Suppression of Heterodera schachtii Populations by Dactylella oviparasitica in Four Soils

The effects of Dactylella oviparasitica strain 50 applications on sugarbeet cyst nematode (Heterodera schachtii) population densities and plant weights were assessed in four agricultural soils. The fungus was added to methyl iodide-fumigated and nonfumigated portions of each soil. The soils were seeded with Swiss chard. Four weeks later, soils were infested with H. schachtii second-stage juveniles (J2). Approximately 1,487 degree-days after infestation, H. schachtii cyst, egg and J2 numbers and plant weights were assessed. In all four fumigated soils, D. oviparasitica reduced all H. schachtii population densities and increased most of the plant weights compared to the nonamended control soils. In two of the nonfumigated soils (10 and SC), D. oviparasitica reduced H. schachtii population densities and increased most plant weight values compared to the nonamended control soils. For the other two nonfumigated soils (44 and 48), which exhibited pre-existing levels of H. schachtii suppressiveness, fungal applications had relatively little impact on H. schachtii population densities and plant weights. The results from this study combined with those from previous investigations suggest that D. oviparasitica strain 50 could be an effective biological control agent.     

Detection of Suppressiveness against Rotylenchulus reniformis in Soil from Cotton (Gossypium hirsutum) Fields in Texas and Louisiana

Rotylenchulus reniformis is a major problem confronting cotton production in the central part of the cotton belt of the United States of America. In this study, the hypothesis that natural antagonists in some cases are responsible for unusually low densities of the nematode in certain fields was tested by assaying soils from 22 selected fields for the presence of transferable agents in pots containing cotton plants. In one field, soil from four different depth ranges was tested. In the first of two types of assays, 1 part nematode infested soil was added to 9 parts test soil that was left untreated or autoclaved before mixing; this mixture was used to fill pots. In the second type of assay, 1 part test soil was added to 9 or 19 parts pasteurized fine sand, and nematodes were introduced in aqueous suspension. In three experiments representing both types of assay, transferable or autoclavable agent(s) from four fields in South Texas suppressed nematode populations by 48, 78, 90 and 95%. In one experiment, transferable agents in five fields in Louisiana suppressed populations from 37 to 66%. Identification and evaluation of these agents for biological control of R. reniformis merits further study.     

Suppression of Cyst Nematode by Natural Infestation of a Nematophagous Fungus

Penetration of cabbage roots by Heterodera schachtii was suppressed 50-77% in loamy sand naturally infested with the nematophagous fungus Hirsutella rhossiliensis. When Heterodera schachtii was incubated in the suppressive soil without plants for 2 days, 40-63% of the juveniles had Hirsutella rhossiliensis spores adhering to their cuticles. Of those with spores, 82-92% were infected. Infected nematodes were killed and filled with hyphae within 2-3 days. Addition of KCl to soil did not increase infection of Heterodera schachtii by Hirsutella rhossiliensis. The percentage of infection was lower when nematodes were touched to two spores and incubated in KCl solution than when nematodes naturally acquired two spores in soil.     

Persistence and Suppressiveness of Pasteuria penetrans to Meloidogyne arenaria Race

The long-term persistence and suppressiveness of Pasteuria penetrans against Meloidogyne arenaria race 1 were investigated in a formerly root-knot nematode suppressive site following 9 years of continuous cultivation of three treatments and 4 years of continuous peanut. The three treatments were two M. arenaria race 1 nonhost crops, bahiagrass (Paspalum notatum cv. Pensacola var. Tifton 9), rhizomal peanut (Arachis glabrata cv. Florigraze), and weed fallow. Two root-knot nematode susceptible weeds commonly observed in weed fallow plots were hairy indigo (Indigofera hirsuta) and alyce clover (Alysicarpus vaginalis). The percentage of J2 with endospores attached reached the highest level of 87% in 2000 in weed fallow, and 63% and 53% in 2002 in bahiagrass and rhizomal peanut, respectively. The percentage of endospore-filled females extracted from peanut roots grown in weed fallow plots increased from nondetectable in 1999 to 56% in 2002, whereas the percentages in bahiagrass and rhizomal peanut plots were 41% and 16%, respectively. Over 4 years, however, there was no strong evidence that endospores densities reached suppressive levels because peanut roots, pods, and pegs were heavily galled, and yields were suppressed. This might be attributed to the discovery of M. javanica infecting peanut in this field in early autumn 2001. A laboratory test confirmed that although the P. penetrans isolate specific to M. arenaria attached to M. javanica J2, no development occurred. In summary, P. penetrans increased on M. arenaria over a 4-year period, but apparently because of infection of M. javanica on peanut at the field site root-knot disease was not suppressed. This was confirmed by a suppressive soil test that showed a higher level of soil suppressiveness than occurred in the field (P ≤ 0.01).     

Molecular Polymorphism and Morphometrics of Species of the Heterodera avenae Group in Syria and Turkey

Molecular characterization of the three most common cereal cyst nematode species of the Heterodera avenae group (H. avenae, H. filipjevi, and H. latipons), originating from various locations in major cereal-cultivating areas in Syria and Turkey, showed distinct restriction fragment patterns of the ITS-rDNA following PCR amplification and RFLP digestion with four endonucleases (Hae III, Hinf I, Ita I, and Pst I). Genetic dissimilarity within H. avenae group populations increased in comparison with H. avenae and other species; it was 0.164 with H. filipjevi and 0.354 with H. latipons populations. No intraspecific polymorphism was observed within H. latipons or H. filipjevi populations. Principal component analysis revealed contrasted correlations among 12 morphological parameters of cysts and juveniles of the three Heterodera species that separated them and distinguished differences within populations of H. latipons. Our results showed a clear separation of the three cyst nematode species on cereal using a conventional method for classification and molecular tests, and confirmed the congruence between genetics and morphological traits.     

Extraction of Cyst Nematodes from Organic Soils

The effects of extraction technique, sample size, soil moisture level, and overflow rate on recovery of Globodera rostochiensis and (or) Heterodera schachtii cysts from organic soils were investigated. A modified Fenwick can (MFC) and an underflow elutriator (UE) described in this paper were evaluated and compared for cyst recovery efficiency and amount of organic flotsam collected. The MFC and UE extracted similar numbers of cysts, but the UE collected 50% less flotsam than the MFC. Sample size was negatively correlated with cyst recovery and positively correlated with amount of flotsam. The amount of flotsam recovered with the MFC was correlated with overflow speed. Presoaking air dried samples for 30 minutes halved the amount of flotsam without affecting cyst recovery. Extracting cysts from wet soil without prior drying resulted in negligible recovery with both extraction techniques. There were no significant differences in cyst recovery of the two genera tested.     

Comparisons of Mitochondrial DNA from the Sibling Species Heterodera glycines and H. schachtii

Restriction fragment patterns of mitochondrial DNA from sibling species of cyst nematodes Heterodera glycines and H. schachtii were examined. Fourteen restriction endonucleases recognizing four, five, and six base-pair sequences yielded a total of 90 scorable fragments of which 10% were shared by both species. Mitochondrial genome sizes for H. glycines and H. schachtii were estimated to be 22.5-23.5 kb and 23.0 kb, respectively. A single wild type mitochondrial genome was identified in all populations of H. glycines examined, although other mitochondrial genomes were present in some populations. The H. schachtii genome exhibited 57 scorable fragments, compared with 33 identified in the H. glycines wild type genome. The estimated nucleotide sequence divergence between the two species was p = 0.145. This estimate suggests these species diverged from a common ancestor 7.3-14.8 million years ago.     

Cereal Cyst Nematode (Heterodera avenae) on Oats. II. Early Root Development and Nematode Tolerance

The effect of Heterodera avenae infestation on early seminal and lateral root growth was examined in four oat genotypes differing in tolerance to H. avenae. Recently emerged seminal roots were inoculated with a range of H. avenae larval densities, then transferred a hydroponic system to remove the effect of later nematode penetration on root development. Intolerance to H. avenae was assessed in terms of impairment of seminal root extension resulting in fewer primary lateral roots emerging from the seminal root below the zone of juvenile penetration. Tolerant plants infested with H. avenae had longer lateral root systems than infested intolerant plants. The decline in lateral root growth below the penetration zone was partly offset by increased growth above. This did not contribute to tolerance, however, as there were no differences between cultivars for this feature. Nematodes induced earlier nodal root emergence in all cultivars. Nodal root development was most advanced on the most tolerant cultivar.     

Nonhost Root Penetration by Soybean Cyst Nematode

A total of 66 plants in 50 species were inoculated with eggs and juveniles of soybean cyst nematode, Heterodera glycines. Roots were stained and observed for penetration and development of the nematode. Twenty-six plants were not penetrated; twenty-three were penetrated, but there was no development of the nematode; eight were penetrated with some nematode development; two were penetrated and had considerable nematode development, but few nematodes, if any, matured; and seven were penetrated with many nematodes maturing. The penetration of nonhosts may imply some susceptibility and that populations eventually would build up on the penetrated plants. Plants not penetrated may be useful as rotation plants because no reproduction would occur.     

Parasitism of the Nematode Heterodera glycines by the Fungus Hirsutella rhossiliensis as Influenced by Crop Sequence

The effect of crop sequence on parasitism of second-stage juveniles (J2) of Heterodera glycines by Hirsutella rhossiliensis was investigated. Data were collected from plots of a long-term crop rotation experiment established in 1982. Crop sequences included (i) continuous monoculture of corn and soybean; (ii) annual rotation of the two crops; and (iii) 1, 2, 3, 4, or 5 years of each crop following 5 years of the other crop. The nematode J2 density and percentage of J2 parasitized by the fungus were determined at planting, midseason, and end of season in 1997 and 1998. A significant effect of the crop sequence on parasitism of J2 was observed at midseason in both years and at end of season in 1998. In plots of first-year soybean following 5 years of corn, fungal parasitism increased from an undetectable level at planting to 2% and 4% of J2 parasitized by ends of season in 1997 and 1998, respectively. Fungal parasitism was similar in plots of second-through-fifth-year soybean after 5 years of corn and in plots of soybean monoculture. Parasitism of J2 in the soybean plots in annual rotation with corn increased from undetectable and 2% at planting to 6% and 23% at midseason in 1997 and 1998, respectively. The effect of crop sequence on the fungal parasitism of J2 may be attributed to a density-dependent relationship between the parasite and its host. Season also affected the fungal parasitism; percentage of J2 parasitized by the fungus was the highest at midseason and the lowest at planting.     

Description and SEM Observations of a New Species of Cyst Nematode Heterodera goldeni (Nematoda: Heteroderidae) Attacking Panicum coloratum in Egypt

A cyst nematode, Heterodera goldeni n. sp., is photographed and described from Qasabagrass roots (Panicum coloratum L.) in Alexandria, Egypt. It is characterized in having second-stage juveniles with body length of 546 µm (450-612), stylet length of 22.6 µm (22-23.5) with anchor-shaped knobs, lateral field with 3 lines, tail 60-75 µm, hyaline tail terminus 38.4 µm (33-43); cysts are lemon-shaped, dark to light brown with an extensive sub-crystalline layer covering the entire cyst, cuticular midbody pattern zig-zag, cysts ambifenestrate, well-developed underbridge with finger-like projections, bullae present, vulva slit measuring 44-48 µm long. Males are absent, and females have heavy punctations on the cuticle. Its relationship to H. graminophila described from Florida and Louisiana and H. leuceilyma described from Florida are discussed. The present known distribution is restricted to Alexandria, Egypt. Its economic importance in rangeland grasses and cultivated crops such as rice is not known.     

Field Efficacy of Verticillium lecanii,Sex Pheromone,and Pheromone Analogs as Potential Management Agents for Soybean Cyst Nematode

A soybean cyst nematode sex pheromone (vanillic acid), chemical analogs of the pheromone, and the fungus Verticillium lecanii were applied in alginate prills (340 kg/ha) to microplots and small-scale field plots as potential management agents for Heterodera glycines on soybean. In 1991 microplot tests, treatment with V. lecanii, vanillic acid, syringic acid plus V. lecanii, or vanillic acid plus V. lecanii lowered midseason cyst numbers compared with the untreated susceptible cultivar control, autoclaved V. lecanii treatment, or aldicarb treatment, At-harvest cyst numbers were lowest with V. lecanii and with vanillic acid treatments. Aldicarb treatment reduced midseason cyst numbers in 1992. There were no differences among seed yields either year. In the field trials, numbers of cysts were reduced one or both years with aldicarb, ferulic acid, syringic acid, vanillic acid, or 4-hydroxy-3-methoxybenzonitfile treatments, or with a resistant cultivar, compared to an untreated susceptible cultivar. Highest yields were recorded after treatment with 4-hydroxy-3-methoxybenzonitrile (1991), methyl vanillate (1992), and aldicarb (1992). These studies indicate that some chemical analogs of vanillic acid have potential for use in soybean cyst nematode management schemes.     

Specific and Functional Diversity of Endophytic Bacteria from Pine Wood Nematode Bursaphelenchus Xylophilus with Different Virulence

Pine wilt disease (PWD) caused by the pine wood nematode (PWN), Bursaphelenchus xylophilus, is one of the most devastating diseases of Pinus spp. The PWN was therefore listed as one of the most dangerous forest pests in China meriting quarantine. Virulence of the PWN is closely linked with the spread of PWD. However, main factors responsible for the virulence of PWNs are still unclear. Recently epiphytic bacteria carried by PWNs have drawn much attention. But little is known about the relationship between endophytic bacteria and virulence of B. xylophilus. In this research, virulence of ten strains of B. xylophilus from different geographical areas in six provinces of China and four pine species were tested with 2-year-old seedlings of Pinus thunbergii. Endophytic bacteria were isolated from PWNs with different virulence to investigate the relationship between the bacteria and PWN virulence. Meanwhile, the carbon metabolism of endophytic bacteria from highly and low virulent B. xylophilus was analyzed using Biolog plates (ECO). The results indicated that ten strains of PWNs showed a wide range of virulence. Simultaneously, endophytic bacteria were isolated from 90% of the B. xylophilus strains. The dominant endophytic bacteria in the nematodes were identified as species of Stenotrophomonas, Achromobacter, Ewingella, Leifsonia, Rhizobium, and Pseudomonas using molecular and biochemical methods. Moreover, S. maltophilia, and A. xylosoxidans subsp. xylosoxidans were the predominant strains. Most of the strains (80%) from P. massoniana contained either S. maltophilia, A. xylosoxidans, or both species. There was a difference between the abilities of the endophytic bacteria to utilize carbon sources. Endophytic bacteria from highly virulent B. xylophilus had a relatively high utilization rate of carbohydrate and carboxylic acids, while bacteria from low virulent B. xylophilus made better use of amino acids. In conclusion, endophytic bacteria widely exist in B. xylophilus from different pines and areas; and B. xylophilus strains with different virulence possessed various endophytic bacteria and diverse carbon metabolism which suggested that the endophytic bacteria species and carbon metabolism might be related with the B. xylophilus virulence.     

Response of Soybean in Cyst Nematode-Infested Soils at Three Soil Water Regimes

Large pot (2 years) and field experiments (1 year) were conducted to determine the response of susceptible soybean Glycine max (L.) Merr. cultivars (Essex and Hutcheson) grown in soybean-cyst-nematode (SCN), Heterodera glycines-infested soils at three soil water regimes. The soil water regimes were irrigation whenever soil water potential ([psi]s) 0.30-m deep was i) -30 kPa (I-30) or ii) - 50 kPa (I-50), and iii) no irrigation. Cyst nematode levels in the pot experiment were either 0 or 20,000 second-stage juveniles (J2) per pot. The field experiment was conducted on soil naturally infested with a population of 145 to 475 cysts L⁻¹ of soil. All growth parameters studied were drastically affected in the presence of SCN under nonirrigated conditions for the large pot tests; however, SCN did not influence growth parameters in the field experiment. Seed yield was lowest in the no irrigation treatment when all treatments were compared in both the pot and field experiments. The infested no irrigation treatment in the pot experiment had the lowest yield among soil water treatments.     

Attempt to Select a Cyst Nematode Population on Soybean Plant Introduction 437654

A method of selecting soybean cyst nematode (Heterodera glycines Ichinohe) on segregating soybean progeny was evaluated for developing a population capable of reproducing on PI 437654. Direct selection on PI 437654 was not possible, since no cysts developed on it. Cysts were selected for 12 nematode generations on F₃ and F₄ plants of Forrest x PI 437654. No cysts of the selected population were produced on PI 437654, but more males were produced on it by the selected population than by the base population. The number of cysts on Forrest and other soybean lines considered to have some of the same genes for resistance increased with selection as expected. The increase in number of males on these other lines with some of the same genes for resistance as Forrest was greater than anticipated, indicating that these lines may have some of the same genes as PI 437654.     

Association of Verticillium chlamydosporium and Paecilomyces lilacinus with Root-knot Nematode Infested Soil

Population densities of Meloidogyne incognita and the nematophagous fungi, Paecilomyces lilacinus and Verticillium chlamydosporium, were determined in 20 northern California tomato fields over two growing seasons. Paecilomyces lilacinus was isolated from three fields, V. chlamydosporium was isolated from one field, and both fungi were isolated from 12 fields. Verticillium chlamydosporium numbers were positively correlated with numbers of M. incognita and P. lilacinus. Paecilomyces lilacinus numbers were positively correlated with V. chlamydosporium numbers, but they did not correlate with M. incognita numbers. The correlation coefficients were low (R < 0.5) but significant (P < 0.05). All P. lilacinus and V. chlamydosporium field isolates parasitized M. incognita eggs in vitro. In a greenhouse study, numbers of V. chlamydosporium and P. lilacinus increased more in soils with M. incognita-infected tomato plants than in soil with uninfected tomato plants. After 10 weeks, the Pf/ Pi of second-stage juveniles in soils infested with P. lilacinus, V. chlamydosporium, and M. incognita was 47.1 to 295.6. The results suggest V. chlamydosporium and P. lilacinus are not effectively suppressing populations of M. incognita in California tomato fields.     

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.