediate control of the peanut root-knot nematode, whereas 1,000 and 5,000 endospores/g of soil each amplified in the host nematode and became suppressive after 3 years.     

Plant-parasitic Nematode Communities and Their Associations with Soil Factors in Organically Farmed Fields in Minnesota

A survey was conducted to determine the assemblage and abundance of plant-parasitic nematodes and their associations with soil factors in organically farmed fields in Minnesota. A total of 31 soil samples were collected from southeast (SE), 26 samples from southwest (SW), 28 from west-central (WC), and 23 from northwest (NW) Minnesota. The assemblage and abundance of plant-parasitic nematodes varied among the four regions. The soybean cyst nematode, Heterodera glycines, the most destructive pathogen of soybean, was detected in 45.2, 88.5, 10.7, and 0% of organically farmed fields with relative prominence (RP) values of 10.3, 26.5, 0.6, and 0 in the SE, SW, WC, and NW regions, respectively. Across the four regions, other common genera of plant-parasitic nematodes were Helicotylenchus (42.6, RP value, same below), Pratylenchus (26.9), Tylenchorhynchus and related genera (9.4), Xiphinema (5.6), and Paratylenchus (5.3). Aphelenchoides, Meloidogyne, Hoplolaimus, Mesocriconema, and Trichodorus were also detected at low frequencies and/or low population densities. The similarity index of plant-parasitic nematodes between two regions ranged from 0.44 to 0.71 and the similarity increased with decreasing distance between regions. The densities of most plant-parasitic nematodes did not correlate with measured soil factors (organic matter, pH, texture). However, the densities of Pratylenchus correlated negatively with % sand, and Xiphinema was correlated negatively with soil pH.     

Alternatives to Fenamiphos for Management of Plant-Parasitic Nematodes on Bermudagrass

Plant-parasitic nematodes can be very damaging to turfgrasses. The projected cancellation of the registration for fenamiphos in the near future has generated a great deal of interest in identifying acceptable alternative nematode management tactics for use on turfgrasses. Two field experiments were conducted to evaluate the effectiveness of repeated applications of several commercially available nematicides and root biostimulants for reducing population densities of plant-parasitic nematodes and (or) promoting health of bermudagrass in nematode-infested soil. One experimental site was infested with Hoplolaimus galeatus and Trichodorus obtusus, the second with Belonolaimus longicaudatus. In both trials, none of the experimental treatments reduced population densities (P ≤ 0.1) of plant-parasitic nematodes, or consistently promoted turf visual performance or turf root production. Nematologists with responsibility to advise turf managers regarding nematode management should thoroughly investigate the validity of product claims before advising clientele in their use.     

Plant-parasitic Nematode Acetylcholinesterase Inhibition by Carbamate and Organophosphate Nematicides

The sensitivity of acetylcholinesterases (ACHE) isolated from the plant-parasitic nematodes Meloidogyne arenaria, M. incognita, and Heterodera glycines and the free-living nematode Caenorhabditis elegans to carbamate and organophosphate nematicides was examined. The AChE from plant-parasitic nematode species were more sensitive to carbamate inhibitors than was AChE from C. elegans, but response to the organophosphates was approximately equivalent. The sulfur-containing phosphate nematicides were poor inhibitors of nematode acetylcholinesterase, but treatment with an oxidizing agent greatly improved inhibition. Behavioral bioassays with living nematodes revealed a poor relationship between enzyme inhibition and expression of symptoms in live nematodes.     

The Future of Nematology: Integration of New and Improved Management Strategies

The potential for managing plant-parasitic nenlatodes by combining two or more control strategies in an integrated program is examined. Advantages of this approach include the use of partially effective strategies and protection of highly effective ones vulnerable from nematode adaptation or environmental risk. Strategies can be combined sequentially from season to season or applied simultaneously. Programs that have several strategies available but that are limited in the true integration of control components are used as examples of current management procedures and the potential for their improvement. These include potato cyst nematodes in northern Europe, soybean cyst nematode in North Carolina, and root-knot nematodes on vegetable and field crops in California. A simplified model of the impact of component strategies on the nematode damage function indicates the potential for combining control measures with different efficacies to give acceptable nematode population reduction and crop protection. The likelihood for additive, synergistic, or antagonistic effects from combining strategies is considered with respect to the biological target and component compatibility.     

Correlation of Edaphic Factors with Plant-parasitic Nematode Population Densities in a Forage Field

Two hundred soil samples from the A_p horizon of a reed canarygrass field overlaying several different but related soils in northern Minnesota were analyzed for plant-parasitic nematodes and 22 edaphic factors. Pratylenchus penetrans was the predominant nematode taxon. Others were Aglenchus agricola, Tylenchorhynchus spp., Heterodera trifolii, Paratylenchus spp., Tylenchus maius, and Criconemella sp. Five nematode taxa, P. penetrans, A, agricola, Tylenchorhynchus spp., H. trifolii, and Paratylenchus spp., were correlated with particle size, Tylenchus maius and Criconemella sp. were correlated with effective cation exchange capacity. Nematode field spatial arrangements were related to a combination of statistically significant positive and negative soil factor effects on the nematode populations. Contour maps derived by geostatistical techniques were used to visually validate statistically significant correlations of nematode and soil data. Contour mapping to supplement traditional statistical techniques can be used to achieve a more holistic approach to studies of nematode-soil interrelationships.     

Nematode Community Structure in a Vineyard Soil

Distribution of the nematode community in a California vineyard was studied over a 13-month period. Omnivorous and microbivorous nematodes were similarly distributed in the root zone, with greatest densities occurring between vine rows and near the soil surface. Greatest densities of plant-parasitic nematodes were found in the vine row, with the individual species differing in their vertical distribution. Total nematode biomass was greatest between rows near the surface. Biomass of plant parasites was greatest in the upper 30 cm of soil in the row, whereas biomass of microbivores was greatest in this region between rows. Of the plant-parasitic nematodes, the variability in distribution among vines was greatest for Paratylenchus hamatus and least for Meloidogyne spp.     

Sensitive PCR Detection of Meloidogyne arenaria, M. incognita, and M. javanica Extracted from Soil

We have developed a simple PCR assay protocol for detection of the root-knot nematode (RKN) species Meloidogyne arenaria, M. incognita, and M. javanica extracted from soil. Nematodes are extracted from soil using Baermann funnels and centrifugal flotation. The nematode-containing fraction is then digested with proteinase K, and a PCR assay is carried out with primers specific for this group of RKN and with universal primers spanning the ITS of rRNA genes. The presence of RKN J2 can be detected among large numbers of other plant-parasitic and free-living nematodes. The procedure was tested with several soil types and crops from different locations and was found to be sensitive and accurate. Analysis of unknowns and spiked soil samples indicated that detection sensitivity was the same as or higher than by microscopic examination.     

Detection and Investigation of Soil Biological Activity against Meloidogyne incognita

Greenhouse experiments with two susceptible hosts of Meloidogyne incognita, a dwarf tomato and wheat, led to the identification of a soil in which the root-knot nematode population was reduced 5- to 16-fold compared to identical but pasteurized soil two months after infestation with 280 M. incognita J2/100 cm³ soil. This suppressive soil was subjected to various temperature, fumigation and dilution treatments, planted with tomato, and infested with 1,000 eggs of M. incognita/100 cm³ soil. Eight weeks after nematode infestation, distinct differences in nematode population densities were observed among the soil treatments, suggesting the suppressiveness had a biological nature. A fungal rRNA gene analysis (OFRG) performed on M. incognita egg masses collected at the end of the greenhouse experiments identified 11 fungal phylotypes, several of which exhibited associations with one or more of the nematode population density measurements (egg masses, eggs or J2). The phylotype containing rRNA genes with high sequence identity to Pochonia chlamydosporia exhibited the strongest negative associations. The negative correlation between the densities of the P. chlamydosporia genes and the nematodes was corroborated by an analysis using a P. chlamydosporia-selective qPCR assay.     

Effect of Compost and Manure Soil Amendments on Nematodes and on Yields of Potato and Barley: A 7-Year Study

A 7-year study located in Prince Edward Island, Canada, examined the influence of compost and manure on crop yield and nematode populations. The compost used in this study consisted of cull waste potatoes, sawdust, and beef manure in a 3:3:1 ratio, respectively. No plant-parasitic nematodes were detected in samples collected from windrow compost piles at 5- and 30-cm depths prior to application on field plots. Low population densities of bacterial-feeding nematodes were recovered from compost windrows at the 5-cm depth. Field plots of potato (Solanum tuberosum cv. Kennebec) received compost applied at 16 metric tonnes per hectare, or beef manure applied at 12 metric tonnes per hectare. An adjacent trial with barley (Hordeum vulgare cv. Mic Mac) received only the compost treatment. In both trials the experimental design was a complete randomized block with four replicates. Data averaged over seven growing seasons indicated that population levels of root-lesion nematodes (primarily Pratylenchus penetrans) were higher in root-zone soil in potato plots treated with either compost or manure compared to the untreated control plots. The soil amendments did not affect root-knot nematode (Meloidogyne hapla) population densities in the potato plots, but clover-cyst nematodes (Heterodera trifolii) were more numerous in the root-zone soils of barley treated with compost compared to the untreated plots. Numbers of bacterial-feeding nematodes (primarily Diplogaster lheritieri) were greater in soil in potato plots treated with manure and in soil around barley roots than in untreated plots. Total yields of potato tubers averaged over seven growing seasons increased by 27% in the plots treated with either compost or manure. Grain yields of barley also were increased by 12% when compost was applied. These results indicated that organic amendments increased crop yields, but the impacts on different nematode species varied and usually increased soil population levels.     

Bacterial rRNA genes associated with soil suppressiveness against the plant-parasitic nematode Heterodera schachtii

The goal of this study was to identify bacteria involved in soil suppressiveness against the plant-parasitic nematode Heterodera schachtii. Since H. schachtii cysts isolated from the suppressive soil can transfer this beneficial property to nonsuppressive soils, analysis of the cyst-associated microorganisms should lead to the identification of the causal organisms. Our experimental approach was to identify bacterial rRNA genes (rDNA) associated with H. schachtii cysts obtained from soil mixtures with various levels of suppressiveness. We hypothesized that we would be able to identify bacteria involved in the suppressiveness by correlating population shifts with differing levels of suppressiveness. Soil treatments containing different amounts of suppressive and fumigation-induced nonsuppressive soils exhibited various levels of suppressiveness after two nematode generations. The 10%-suppressive-soil treatment contained numbers of eggs per gram of soil similar to those of the 100%-suppressive-soil treatment, indicating that the suppressive factor(s) had been transferred. Bacterial rDNA associated with H. schachtii cysts were identified using a culture-independent method termed oligonucleotide fingerprinting of rRNA genes. Bacteria from five major taxonomic groups (Actinobacteria, Cytophaga-Flexibacter-Bacteroides, alpha-Proteobacteria, beta-Proteobacteria, and gamma-Proteobacteria) were identified. Three bacterial rDNA groups contained clones that were more prevalent in the highly suppressive soil treatments than in the less suppressive treatments, indicating a potential involvement in the H. schachtii suppressiveness. When these three groups were examined with specific PCR analyses performed on H. schachtii cysts that developed in soils treated with three biocidal compounds, only one bacterial rDNA group with moderate to high sequence identity to rDNA from several Rhizobium species and uncultured alpha-proteobacterial clones was consistently associated with the highly suppressive treatments. A quantitative PCR analysis confirmed the association of this Rhizobium-like rDNA group with the H. schachtii suppressiveness.     

Drip Irrigation as a Delivery System for Infestation of Field Plots with Nematodes

A drip irrigation delivery system was used to infest field sites with the plant-parasitic root-knot nematodes, Meloidogyne incognita. Juvenile or egg inocula passed through the system without blockage of emitters or harm to the nematodes. Field sites so infested were available for experimentation. Delivery of approximately 5 x 10⁴ to 10⁵ juveniles or 10⁵ to 3 x 10⁵ eggs per emitter through the drip system resulted in heavy root galling of tomatoes planted next to the drip emitters. Nematodes feeding on bacteria (Acrobeloides sp.) and on fungi (Deladenus durus) also were successfully applied through the drip system. This method has potential for uniformly infesting experimental sites with plant-parasitic or entomogenous nematodes and for manipulation of nematode community structure for soil ecological studies.     

Use of Chitin for Controlling Heterodera avenae and Tylenchulus semipenetrans

The nematicidal effect of chitin, relative to other pesticides, was evaluated against two plant-parasitic nematodes, Heterodera avenae and Tylenchulus semipenetrans. Wheat seedlings, grown in soils artificially or naturally infested with H. avenae, were treated with 0.4% (w/w) ClandoSan (CLA) prepared from crustacean chitin, aldicarb (Temik 15G), or ethylene dibromide (EDB 90EC). The CLA treatment significantly increased wheat straw, ear, and average grain dry weights of nematode-infected plants, compared with the other two treatments. In an experiment covering two consecutive seasons, all three treatments reduced the number of cysts in the soil by 60%. In a one-season experiment, CLA reduced the number of cysts by 51% and aldicarb or EDB reduced cyst number by about 40%. A reduction of 50-90% in T. semipenetrans population densities on roots of two citrus rootstocks was recorded following an application of 0.2% (w/w) CLA to the soil.     

Ecological Biogeography of the Terrestrial Nematodes of Victoria Land,Antarctica

The terrestrial ecosystems of Victoria Land, Antarctica are characteristically simple in terms of biological diversity and ecological functioning. Nematodes are the most commonly encountered and abundant metazoans of Victoria Land soils, yet little is known of their diversity and distribution. Herein we present a summary of the geographic distribution, habitats and ecology of the terrestrial nematodes of Victoria Land from published and unpublished sources. All Victoria Land nematodes are endemic to Antarctica, and many are common and widely distributed at landscape scales. However, at smaller spatial scales, populations can have patchy distributions, with the presence or absence of each species strongly influenced by specific habitat requirements. As the frequency of nematode introductions to Antarctica increases, and soil habitats are altered in response to climate change, our current understanding of the environmental parameters associated with the biogeography of Antarctic nematofauna will be crucial to monitoring and possibly mitigating changes to these unique soil ecosystems.     

Counting Nematodes with a Microplate Reader

The feasibility of counting plant-parasitic nematodes in aqueous suspensions by measuring light transmittance through aqueous suspensions with an ELISA microplate reader was explored. Absorbance readings for eggs or vermiform stages of three species were linearly related (R² > 0.99) to concentrations between 0 and 10,000 nematodes/ml. Coefficients of variation ranged from 12-23%, depending on the species and developmental stage used. The method, therefore, was at least as accurate as direct counts of nematodes in aliquots on a microscope and more than 100 times as fast. The method should have direct application in research programs on plant resistance to nematodes, nematode population dynamics, and nematode behavior.     

An insight into critical endocycle genes for plant-parasitic nematode feeding sites establishment

Root-knot and cyst nematodes are biotrophic parasites that invade the root apex of host plants and migrate toward the vascular cylinder where they cause the differentiation of root cells into galls (or root-knots) containing hypertrophied multinucleated giant-feeding cells, or syncytia, respectively. The precise molecular mechanisms that drive the formation of such unique nematode feeding sites are still far-off from being completely understood. The diverse gene expression changes occurring within the host cells suggest that both types of plant-parasitic nematodes modulate a variety of plant processes. Induction and repression of genes belonging to the host cell cycle control machinery have shown to be essential to drive the formation of such specialized nematode feeding cells. We demonstrate that nematodes usurp key components regulating the endocycle in their favor. This is illustrated by the involvement of anaphase-promoting complex (APC) genes (CCS52A and CCS52B), the endocycle repressor DP-E2F-like (E2F/DEL1) gene and the ROOT HAIRLESS 1 PROTEIN (RHL1), which is part of a multiprotein complex of the toposiomerase VI, in the proper formation of nematode feeding sites. Altering the expression of these genes in Arabidopsis plants by down- or overexpressing strategies strongly influences the extent of endoreduplication in both types of nematode feeding site leading to a disturbance of the nematode’s life cycle and reproduction.