Initial Nematode Population Density Research Articles

Impact of Pratylenchus penetrans on Soybean Grown in Wisconsin, U.S.A.

Models were developed to quantify the impact of Pratylenchus penetrans to the early season growth and yield of soybean in field and greenhouse environments and to estimate yield loss because of P. penetrans in Wisconsin. There was a negative linear relationship between initial nematode population densities (Pi) and shoot and total plant weight at V2 and yield, pod number, seed number, and seed mass at harvest in the field. Relative yield loss, modeled for the second year of the field experiment, suggested a loss of 4.5% for yield and between 2.4 and 2.8% for yield components at the mean field Pi value. Negative linear relationships were demonstrated for the relative loss in those variables as well as for harvest index and shoot, root, and total plant weight at harvest in the greenhouse. Stress imposed by P. penetrans began within 2 weeks after planting and continued through harvest. Estimates of the percent loss attributed to each nematode Pi value were 0.020% for yield, 0.015% for pod number, and 0.017% for seed number. Pratylenchus spp. was the most widely prevalent pest nematode among samples submitted to a statewide nematode testing program. Molecular identification of a subset of 63 samples suggested 15% were infested with P. penetrans at a mean Pi value of 197 P. penetrans per 100 cm3 soil. Yield loss because of P. penetrans, estimated from prevalence data and our empirical greenhouse model, ranged from 0.23 to 2.76% among Wisconsin's agricultural districts. The cumulative impact for all Pratylenchus spp. is likely much greater, given this loss estimate does not account for the monoecious species present in 79% of the samples.

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.

Genetic variation and population dynamics of the cereal cyst nematode, Heterodera filipjevi in wheat areas of Bolu, Turkey

The cereal cyst nematode, Heterodera filipjevi (Madzhidov) Stelter, was detected in Turkey in 1996 and since then it has been increasingly reported to cause losses in wheat yields. Cereal cyst nematode populations have a wide genetic diversity which makes it difficult to be managed. The objectives of this research were to investigate the genetic variation of 23 cyst nematode populations collected from wheat growing areas of Bolu Province, utilising sequenced ITS-rDNA regions, and monitoring main nematode isolate population dynamics in field conditions. The results of ribosomal DNA sequencing revealed that all cyst populations were H. filipjevi and that little genetic variation was present among populations. The highest number of second-stage juveniles was observed between April and beginning of May in a field experiment; white females were visible on the roots in late May. The hatching process was positively correlated with low temperatures, but there were inverse correlations between reproductive factors and initial nematode population densities.

Population Dynamics and Damage Potential of Meloidogyne hapla to Rose Rootstock Species

AbstractThe relationship between initial population densities (Pi) of Meloidogyne hapla on growth of three rose rootstocks (Rosa corymbifera ‘Laxa’, R. multiflora and R. canina ‘Inermis’) and nematode population development was studied. Each plant species was inoculated with ranges of nematode densities of 0, 0.062, 0.125, 0.25, 0.50, 1, 2, 4, 8, 16, 32, 64 and 128 second‐stage juvenile/g soil and were allowed to grow for 9 weeks. Seinhorst yield model was fitted to total fresh biomass data of the rootstocks. The tolerance limits (T) were 0.04, 0.09 and 0.01 J2/g soil and the minimum yield (m) 0.65, 0.47 and 0.43 for R. corymbifera ‘Laxa’, R. multiflora and R. canina ‘Inermis’, respectively. The reproductive factor (Pf/Pi) was highest at low initial nematode densities for all rootstocks and then decreased to below maintenance level with increasing initial population densities. Root gall severity consistently increased with initial nematode population density. Furthermore, number of root galling showed a strong positive relationship with final nematode population per gram root fresh weight. The relation between Pi and Pf was also fitted to the Seinhorst population model (Pf = (M*Pi)/Pi + M/a). Rosa multiflora supported the population of M. hapla to a maximum population density (M) of 27.53 J2/g soil with an estimated average maximum multiplication rate (a) of 24.39. For R. corymbifera ‘Laxa’ and R. canina, the maximum multiplication rate was 4.34 and 3.62 and the maximum population density 6.08 and 4.78 J2/g dry soil, respectively. Hence, it was demonstrated that all three rootstocks were susceptible to even low initial nematode densities and therefore are considered good hosts for M. hapla.

Effect of Ditylenchus dipsaci Kühn, 1857 (Tylenchida: Anguinidae) on onionyield in Karaman Province, Turkey

This study was conducted to investigate the effect of the stem and bulb nematode Ditylenchus dipsaci Kühn (Tylenchida: Anguinidae) on yield parameters of some onion varieties in Karaman Province, Turkey. Experiments were established in a naturally infested onion field during two consecutive years (2012-2013). Nematode population development was followed at the beginning and end of the experiment. The reproduction rate was also calculated. Onion yield and the representative average diameter and length of the onion cultivar bulbs were determined. Yield parameters of six onion varieties were compared on nematicide-treated and untreated control plots in the experiments. Nematode populations significantly decreased in the nematicide-treated plots at the beginning of the experiment during both growing seasons. The average population density of D. dipsaci was 104 individuals/100 g dry soil in the first year and 68 individuals/100 g dry soil in the second year of the experiment. The initial and final nematode populations were significantly positively correlated with Betapanko, Pan88, and Local variety1 cultivars, whereas they were negatively correlated with the Valenciana cultivar during the first year. This suggests that Betapanko, Pan88, and Local variety1 were susceptible to D. dipsaci, whereas the Valenciana cultivar could have some resistance. The overall reproduction rate of D. dipsaci was negatively correlated with initial population density and was primarily <1, which presumably represented a downward trend in nematode reproduction due to environmental conditions at the end of the experiment in September. Betapanko and Pan88 were susceptible varieties, and stem and bulb nematode caused yield losses of up to 12.93% and 15.33%, respectively; these varieties were considered intolerant. The yield of Local variety1 was positively related with initial nematode population density and it was categorized as tolerant to D. dipsaci.

Pathogenicity and Host-Parasite Relationships of Heterodera cruciferae in Cabbage.

Stunted cabbage ('Lupini') associated with severe soil infestations by a cyst-forming nematode were observed in large patches of open fields in Castellaneta, province of Taranto, southern Italy. Morphological traits based on mature cysts, males, and second-stage juveniles (J2s) and molecular analysis of ribosomal DNA (D2 to D3 expansion segments of 28S and internal transcribed spacer [ITS]1 region) were used to identify the species. ITS1 sequence information supported the identity of Heterodera cruciferae, also showing a high degree of similarity to other species of the Heterodera Goettingiana group, including H. goettingiana, H. carotae, and H. urticae. Nematodes successfully established permanent feeding sites in cabbage roots which caused cellular alterations in the root cortex, endodermis, pericycle, and vascular cylinder by inducing typical multinucleate syncytia. Syncytial cytoplasm was granular and dense, with variously sized vacuoles and hypertrophied nuclei with nucleoli. Cabbage plant growth was also reduced in pathogenicity tests. The relationship between the initial nematode population density in soil and shoot plant weight was well described by the Seinhorst's equation. Tolerance limits with respect to shoot plant weight of cabbage to H. cruciferae was estimated as 1.50 units of eggs plus J2s/cm3 of soil. The minimum relative value (m) for plant height was 0.71 at an initial nematode population density of (Pi) ≥ 64 units of eggs plus J2s/cm3 of soil. The maximum nematode reproduction rate (Pf/Pi) was 4.6 times that of the initial population density of 8 units of eggs plus J2s/cm3 of soil.

Host-pathogen interaction of root-knot nematode Meloidogyne incognita on pepper in the southeast of Spain

The feeder roots of pepper plants (cv. California Wonder) in Campo de Cartagena (southeast Spain) were found to be severely infected by Meloidogyne incognita. Morphometric traits, differential host test and DNA analysis based on PCR were used to characterize the nematode. Naturally and artificially infected pepper plants showed severe yellowing and stunting, with heavily deformed and damaged root systems. Root galls were spherical and commonly contained more than one female and egg masses with eggs. Typical giant cells with a granular cytoplasm and many hypertrophied nuclei were observed in histological preparations. The relationship between initial nematode population density (Pi) and pepper plant growth was tested in greenhouse experiments with inoculum levels that varied from 0 to 64 eggs and second-stage juveniles (J2) ml−1 soil. A Seinhorst model was fitted to plant height and top fresh weight data of inoculated and non-inoculated plants. The tolerance limit with respect to plant height and fresh top weight of pepper to M. incognita was estimated as 0.85 eggs and J2 ml−1 soil. The minimum relative values (m) for plant height and top fresh weight were 0.15 and 0.16, respectively, at Pi ≥ 64 eggs and J2 ml−1 soil. The maximum nematode reproduction rate (Pf/Pi) was 315.4 at an initial population density (Pi) of 4 eggs and J2 ml−1 soil. The obtained results could be used as a base to establish field experiments that allow strategies to prevent surpassing the threshold of nematodes in fields that are infested.

A survey of nematodes associated with pyrethrum in Tasmania, Australia, and the susceptibility of pyrethrum cultivars to root-lesion nematode

In a survey of fields in northern Tasmania, Australia, with various cropping histories, root-lesion nematode (Pratylenchus spp.) was detected in soil from all 99 fields sampled before planting pyrethrum. Population densities were generally low, but seven fields had more than 600 Pratylenchus/200mL soil, with the maximum density being 3930/200mLsoil. Root-knot nematode (Meloidogyne spp.) was detected in 20 fields, with only three having population densities greater than 50/200mL soil. Pratylenchus spp. from a subset of 31 fields were identified to species level, with Pratylenchus crenatus, P. penetrans, P. neglectus and P. Thornei occurring in 27, 10, 2 and 3fields, respectively. In pyrethrum crops aged 1–2 years, root-lesion nematode was recovered from all 70 fields sampled, with 18 fields having more than 300/200mL soil. The highest population density was 960/200mL soil. Eighteen fields had more than 200/g fresh weight of root with a maximum of 786/g fresh weight of root. P. Crenatus and P. Penetrans were extracted from roots of pyrethrum transplants grown for ∼3 months in field soil from 8 and 4 of 10 fields, respectively, indicating that pyrethrum was a host of these species. In two pot experiments conducted in the glasshouse, with four commercial pyrethrum cultivars, P. Penetrans multiplication rates (the ratio of final to initial nematode population densities) ranged from 2.7 to 7.9 and 2.2 to 6.6.

Pathogenicity and host‐parasite relationships of the root‐knot nematode Meloidogyne incognita on celery

Pathogenicity and host‐parasite relationships in root‐knot disease of celery (Apium graveolens ) caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Naturally and artificially infected celery cv. D’elne plants showed severe yellowing and stunting, with heavily deformed and damaged root systems. Nematode‐induced mature galls were spherical and/or ellipsoidal and commonly contained more than one female, males and egg masses with eggs. Feeding sites were characterized by the development of giant cells that contained granular cytoplasm and many hypertrophied nuclei. The cytoplasm of giant cells was aggregated along their thickened cell walls and consequently the vascular tissues within galls appeared disrupted and disorganized. The relationship between initial nematode population density (Pi) and growth of celery plants was tested in glasshouse experiments with inoculum levels that varied from 0 to 512 eggs and second‐stage juveniles (J2) mL−1 soil. Seinhorst's model y = m + (1 – m)zP–T was fitted to height and top fresh weight data of the inoculated and control plants. The tolerance limit with respect to plant height and fresh top weight of celery to M. incognita race 1 was estimated as 0·15 eggs and J2 mL−1 soil. The minimum relative values (m) for plant height and top fresh weight were 0·37 and 0·35, respectively, at Pi ≥ 16 eggs and J2 mL−1 soil. The maximum nematode reproduction rate (Pf/Pi) was 407·6 at an initial population density (Pi) of 4 eggs and J2 mL−1 soil.

Pathogenicity and Host-Parasite Relationships of Meloidogyne arenaria in Sweet Basil.

Stunted growth of sweet basil (cv. Genovese) associated with large patches and severe soil infestations by the root-knot nematode Meloidogyne arenaria race 2 were observed in open fields at the end of the crop-growing season in Albenga, province of Savona, Northern Italy by early September 2007. Observed morphological traits of nematode life stages and results of analysis of isozyme electrophoretic patterns and differential host tests were used for nematode species and race identification. Nematode-induced mature galls (either in naturally infected plants or in artificially inoculated root tissues) were spherical or ellipsoidal and confluent along the root axis, containing usually several females, males, and egg-masses with eggs. Feeding sites were characterized by the development of giant cells that contained several hypertrophied nuclei and nucleoli. Giant cell cytoplasm was aggregated along the thickened cell walls. Vascular elements within galls appeared disorganized and disrupted. Due to the heavy damage observed, the relationships between the initial population density and growth of basil plants was also tested in a greenhouse experiment in which inoculum levels varied from 0 to 512 eggs and juveniles/cm3 of soil. Height and top fresh weight data of the inoculated and control plants was fitted to Seinhorst's model. Tolerance limits with respect to plant height and fresh top weight of basil cv. Genovese plants to M. arenaria race 2 were estimated as 0.15 eggs and juveniles/cm3 of soil. The minimum relative values (m) for plant height and top fresh weight were 0.39 and 0.19 at initial nematode population density (Pi) ≥16 eggs and juveniles/cm3 of soil, respectively. The maximum nematode reproduction rate (Pf/Pi) was 448.7 times at an initial population density of 4 eggs and second-stage juveniles/cm3 of soil.

오이 시설재배시 고구마뿌리혹선충의 경제적피해한계

고구마 뿌리혹선충(Meloidogyne incognita)의 시설재배 오이에 대한 경제적 피해한계를 조사하기 위하여 비닐하우스 내에 합판으로 만든 박스(<TEX>$30{\times}40{\times}15cm$</TEX>)를 묻고 박스 내 토양 <TEX>$100cm^3$</TEX>당 뿌리혹선충 유충의 밀도를 0, 10, 30, 100, 300, 600마리로 조정된 토양을 넣었다. 각 박스에는 '장수신토좌'에 접붙인 '슈퍼만촌'오이 23일 묘를 8월 1일 1포기씩 심고 11월까지 각 구당 오이 수확량을 조사하였다. 뿌리혹선충의 유충밀도와 수량과의 회귀식은 <TEX>$Y=0.82-0.04{\cdot}Log_{10}$</TEX>(Pi+1)로 방제 비용을 감안한 경제적 피해 허용 수준은 정식전 뿌리혹선충 유충의 밀도가 토양 <TEX>$1,000cm^3$</TEX>당 5마리이면 살선충제 입제 살포가 경제적이고, 유충의 밀도가 토양 <TEX>$1,000cm^3$</TEX>당 25마리 이상이면 훈증제를 처리하는 것도 고려해 볼 수 있다. 오이 시설재배시 뿌리혹선충의 피해를 방지하기 위해서는 최소한 정식 수개월 전에 미리 토양 선충 밀도를 조사해야 할 것이다. To determine the economic threshold level of Meloidogyne incognita on cucumber in greenhouse conditions, cucumber seedlings(Cucumis sativa L. cv. 'Super Manchon') grafted on 'Jangsushintozoa'(Cucurbita maxima x Cu. moschata) were planted in wooden boxes(<TEX>$30{\times}40{\times}15cm,\;L{\times}W{\times}D$</TEX>) under a plastic house in August 01 and harvested from Sep. 01-Oct. 30, 2006. The initial nematode population densities(Pi) in the wooden boxes were adjusted to 0, 10, 30, 100, 300, and 600 second-stage juveniles(J2)/100 <TEX>$cm^3$</TEX> soil. The relationship of total fruit yield to Pi level could be adequately described by a linear regression equation, <TEX>$Y=0.82-0.04{\cdot}Log_{10}$</TEX>(Pi+1). Initial nematode densities(Pi) before planting in excess of 5 J2/1,000 <TEX>$cm^3$</TEX> soil caused in total yields loss that is equivalent to the costs of granular nematicide application; Pi level in excess of 25 J2/1,000 <TEX>$cm^3$</TEX> soil caused in total yields loss that exceed the costs for application of fumigants at current control costs. We propose two different management strategies depending on nematode densities, (i) application of non-fumigant granular nematicides for M. incognita Pi level of 5 J2/1,000 <TEX>$cm^3$</TEX> soil and (ii) fumigant treatment with Pi level over 25 J2/1,000 <TEX>$cm^3$</TEX> soil. Soil samples to determine population density of Meloidogyne spp. for advisory purposes should be taken several months before planting time, which allows a period of time enough for implementing management procedures.

Pathogenicity of the root‐knot nematode Meloidogyne javanica on potato

Host–parasite relationships and pathogenicity of Meloidogyne javanica on potatoes (newly recorded from Malta) were studied under glasshouse and natural conditions. Potato cvs Cara and Spunta showed a typical susceptible reaction to M. javanica under natural and artificial infections, respectively. In potato tubers, M. javanica induced feeding sites that consisted of three to four hypertrophied giant cells per adult female. Infection of feeder roots by the nematode resulted in mature large galls which usually contained at least one mature female and egg mass. In both tubers and roots, feeding sites were characterized by giant cells containing granular cytoplasm and many hypertrophied nuclei. Cytoplasm in giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density (P) [0–64 eggs + second‐stage juveniles (J2s) per cm3 soil] and growth of cv. Spunta potato seedlings was tested under glasshouse conditions. A Seinhorst model [y = m + (1 − m)z(P−T)] was fitted to fresh shoot weight and shoot height data of nematode‐inoculated and control plants. Tolerance limits (T) for fresh shoot weight and shoot height of cv. Spunta plants infected with M. javanica were 0·50 and 0·64 eggs + J2s per cm3 soil, respectively. The m parameter in that model (i.e. the minimum possible y‐values) for fresh shoot weight and shoot height were 0·60 and 0·20, respectively, at P = 64 eggs + J2s per cm3 soil. Root galling was proportional to the initial nematode population density. Maximum nematode reproduction rate was 51·2 at a moderate initial population density (P = 4 eggs + J2s per cm3 soil).

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.

Host–parasite relationships of Meloidogyne incognita on spinach

Host–parasite relationships in root‐knot disease of spinach caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Nematode‐induced mature galls were large and usually contained one or more females and egg masses with eggs. Feeding sites were characterized by the development of giant cells containing granular cytoplasm and many hypertrophied nuclei. The cytoplasm in these giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density (Pi) in a series from 0–128 eggs and second‐stage juveniles per cm3 soil and growth of spinach cv. Symphony F1 seedlings was tested under glasshouse conditions. A Seinhorst model [y = m + (1 − m)zP–T] was fitted to fresh top‐ and total plant‐weight data for inoculated and control plants. Tolerance limits (T) of spinach cv. Symphony F1 to M. incognita race 1 for fresh top and total plant weights were 0·25 and 0·5 eggs and second‐stage juveniles per cm3 soil, respectively. The minimum relative values for fresh top and total plant weights were zero in both cases at Pi ≥ 32 eggs and second‐stage juveniles per cm3 soil. Root galling was least at low initial population densities and greatest at 16 eggs and second‐stage juveniles per cm3 soil. Maximum nematode reproduction rate was 33·1‐fold at the lowest Pi.

Host-parasite relationships in tobacco plants infected with a root-knot nematode (Meloidogyne incognita) population from the Azores

During a nematode survey, severe infections of tobacco feeder roots and heavy soil infestations byMeloidogyne incognita race 1 were found in S. Miguel (Azores islands, Portugal). This is the first record ofM. incognita infection of tobacco in Azores. Morphology of various life stages, analysis of the esterase electrophoretic pattern and differential host tests were used for nematode characterization and identification. Nematode-induced mature galls were spherical and/or ellipsoidal and usually contained more than one female, males and egg masses with eggs. Feeding sites were characterized by the development of giant cells that contained granular cytoplasm and many hypertrophied nuclei. Giant cell cytoplasm was aggregated along a thickened cell wall. Vascular tissues within galls appeared disorganized. The relationship between the initial nematode population density and growth of tobacco plants was tested in a glasshouse experiment in which inoculum levels varied from 0 to 512 eggs and juveniles (J2) cm−3 of soil. Seinhorst’s model was fitted to height and top fresh weight data of the inoculated and control plants. Tolerance limits with respect to plant height and fresh top weight of tobacco cv. ‘Erzegovina’ plants toM. incognita race 1 were estimated as 1.25 eggs and J2 cm−3 of soil. The maximum nematode reproduction rate was 404.7 at an initial population density of 4 eggs and J2 cm−3 of soil.

Tolerance of Rosa rootstocks and species to Pratylenchus penetrans

AbstractA method for screening the tolerance of Rosa rootstocks and species to Pratylenchus penetrans was developed after observing the effects of nutrition and initial nematode population density on damage to Rosa corymbifera cv. Laxa. The correlation between initial P.penetrans densities and the weight of host plants fitted (R2 &gt; 0.90) the Seinhorst model, Y = ym for Pi ≤ T, and Y = ym · m + ym(1 – m)z(Pi–T). All inoculated plants of R. virginiana and R. eglanteria survived while there were differences in the mortality after inoculation of 12 other Rosa rootstocks or species. Pratylenchus penetrans caused significant reductions in the numbers of the leaves and roots, and in fresh weights of shoots, roots and whole plants of all 14 accessions. Among these species and cultivars, reductions in root number were least for R. virginiana whereas fresh weights of roots, shoots and whole plants were least affected for R. eglanteria, and the greatest reductions were of R. corymbifera cv. Laxa and R. glauca. Plant mortality and the reduction of shoot and plant weights of surviving plants were positively correlated with the reduction in numbers of secondary roots. The numbers of secondary roots at inoculation were negatively correlated with the degree by which plant weight was reduced and with plant mortality observed 50 days later. Differences in tolerance among species as observed by differences in reduction of plant growth parameters were confirmed by differences of the parameters derived from applying the Seinhorst model to the species.

Host-Parasite Relationships in Root-Knot Disease of White Mulberry.

Severe infections of white mulberry feeder roots and heavy soil infestations by Meloidogyne arenaria race 2 were found in southern Spain. This is the first record of M. arenaria on white mulberry in Europe. Morphometric observations, analysis of the esterase electrophoretic pattern, and artificial inoculations of race differentials were used to characterize nematodes. Nematode-induced mature galls were spherical and usually contained one or more females, males, and egg masses with eggs. Feeding sites were characterized by the development of giant cells that contained granular cytoplasm and many hypertrophied nuclei. Giant cell cytoplasm was aggregated along a thickened cell wall. Vascular tissues within galls appeared disorganized. The relationship between the initial nematode population density (Pi) in a series from 0 to 1,024 eggs and juveniles/cm3 soil and growth of white mulberry seedlings was tested in the greenhouse. A Seinhorst model was fitted to plant height and top fresh weight. Tolerance limits of white mulberry to M. arenaria race 2 for plant height and top fresh weight were, respectively, 1.1 and 1.38 eggs and juveniles/cm3 soil. The minimum relative values for plant height and top fresh weight were 0 at Pi ≥ 64 and Pi ≥ 128 eggs and juveniles/cm3 soil, respectively. Maximum nematode reproduction rate was 435-fold at the lowest Pi.

Growth and development of plants with potential for use as trap crops for potato cyst nematodes and their effects on the numbers of juveniles in cysts

SummaryGrowth and development of three plant accessions with potential for use as trap crops for potato cyst nematodes (PCN), Solanum sisymbriifolium and two varieties of S. nigrum, were studied under 12 h and 17 h photoperiods. In pot experiments, rate of plant emergence, plant height, and shoot and root mass were greater for the S. nigrum varieties ‘90‐4750‐188’ and ‘88‐4750‐061’ than for S. sisymbriifolium and markedly greater than for a S. nigrum variety found as a weed of arable fields in The Netherlands. However, the last mentioned S. nigrum variety produced the most berries. Plant height and shoot weight of all the S. nigrum varieties were greater under the longer photoperiod, whereas the root mass was hardly affected. Plant height and shoot weight of S. sisymbriifolium also were greater under the longer photoperiod but the root weight was less. Under field conditions, with sowing dates from the end of March to mid August, S. sisymbriifolium and S. nigrum‘90‐4750‐188’ grew better than S. nigrum‘88‐4750‐061’. In contrast to S. nigrum, S. sisymbriifolium appeared resistant to night frosts in autumn. The stubbles of both S. sisymbriifolium and S. nigrum showed good regrowth after cutting the plants 5 or 10 cm above the soil surface 11 wk after sowing. In a pot experiment, all the plant accessions strongly reduced the numbers of juveniles in cysts compared with flax. Tolerance to Globodera rostochiensis of S. sisymbriifolium and S. nigrum‘90‐4750‐188’ was investigated in pot experiments under glasshouse conditions in sandy soil at pH 4.8 and 6.0. At soil infestation levels ranging from 0 to 56 juveniles ml−1 soil, S. sisymbriifolium appeared much more tolerant than S. nigrum‘90‐4750‐188’. Shoot yield of S. nigrum decreased markedly with increasing soil infestation and root weight also decreased, except at pH 4.8 and light infestation levels. Both S. sisymbriifolium and S. nigrum grew better at soil pH 4.8 than 6.0. The proportion of lateral roots in the total root mass increased in both species with increasing PCN infestation and soil pH. However, although the proportion of lateral roots in plants grown at soil pH 6.0 was greater at PCN infestations up to 14 juveniles ml−1 soil, the proportion of laterals in S. nigrum was considerably less at PCN infestations of 56 juveniles ml−1 soil. The proportion of PCN juveniles hatching was similar for the two species and decreased slightly with increasing initial nematode population densities.

The effects of nematode grazing on nitrogen mineralization during fungal decomposition of organic matter

Soil fungi and nematodes isolated from a long-term Sustainable Agriculture Farming Systems (SAFS) project research site at the University of California, Davis were investigated in microcosm systems. Nitrogen-free sand in the columns was amended with ground alfalfa and cellulose, with total N held constant, to create C-to-N ratios of 11:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1 and 45:1. Nitrogen mineralization by Aphelenchus avenae and Aphelenchoides composticola feeding on Rhizoctonia solani and Trichoderma sp. was determined by measuring ammonium and nitrate concentrations in the leachate from the columns at 3-d intervals. Nematode population numbers and the fatty acid 18:2 ω6c, a fungal biomass indicator, were monitored by destructive sampling on d 0, 7, 14 and 21. For R. solani, but not Trichoderma sp., there was significantly more N extracted from columns in the presence of either nematode species than in the absence of nematodes. Average N-mineralized nematode −1 d −1 was 1.8 ng for A. avenae and 3.3 ng for A. composticola when feeding on R. solani. As the C-to-N ratios of organic substrates increased, total mineral N decreased with R. solani alone, but in general remained the same in the presence of nematodes. Initial and average nematode population densities were significantly higher in columns containing R. solani than in those with Trichoderma sp. Both nematode species reduced the fungal fatty acid 18:2 ω6c in Trichoderma columns on d 21. The fatty acid 18:2 ω6c was lower in columns containing both R. solani and A. composticola on d 0 and 7 and higher on d 14 and 21 than those in the absence of nematodes.

Environmental differences and their effect on the assessment of quantitative resistance to potato cyst nematodes1

Experiments assessing the quantitative resistance of potato clones to potato cyst nematodes tend to produce variable results because of genotype‐environment interactions. Despite these interactions clones are ranked in the same way even when a major environmental factor such as initial nematode population density is changed. It is also argued that as the reproductive rate is so environmentally sensitive the use of fixed levels of nematode multiplication rate as criteria for classifying clones is inappropriate. It is proposed that standard resistant and partially resistant clones be included in all assessment tests. They would act as delineators of resistance classes and would allow easy comparisons to be made between tests whether conducted in glasshouse or field.