Final Nematode Population Densities Research Articles

Evaluation of Wheat Cultivars and Germplasm Lines for Resistance to Pratylenchus neglectus Populations Collected in North Dakota.

Root-lesion nematode (RLN; Pratylenchus neglectus) is a migratory endoparasite and a major soilborne pathogen that affects wheat (Triticum spp.) production worldwide. Genetic resistance is one of the most economical and effective ways to manage P. neglectus in wheat. This study evaluated 37 local cultivars and germplasm lines in seven greenhouse experiments, including 26 hexaploid wheat, six durum wheat, two synthetic hexaploid wheat, one emmer wheat, and two triticale for P. neglectus resistance from 2016 to 2020. North Dakota field soils infested with two RLN populations (350 to 1,125 nematodes per kilogram of soil) were used for resistance screening under controlled greenhouse conditions. The final nematode population density for each cultivar and line was counted under the microscope to categorize the resistance ranking of these entries as resistant, moderately resistant, moderately susceptible, and susceptible. Out of the 37 cultivars and lines, one was classified as resistant (Brennan); 18 were moderately resistant (Divide, Carpio, Prosper, Advance, Alkabo, SY Soren, Barlow, Bolles, Select, Faller, Briggs, WB Mayville, SY Ingmar, W7984, PI 626573, Ben, Grandin, and Villax St. Jose); 11 were moderately susceptible; and seven were susceptible to P. neglectus. The resistant to moderately resistant lines identified in this study could be used in breeding programs after the resistance genes or loci are further elucidated. This research provides valuable information about P. neglectus resistance among wheat and triticale cultivars used in the Upper Midwest region of the United States.

Population dynamics and damage threshold of Pratylenchus n. sp. and Meloidogyne javanica on finger millet

Summary Finger millet (Eleusine coracana) is a staple crop for subsistence farmers and is primarily cultivated in arid and semi-arid regions. Nevertheless, there are limited data available on the occurrence of plant-parasitic nematodes that specifically target finger millet, and the damage arising from such infections. In this study, we investigated the population dynamics, damage threshold and tolerance of the lesion nematode, Pratylenchus n. sp., and the root-knot nematode, Meloidogyne javanica, on finger millet ‘P-224’. We used 11 initial population densities (Pi) of second-stage juveniles (J2): 0, 0.125, 0.25, 0.5, 1, 2, 4, 8, 16, 32 and 64 J2 (g soil)−1 for M. javanica and the same densities of mixed life-stages for Pratylenchus n. sp. The logistic growth model, Seinhorst yield loss and population dynamic models were fitted to the shoot height, plant biomass and final nematode population density. Based on the population dynamics model parameters, maximum multiplication rates and maximum population densities for each nematode species were estimated. The maximum multiplication rates (a) were 32.39 and 17.46, whilst the maximum population densities (M) were 18.83 nematodes (g soil)−1 and 19.78 J2 (g soil)−1 for Pratylenchus n. sp. and M. javanica respectively. The maximum height reached (C) was negatively affected by increasing Pi for both nematode genera, while the rate of growth (B) and the time to reach was not affected based on the logistic model. A tolerance limit (T) of 1.70 nematodes (g soil)−1 and 0.65 J2 (g soil)−1; relative minimum grain yield (m) of 0.23 and 0.40 for Pratylenchus n. sp. and M. javanica was found, respectively. The results indicate that ‘P-224’ is a good host for both Pratylenchus n. sp. and M. javanica, which significantly reduce yield. These findings can be used as a base to develop effective nematode management strategies for finger millet.

Population Development of the Root-Lesion Nematode Pratylenchus dakotaensis on Soybean Cultivars.

Two greenhouse experiments were conducted with the new species of root-lesion nematode, Pratylenchus dakotaensis, discovered in North Dakota, USA in 2017 and named in 2021. Experiment 1 was conducted to ascertain resistance levels of soybean (Glycine max [L.] Merr.) cultivars to the new species P. dakotaensis and Experiment 2 was conducted to evaluate the population levels of P. dakotaensis in roots versus soil. A total of 20 soybean cultivars, and the positive control, cultivar Barnes, accompanying a nonplanted control, were evaluated in both experiments; they each had five replicates and the experiments were repeated. Among the 20 soybean cultivars evaluated, a cultivar with the ID# 13 consistently produced the greatest population densities of P. dakotaensis across all the experiments and therefore it was selected as the susceptible check. The ratio of the final nematode population density of a test cultivar relative to the final population density of the susceptible check was used to scale resistance rating. Combined results of all the trials indicated that seven of the cultivars were moderately resistant, nine of the cultivars plus 'Barnes' were moderately susceptible, and four of the cultivars were susceptible. However, none of the cultivars tested were classified as resistant. Analysis of P. dakotaensis population levels revealed that ≥50% of the population resided in the root habitat and the remaining nematodes were recovered from soil at 9 weeks after planting for a majority of the cultivars tested. These results provide an insight into the virulence of P. dakotaensis on commercial soybean cultivars.

Resistance of Wheat Genotypes to Root-Lesion Nematode (Pratylenchus thornei) Can be Used to Predict Final Nematode Population Densities, Crop Greenness, and Grain Yield in the Field.

The root-lesion nematode Pratylenchus thornei is a major pathogen of wheat (Triticum aestivum) in many regions globally. Resistance of wheat genotypes to P. thornei can be determined from final nematode population densities in glasshouse experiments but combining results across multiple experiments presents challenges. Here, we use a factor analytic method for multiexperiment analysis of final population densities of P. thornei for 1,096 unique wheat genotypes in 22 glasshouse experiments. The resistance to P. thornei of the genotypes was effectively represented by a two-factor model with rotation of the axes to a principal components solution. Principal axes 1 and 2 (PA1 and PA2) accounted for 79 and 11% of the genetic variance, respectively, over all experiments. Final population densities of P. thornei as empirical best linear unbiased predictors (PA[1+2]-eBLUPs) from the combined glasshouse experiments were highly predictive (P < 0.001) of final nematode population densities in the soil profile, crop canopy greenness (normalized difference vegetation index), and grain yield of wheat genotypes in P. thornei-infested fields in the Australian subtropical grain region. Nine categories of resistance ratings for wheat genotypes from resistant to very susceptible were based on subdivision of the range of PA(1+2)-eBLUPs for use in growers' sowing guides. Nine genotypes were nominated as references for future resistance experiments. Most (62%) Australian wheat genotypes were in the most susceptible three categories (susceptible, susceptible to very susceptible, and very susceptible). However, resistant germplasm characterized in this study could be used in plant breeding to considerably improve the overall resistance of Australian wheat crops.

Influence of Nemafric-BL phytonematicide on suppression of Meloidogyne incognita and growth of zucchini under greenhouse conditions

Zucchini or squash (Cucumis pepo), also referred to as the green baby marrows, are highly susceptible to Meloidogyne species, without any genotype with resistance to the nematode genus. Cucurbitacin-containing phytonematicides are being used as alternatives to methyl bromide in various crops, with limited information on whether the product could induce auto-allelopathy in C. pepo cultivars. The objective of the study was to investigate the effects of Nemafric-BL phytonematicide on suppression of M. incognita and growth of zucchini under greenhouse conditions. Zucchini cv. ‘Cartesa’ seedlings were inoculated with 5000 eggs and second-stage juveniles (J2) of M. incognita and then managed at weekly basis using Nemafric-BL phytonematicide at 0, 2, 4, 8, 16 and 32%. Eight weeks after the treatments, relative to untreated control, the treatments reduced eggs (95–100%) and J2 (99–100) in roots, J2 (86–110%) in soil and final nematode population densities (97–100), but increased dry fruit mass (5–14%) and fruit number (66–137%), without affecting other plant variables. Nemafric-BL phytonematicide could be used in managing population densities of Meloidogyne species in zucchini production under greenhouse conditions without inducing phytotoxicity.

Effects of initial nematode population density and water regime on resistance and tolerance to the rice root-knot nematode Meloidogyne graminicola in African and Asian rice genotypes

ABSTRACTThe rice root-knot nematode (RKN), Meloidogyne graminicola, is an important pathogen affecting rice production in South and Southeast Asia. Efficacy of resistance and tolerance in selected M. graminicola-resistant African rice genotypes TOG5674, TOG5675 and CG14 and -susceptible Asian rice genotypes IR64 and UPLRi-5 were examined under a range of initial population densities (Pi) and water regimes. Resistance to M. graminicola in resistant rice genotypes was not broken with increasing pathogen pressure (Pi = 15,000 to 60,000 J2/plant). Resistant rice genotypes were even tolerant to the damage and yield loss caused by high pathogen pressure. On the other hand, increasing Pi levels caused more damage on susceptible rice genotypes. Final nematode population densities in the root systems of resistant and susceptible rice genotypes were significantly lower under flooded conditions than under upland and drought conditions. TOG5674, TOG5675 and CG14 were more tolerant to M. graminicola infection even when grown under upland and drought conditions while IR64 and UPLRi-5 were highly sensitive.

Responses of the citrus nematode population densities post-application of Nemarioc-AL phytonematicide and aldicarb synthetic nematicide

Density-dependent growth patterns suggested that under phytonematicides population densities of plant-parasitic nematodes could either be stimulated, neutral or inhibited, thereby resulting in ‘inconsistent results' concept. The objective of this study was to determine whether post-application sampling of the citrus nematode (Tylenchulus semipenetrans Cobb) population densities under untreated control, Nemarioc-AG phytonematicide and aldicarb on rough lemon (Citrus jambhiri Lush. ) seedlings could provide quantitative evidence of ‘inconsistent results'. Uniform three-month old C. jambhiri seedlings in plastic pots containing 2700 ml growing mixture were each inoculated with 10000 T. semipenetrans eggs and second-stage juveniles (J2). A week after inoculation, seedlings were split into three groups of seventy each and subjected to once-off treatment of untreated control, 2 g Nemarioc-AG phytonematicide and 2 g aldicarb. Five seedlings from each group were randomly placed on greenhouse benches to serve as a sampling block, with a total of 14 blocks. The entire block was sampled on a weekly basis and assessed for final nematode population density (Pf). After the final assessment, Pf under untreated control over increasing sampling intervals exhibited positive quadratic relations, with the model explaining 90% associations, with optimum Pf of 13804 eggs and J2 at six weeks post-application. In contrast, Pf over increasing sampling intervals under phytonematicide and aldicarb exhibited negative quadratic relations, with the model explaining 95 and 92% associations, respectively. The two had minimum Pf at 974 eggs and J2 and 2205 eggs and J2, respectively, each at approximately six weeks. In conclusion, opposite temporal cyclic nematode population growth patterns post-application of nematicidal products and untreated control provided empirically-based evidence of ‘inconsistent results' in nematode management under phytonematicides.

Potential existence of multiple Tylenchulus semipenetrans biotypes in South Africa

The existence of multiple Tylenchulus semipenetrans biotypes within a citrus-producing region complicates the selection of nematode-resistant rootstocks for managing the slow decline of citrus. The potential existence of multiple T. semipenetrans biotypes in South Africa was investigated under greenhouse conditions on differential host plants using nematode isolates from 18 citrus-producing districts with at least six diverse soil groups and three different climates. After three nematode generations, the regional effects had no influence on final nematode population densities, whereas the differential host plants significantly segregated the nematode reproductive potentials. On Olea europaea reproductive potentials (eggs and juveniles/g) were zero, whereas those on Citrus jambhiri and Poncirus trifoliata ranged from 79 to 482 and 86 to 433, respectively. All T. semipenetrans isolates, regardless of the soil group and climate type, behaved similarly to the Poncirus biotype, which therefore rules out the existence of multiple T. semipenetrans biotypes in South Africa.

Effects of the root-knot nematodes Meloidogyne incognita and M. javanica on olive plants growth in glasshouse conditions

Abstract The influence of ten initial population levels (0, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 second stage juveniles/cm3 soil) of Meloidogyne incognita and M. javanica on olive cultivar Zard growth was studied in two pot trials. Ten month old self-rooted olive cuttings were individually transplanted into 2 000 cm3 pot and inoculated with the defined initial populations, of both nematode species. Plants were grown in glasshouse for 13 months, then they were uprooted and plant growth (percent growth increase of main shoot length, number of nodes on main shoot, top and root fresh and dry weights and root length) and nematode parameters (root gall index, J2/g root, final nematode population density and reproduction rate Pf/Pi) were recorded. Results showed that cv. Zard was more susceptible to M. javanica than to M. incognita. A significant reduction of main shoot length growth 37.6 % and 10.7 % was observed at 0.1 and 12.8 juveniles/cm3 soil of M. javanica and M. incognita, respectively, in comparison to uninfested plants. Root systems of olive plants grown in M. incognita or M. javanica infested soils were galled within the gall index range 1.4–6. No significant differences were observed in the number of nodes on main shoot, top and root fresh weights and root dry weight at high levels of M. incognita Pi. A tolerance limit (T) of 0.4 juveniles/cm3 soil was estimated for olive plants cv. Zard to M. javania. The use of resistant olive rootstock or selected cultivars is recommended to minimize or to limit damage of nematode infections in nurseries and to prevent secondary attacks of soil borne plant pathogens especially Verticillium dahliae.

English

Bio-pesticides, when used as a post-planting pesticide, are limited by their potential ability to suppress the pest and their degree of phytotoxicity. Baseline information on the suitability of fermented crude extracts (FCE) of Cucumis africanus fruit as a post-planting bio-nematicide was determined on Meloidogyne incognita and growth of tomato in pot trials. Seedlings were inoculated with 1,130 eggs and juveniles/pot, while FCE were applied weekly at 0, 10, 20, 30, 40, 50 and 60% dilutions. At harvest, nematode and plant data were subjected to analysis of variance and the curve-fitting allelochemical response data (CARD) computer-based model, respectively. Relative to inoculation level, final nematode population density (Pf) at the same level of inoculation was reduced, while increasing dosages had no effect on Pf and reproductive factor values. Since increasing dosages had no effect on Pf, the material could possibly still reduce this variable when applied below 10% dilutions. Dosages used were phytotoxic to tomato plants, but CARD model demonstrated that the material stimulated plant growth at dosages below 10%. In conclusion, baseline information from CARD model suggested that FCE from C. africanus fruit have the potential to serve as a bio-nematicide and bio-fertiliser on tomato provided the material was applied below 10% dilutions. Key words : Cucumis africanus , bionemagation, effective microbe organisms, ground leaching technology, root-knot nematode.

Optimizing application frequency of diluted (3%) fermented Cucumis africanus fruit in tomato production and nematode management

A microplot experiment was conducted to determine application interval of fermented crude extracts of wild watermelon (Cucumis africanus) fruit for optimum growth of tomato (Solanum lycopersicum) plants and suppression of the southern root-knot (Meloidogyne incognita) nematode. Four-week-old tomato cultivar ‘Floradade’ seedlings were each inoculated with 5000 second-stage juveniles of M. incognita race 2 with 3% dilution of fermented crude extracts applied through drip irrigation at 1, 2, 3 and 4 times per 30-day month. At 56 days after the treatment, plant variables and increasing application frequencies had quadratic relationships, with the model explaining 63–99% total treatment variation in variables. Mean integrated optimum application frequency was 2.4 weeks, which translated to an optimum 17-day application interval. At this interval, final nematode population densities were low. In conclusion, at 3% dilution fermented crude extracts from C. africanus fruit could be applied at 17-day interval for interruption of the nematode life cycle and improvement of tomato plant growth.

Suitability of Brassica oleracea leaves in managing Meloidogyne incognita through the ground leaching technology system under microplot conditions

The ground leaching technology (GLT) system was introduced in low-input farming systems in response to the withdrawal of methyl bromide from agrochemical markets and the amelioration of conventional organic amendment drawbacks in managing plant-parasitic nematodes in olericulture. The primary indicator for success in GLT system is that potent nematicidal chemicals be released through leaching by water, while in conventional organic amendments potent chemicals are released through microbial degradation. Due to widespread availability of cabbage (Brassica oleracea) leaves in local markets as a waste, the material would be readily available for use in GLT systems in peri-urban agriculture. The objective of the study was to determine whether crude extracts of B. oleracea leaves were suitable for use in GLT system for managing the root-knot (Meloidogyne spp.) nematodes and improving growth of tomato (Solanum lycorpersicum). A 2×2×2 factorial experiment, with the main factors being Bacillus species, crude extracts of B. oleracea leaves and Meloidogyne incognita, was conducted over two seasons under microplot conditions. At harvest, Bacillus×Brassica×nematode interaction reduced final nematode population densities, fresh fruit yield, dry shoot mass, plant height and stem diameter, while Bacillus× Brassica interaction increased tomato growth variables. Incidentally, Brassica alone reduced the productivity of tomato. Overall, the efficacy of crude extracts of B. oleracea leaves in nematode suppression and tomato productivity was dependent upon microbial activities and therefore, the material was not suitable for use in GLT systems.

Mean Dosage Stimulation Range of Allelochemicals from Crude Extracts of Cucumis africanus Fruit for Improving Growth of Tomato Plant and Suppressing Meloidogyne incognita Numbers

Successful utilisation of allelochemicals in management of plant-parasitic nematodes depends on their degree of phytotoxicity. Conventional methods of determining phytotoxicity are tedious, with inconsistent results. Plants respond to increased dosages of allelochemicals in a density-dependent growth pattern, which allows the use of the Curve-fitting Allelochemical Response Data computer-based model to determine the mean dosage stimulation range of used allelochemicals. The CARD modelling was used to determine the stimulation range of fermented dried crude extracts of wild cucumber (Cucumis africanus) fruit for improving growth of tomato (Solanum lycopersicon) plants, each infested with 1500 eggs and juveniles of the southern root-knot (Meloidogyne incognita) nematode. Dilutions at 0, 2, 4, 8, 16, 32 and 64% were applied weekly through irrigation system. At 56 days after treatment, CARD demonstrated density-dependent growth patterns as dosages increased. The mean dosage stimulation range of diluted fermented crude extracts, computed from CARD biological indices, was 2.64% dilution for tomato plant. Since at 2% dilution, the material reduced final nematode population density of M. incognita by 90%. The 2.64% was suitable for stimulation of tomato plant and suppression of nematode numbers.

Nematicidal and fertilizing effects of chicken manure, fresh and composted olive mill wastes on organic melon

Abstract The fertilizing and nematicidal effects of three organic amendments were evaluated in a pot experiment on melon plants infested by the root-knot nematode Meloidogyne incognita. A soil artificially infested with 4 eggs and juveniles/ ml soil of the nematode was amended with: a) virgin olive pomace (VOP); b) composted olive pomace (COP); c) chicken manure based fertilizer (CM) and d) chicken manure based fertilizer combined with the biological control agent Paecilomyces lilacinus strain 251, brand name BioAct WG (CMB). VOP was applied at doses of 11 (VOP-A), 22 (VOP-B) and 44 t/ha (VOP-C); COP at 4.5 (COP-A), 9 (COP-B) and 18 t/ha (COP-C); CM at 3 t/ha and CMB at 3 t/ha combined with 4 kg/ha of BioAct WG. Untreated soil was used as control. The treatments CM, CMB, VOP-B and COP-B were established on the basis of N requirement of melon plants (120 kg/ha) taking into account soil and amendments N availability. Two weeks later amendment application and nematode inoculation, the soil was poured in 4.8 l clay pots which were arranged in a greenhouse according to a randomized block design with ten replications for each treatment. A one-month old melon seedling (cv. Galia) was transplanted in each pot and organic farming management practices were used during the growing period. At the end of the experiment, 60 days after transplant, plants were uprooted and height, fresh and dry shoot and root weights were recorded. Root gall index, on the roots, caused by the nematode attack, was estimated according to a 0–5 scale. Final nematode population density and reproduction rate were also calculated for each pot. All data were subjected to statistical analysis of variance (ANOVA) and means compared according to Least Significant Difference’s Test. Nematode population and root infestation were significantly suppressed by the addition of all amendments, compared to untreated control. However, CM and CMB resulted in a total more suppressive effect and in a significantly higher plant growth in comparison to all the other treatments. A significant correlation was found between root gall index and eggs and juveniles/g root and final nematode population density. No signifycant correlations were found between nematological parameters or plant growth parameters and amendment doses.

Interactive effects of Meloidogyne incognita race 2, Bradyrhizobium japonicum and crude extracts of Cucumis myriocarpus fruit on Vigna unguiculata

Crude extracts of wild cucumber (Cucumis myriocarpus) fruit, with scant information of their effects on nodulation and growth of cowpea (Vigna unguiculata), had been touted as having potential to serve as bio-nematicide and bio-fertilizer in low-input farming systems. Interactive effects of the southern root-knot nematode (Meloidogyne incognita), Bradyrhizobium japonicum and crude extracts of C. myriocarpus fruit on final nematode population density, nodulation and growth of cowpea variety Eureka were investigated in a 2 × 2 × 2 factorial experiment under greenhouse conditions. At 70 days after the treatments, significant (P ≤ 0.05), first order interactions in Nematode × Cucumis reduced final nematode population densities and nodule numbers by 79% and 20%, respectively. However, in the absence of nematodes, Cucumis increased nodule numbers by 9%. Similarly, Nematode × Bradyrhizobium reduced fresh shoot mass, dry shoot mass and nodule mass by 57%, 59% and 64%, respectively. However, in the absence of nematodes, Bradyrhizobium increased the variables by 50%, 42% and 35%, respectively. Compared to nematode alone, the two interactions had a relative ameliorative effect on the variables measured. Consequently, crude extracts of C. myriocarpus fruit are suitable for use in managing population densities of M. incognita on cowpea production.

Screening Musa germplasm for resistance to burrowing nematode populations from Uganda

Nematode pathogenicity experiments on Musa host plants were carried out in pot trials. Both the final nematode population densities and percentages root necrosis on different host plants were higher for the Radopholus similis population from Mbarara than for the populations from Namulonge, Ikulwe and Mukono (Uganda). The R. similis population from Mbarara managed to break the resistance of the diploid banana Pisang Jari Buaya, a known worldwide source of resistance to R. similis. The diploid banana hybrid TMB2× 9128-3 and the triploid dessert banana Yangambi km5 showed resistance against the four R. similis populations. These results indicate that differences in pathogenicity among different R. similis populations exist and should be taken into consideration for plantain and banana germplasm enhancement.

The reaction of soybean and other legume crops to root-knot nematodes (Meloidogynespp.), and implications for growing these crops in rotation with sugarcane

The reaction of soybean, mungbean, peanut, lablab, cowpea and velvet bean to two species of root-knot nematode (Meloidogyne javanica and M. incognita) was assessed by bare fallowing a field after sugarcane harvest and planting the legumes 4 and 12weeks later. Leichhardt, the most widely grown soybean cultivar in the Queensland sugar industry and three other soybean cultivars were included whereas the other legumes were represented by only one cultivar. The severity of nematode galling was assessed 7 weeks after planting and final nematode population densitieswere measured at ∼14weeks. The roots of mungbean, lablab and all soybean cultivars except cv. Stuart were severely galled by root-knot nematode at both planting times; however, because root-knot nematode populations declined rapidly in the bare fallow that followed the sugarcane harvest, galling was less severe in the second planting compared with the first planting. Peanut, velvet bean and cowpea cv. Meringa showed little or no nematode damage at either planting time. Final nematode population densities were relatively high following the heavily galled crops but were similar to an adjacent bare fallow after peanut, velvet bean and soybean cv. Stuart. In a separate glasshouse inoculation study, soybean cv. Leichhardt was badly damaged by a population of M. incognita from the field site but was relatively tolerant to M. javanica and a population of M. incognita from another region (Bundaberg). In another glasshouse test, M. javanica and the Bundaberg population of M. incognita did not increase significantly on soybean cv. Leichhardt or a range of tropical soybean lines after 8 weeks. These results indicate that soybean is generally suitable for rotation with sugarcane but that local populations of root-knot nematode may damage certain cultivars in sandy soils.

A microplot study on Ditylenchus africanus population densities and damage symptoms on groundnut in relation to commercial yields

AbstractOptimum kernel quality is important at all levels of groundnut production and utilisation because of its value as a direct or processed food. Kernel quality can be reduced by many factors but in South Africa the groundnut pod nematode, Ditylenchus africanus, is responsible for severe losses in groundnut crops. The objective of this study was to evaluate in microplots the effect of D. africanus on groundnut yield in terms of commercial grading principles. Differential initial population densities (Pi) of between zero to 7000 and zero to 20000 nematodes per plant were inoculated, respectively, in two separate trials over two consecutive seasons. Significant non-linear relationships were recorded between Pi and nematode densities at harvest (Pf), between Pi and downgrading of kernels (% unsound, blemished and soiled (UBS)) and between % UBS and income earned per tonne. No relationships were found between initial or final nematode population densities and kernel fresh weight. Sufficient evidence was provided that damage by D. africanus on groundnut could be expressed in terms of commercial yield grading stipulations. Therefore, it is possible to estimate the effect of this nematode in monetary terms and to consider control measures in terms of financial gains or losses.

Control of the soybean cyst nematode by the fungi Hirsutella rhossiliensis and Hirsutella minnesotensis in greenhouse studies

The biological control effectiveness of Hirsutella rhossiliensis and Hirsutella minnesotensis, two endoparasitic nematophagous fungi, was evaluated using solid cultures of corn grits against the soybean cyst nematode, Heterodera glycines, in greenhouse experiments. Both fungi, either applied at planting or 2 weeks before planting, were highly effective in control of the nematode. The biological control effectiveness increased with increasing inoculation level of the solid culture from 0.04 to 1% (corn grits:soil, w/w). Control differed between the two species and among isolates. H. rhossiliensis was more effective than H. minnesotensis, and H. rhossiliensis isolate OWVT-1 appeared to be the most effective among all isolates tested. In some experiments, the isolate OWVT-1 almost completely controlled the nematode population. Soil heating treatment increased the effectiveness of the fungi as compared with no-heating treatment. The percentage of H. glycines second-stage juveniles (J2) parasitized by the fungi at the end of the experiment (60 days after planting) was not correlated with level of biological control of the nematode. Parasitism of J2 by H. rhossiliensis at the end of experiment was negatively correlated with fungal inoculation level and positively correlated with the final nematode population densities. H. minnesotensis appeared to establish slower in soil than H. rhossiliensis.

Systemic Resistance in Tomato Induced by Biocontrol Bacteria Against the Root‐Knot Nematode, Meloidogyne javanica is Independent of Salicylic Acid Production

AbstractSalicylic acid (SA)‐mediated induction of systemic resistance by Pseudomonas aeruginosa strain 7NSK2 and P. fluorescens strain CHA0 against soil‐borne fungi and viruses have been reported. The role of SA biosynthesis in the enhancement of defence mechanism against plant‐parasitic nematodes by these bacterial strains in tomato is not known. To better understand the importance of SA in rhizobacteria‐mediated suppression of root‐knot nematodes, biocontrol potential of SA‐negative or SA‐overproducing mutants against Meloidogyne javanica was evaluated with their respective wild type counter parts. Culture supernatant of 7NSK2, CHA0 and their respective mutants caused significant mortality of M. javanica juveniles in vitro. SA deletion in 7NSK2 and SA overproduction in CHA0 did not influence bacterial efficacy to cause nematode deaths. Similarly, culture supernatants resulting from King's B liquid medium amended with FeCl3 did not influence nematicidal activity of the bacterial strains. Strain CHA0 induced juvenile deaths more than 7NSK2 did. In pot experiments, the bacterial strains applied in unsterilized sandy loam soil markedly reduced final nematode population densities in roots and subsequent root‐knot infection in tomato seedlings. SA‐negative or overproducing derivatives prevented tomato roots in kinetics similar to those with their respective wild types. When soil iron concentration was lowered by the addition of ethylenediamine di(o‐hydroxyphenylacetic acid), nematode biocontrol by the bacterial strains (both wild type and mutants) remained unaltered. To understand the mechanism involved in rhizobacteria‐mediated suppression of root‐knot nematode in tomato, bacterial performance was assessed in a split root trial in which one‐half of the root system was treated with bacterium while the other inoculated with nematode. Compared with the controls, application of the bacterial cell suspension to one‐half of the root system lowered the populations of root‐knot nematode in non‐bacterized nematode‐treated sections indicating enhanced defence in the non‐bacterized half. With respect to nematode infection, mutants induced systemic resistance to a similar extent as that caused by the wild types in both wild type tomato and NahG tomato plants. It is concluded that fluorescent pseudomonads induce systemic resistance against root‐knot nematode via a signal transduction pathway, which is independent of SA accumulation in roots.