Distribution of Pratylenchus scribneri between Root and Soil Habitats

The abundance of Pratylenchus scribneri in soil and root habitats was compared in potato and corn plots during 1986-88. Nematodes were extracted from 100-cm³ soil samples and the roots contained within the samples. The percentage of the population recovered from soil, similar among years and crops, averaged ca. 50% at the beginning and end of the growing season and ca. 20% from early to late season. Proportionately more adults and fourth-stage juveniles than younger stages were located outside roots until harvest. In a related study, nematodes were isolated from the roots, root surfaces, and soil associated with roots of whole corn and potato plants sampled from the field. Nematode population estimates calculated from the whole plant samples were generally lower than those based on soil cores, but showed similar patterns of population growth. Nematode density per gram dry weight was highest in roots, intermediate on root surfaces, and lowest in soil. Estimates of the absolute abundance of nematodes in each of the three habitats were highest in roots or soil, depending on the sampling date, and lowest on root surfaces. This study demonstrates that P. scribneri inhabits soil environments even when host roots are present and illustrates the importance of considering all possible habitats when estimating the size of Pratylenchus spp. populations.     

Winter Survival of Pratylenchus scribneri

Population densities of Pratylenchus scribneri in a Plainfield loamy sand soil were sampled from 1 October to 1 May for 4 years. From May to October of each year, the site was planted to Russet Burbank potato and Wis 4763 corn. Percentages of change in population densities of nematodes were computed on the basis of number of nematodes present on 1 October. The decline of P. scribneri between growing seasons was nonlinear, with most mortality occurring in the autumn before the soil froze. Winter survival, defined as the percentage of change in population densities from 1 October to 1 May the following year, ranged from 50 to 136% for nematodes in corn plots and from 15 to 86% for nematodes in potato plots. There was no difference in survival of nematodes of different life stages or among root and soil habitats. Winter survival of nematodes was density-dependent in 3 of 4 years in corn plots and in 1 of 4 years in potato plots. Although predators were present, their abundance was not correlated with the winter survival of nematodes. Cumulative and average snow cover was correlated with the survival of nematodes associated with corn but not with potato. No relationships between other climatic factors and survivorship were detected.     

Nematode Numbers and Crop Yield in a Fenamiphos-Treated Sweet Corn-Sweet Potato-Vetch Cropping System

Nematode population densities and yield of sweet corn and sweet potato as affected by the nematicide fenamiphos, in a sweet corn-sweet potato-vetch cropping system, were determined in a 5-year test (1981-85). Sweet potato was the best host of Meloidogyne incognita of these three crops. Fenamiphos 15G (6.7 kg a.i./ha) incorporated broadcast in the top 15 cm of the soil layer before planting of each crop increased (P ≤ 0.05) yields of sweet corn in 1981 and 1982 and sweet potato number 1 grade in 1982 and 1983. Yield of sweet corn and numbers of M. incognita second-stage juveniles (J2) in the soil each month were negatively correlated from planting (r = - 0.47) to harvest (r = -0.61) in 1982. Yield of number 1 sweet potato was inversely related to numbers of J2 in the soil in July-October 1982 and July-September 1983. Yield of cracked storage roots was positively related to the numbers of J2 in the soil on one or more sampling dates in all years except 1985. Some factor(s), such as microbial degradation, resistant M. incognita development, or environment, reduced the effect of fenamiphos.     

Abundance and Vertical Distribution of Longidorus breviannulatus Associated with Corn and Potato

Longidorus breviannulatus was detected in a field planted to corn after 13 years of potato. Nematode populations were maintained in this field in adjacent corn and potato plots for 2 years but did not increase significantly on either crop. Population levels increased until approximately 60 days after planting and then declined until the end of the growing season. Overwinter mortality was negligible. The vertical distribution of the nematode population changed during the course of the season. More nematodes were recovered from depths of 0-15 cm in early season samples and from depths of 15-30 cm in late season samples. Data indicated that this redistribution was due to nematode migration.     

Effect of Tillage System and Irrigation on Population Densities of Plant Nematodes in Field Corn

Soil populations of plant-parasitic nematodes were monitored bimonthly for 18 months in irrigated and nonirrigated corn plantings using four production systems: conventional and minimum tillage with crop residue returned and minimum tillage with 60% or 90% of previous corn crop residue removed. Populations of Meloidogyne incognita, Scutellonema brachyurum, Pratylenchus scribneri, and Paratrichodorus christiei varied among the tillage, nematicide, and irrigation treatments. Meloidogyne incognita and P. christiei populations were not significantly affected by tillage method, but S. brachyurum populations were highest during April 1981 and 1982 in minimum tillage treatments where crop debris was not removed. In contrast, S. brachyurum populations were lowest during the same period in minimum tillage plots where 90% of previous crop debris had been removed or where residues were incorporated with conventional tillage. Populations of P. scribneri were lowest in minimum tillage during August 1981 and April 1982. Regardless of tillage system, corn yields in all nonirrigated plots were increased during 1982 by application of carbofuran (2.24 kg a.i./ha). No yield increases were observed following nematicide application in 1981.     

Reproduction of Pratylenchus penetrans on Potato and Crops Grown in Rotation with Potato

The relative suitability of potato and crops frequently grown in rotation with potato as hosts for Pratylenchus penetrans was evaluated. Suitability of rye, wheat, corn, oat, sorgho-sudangrass, and potato were compared in pot studies based on ratios of final population : initial population density and densities of nematodes in roots at harvest. Population densities increased more on potato, oat, and corn than on rye, wheat, and sorgho-sudangrass. There were no differences among the four rye cultivars or between the two oat cultivars in host suitability. Population increases were not related to root weight or consistently to nematode densities in roots. Although rye and wheat were equally suitable hosts in pot studies, P. penetrans increased more on wheat than on rye in a field study, indicating that reproduction was reduced or mortality was increased on rye under field conditions.     

Species of Pratylenchus Associated with Solanum tuberosum cv Superior in Ohio

Seventy-three Ohio fields comprising ca. 440 ha of cv Superior potatoes were surveyed in 1977 for plant-parasitic nematodes. Of eight genera of plant-parasitic nematodes, Pratylenchus was found most frequently, occurring in 65% of the soil samples and 84% of the root samples. Populations of Pratylenchus were consistently higher than populations of the other nematode genera. The six species of Pratylenchus extracted from potato roots, in descending order of frequency, were P. crenatus, P. penetrans, P. scribneri, P. alleni, P. thornei, and P. neglectus. Prevalence of these Pratylenchus species in Ohio potato fields suggests that they could be involved with vascular wilt fungi in premature death of cv Superior potato vines known in Ohio as "early dying."     

Survival of Heterodera zeae in Soil in the Field and in the Laboratory

Eggs and (or) second-stage juveniles (J2) inside cysts of Heterodera zeae survived over winter in the field with no detectable mortality at all six depths to 30 cm from which soil samples were collected between corn stubble in the row at 4-8-week intervals. Few or no free J2 were recovered from soil collected in January-April from the top 5 cm, but some were recovered at all samplings from soil collected at greater depths. Emergence of J2 from cysts and numbers of females developing on corn roots in bioassays of cysts increased substantially between January and April. Cyst numbers in a fallow area of the corn field did not decline at any depth to 30 cm during 20 months. Free soil J2, J2 emerged from cysts, and females from the bioassay of cysts were highest at the first soil sampling in July after 10 months of fallow; numbers of nematodes in all three categories declined thereafter, but a few were still detectable after 20 months of fallow. Some cysts were still being recovered after 51 months from naturally infested field soil stored moist in the laboratory at 2 C and 24 C. Females were produced in the bioassays of cysts recovered from soil stored for 38 months at 24 C and for 32 months at 2 C. No free J2 were recovered from soil after 1 month of storage at -18 C, but even after 7 months storage J2 emerged from cysts that were recovered and many females developed in bioassays of those cysts.     

Spatial-temporal Patterns of Meloidogyne konaensis on Coffee in Hawaii

Population densities ofMeloidogyne konaensis were determined in March and July of 1991 and 1992 on coffee cultivars Guatemalan and 502, and on four rootstocks (Purpuree, Congensis, Deweveri, and Kaffe) with Guatemalan or 502 as a scion. Three-dimensional spatial patterns were characterized on roots of Guatemalan and Deweveri. Population densities differed among rootstocks (P < 0.05) and times (P < 0.01). The greatest number of second-stage juveniles (J2) occurred on Guatemalan and fewest J2 on Purpuree and Deweveri rootstocks. More nematodes were found in March than in July of both years. The spatial distribution varied by positions and depths on Guatemalan. The highest nematode population density occurred at 60 cm from the base of the tree and 15-45 cm deep. Numbers of nematodes were relatively low at all positions and all depths on the Guatemalan-Deweveri combination.     

Newly Discovered Transmission Pathway of Bursaphelenchus xylophilus from Males of the Beetle Monochamus alternatus to Pinus densiflora Trees via Oviposition Wounds

The transmission of Bursaphelenchus xylophilus from Monochamus alternatus males to Pinus densiflora trees via oviposition wounds has been determined. Nematode-infested males, with mandibles fixed experimentally to prevent feeding, were placed for 48 hours with pine bolts containing oviposition wounds that had been made by nematode-free females. After removal of the nematode-infested males, the pine bolts were held for 1 month and then examined for the presence of nematodes. Reproducing nematode populations were recovered from pine bolts that were exposed to male beetles carrying a high number of nematodes. No reproducing nematode population could be recovered from pine bolts exposed to beetles with a small number of nematodes. Nematode reproduction in the pine bolts was not related to the number of oviposition wounds per bolt. Fourth-stage dispersal B. xylophilus juveniles, collected from beetle body surfaces, were inoculated on pine bolt bark 0, 5, 10, and 15 cm away from a single artificial, small hole. These dauer juveniles successfully entered some bolts. The probability of successful nematode reproduction decreased with increased distance between inoculation point and artificial hole. The results indicated that B. xylophilus can move a significant distance to oviposition wounds along the bark surface and enter a tree via the wounds. The new transmission pathway is considered important for the nematode to persist in pine forests such as in North America where pine wilt disease does not occur.     

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.     

Extraction Efficiency of Belonolaimus longicaudatus from Sandy Soil

Numbers of Belonolaimus longicaudatus extracted from sandy soils (91-92% sand) by sieving and centrifugation were only 40-55% of those extracted by sieving and incubation on a Baermann tray. Residues normally discarded at each step of the sieving plus Baermann tray extraction procedure were examined for nematodes to obtain estimates of extraction efficiencies. For third-stage and fourth-stage juveniles, males, and females, estimates of extraction efficiency ranged from 60 to 65% in one experiment and 73 to 82% in another. Estimated extraction efficiencies for second-stage juveniles were lower (33% in one experiment, 67% in another) due to losses during sieving. When sterilized soil was seeded with known numbers of B. longicaudatus, 60% of second-stage juveniles and 68-76% of other stages were recovered. Most stages of B. longicaudatus could be extracted from these soils by sieving plus Baermann incubation with an efficiency of 60-70%.     

Age Structure and Community Diversity of Nematodes Associated with Maize in Iowa Sandy Soils

Age structure of nematode populations around maize growing in sandy soils in Iowa was studied at soil depths of 0-15and 15-30 cm for 2 years. Numbers of Longidorus breviannulatus were generally greater at 0-15 cm than at 15-30 cm deep until mid to late season. The decline in numbers of females as the season progressed indicates that fecundity slowed and is evidence of only one generation per year. Peak populations of Pratylenchus scribneri and Xiphinema americanum occurred in late August or early September. Adults of Hoplolaimus galeatus were few in the roots but common in the soil, indicating that fertilization occurred mostly in the soil. Numbers of P. scribneri were generally greater at the lower depth, especially late in the season. Community diversity (H'') was less when nematode biomass was used instead of numbers. Numbers of H. galeatus did not decline over the winter. Numbers of L. breviannulatus, P. scribneri, and X. americanum declined significantly over the winter, but not between spring cultivation and planting.     

Comparison of Populations of Pratylenchus brachyurus Based on Isozyme Phenotypes

Enzymes from females of five Pratylenchus brachyurus populations and one P. scribneri population were analyzed by isoelectric focusing electrophoresis. Of the 18 enzyme systems investigated, only malate dehydrogenase (MDH), phosphoglucomutase (PGM), and phosphoglucose isomerase (PGI) were detected from all five P. brachyurus populations and P. scribneri. Faint bands were detected for isocitrate dehydrogenase and phosphogluconate dehydrogenase from one P. brachyurus population. Three distinct phenotypic groups were found in the MDH and PGM systems for P. brachyurus populations, but only a single electromorph was detected for PGI. Multiple electromorphs for MDH, PGM, and PGI were detected for P. scribneri; there was no similarity among these patterns with those from P. brachyurus. No phenotypic differences in PGI were observed between females and mixed juveniles of one population of P. brachyurus.     

Comparison of Three Methods for Estimating the Number of Entomopathogenic Nematodes Present in Soil Samples

Numbers of Steinernema sp. (CB2B) and S. carpocapsae (Agriotos) exponentially declined after application into a clay loam soil. Over a 35-day sampling period, Steinernema sp. (CB2B) was more persistent than S. carpocapsae (Agriotos). The presence or absence of the second-stage cuticle on the third-stage juveniles (J3) at the time of application did not alter the rate of population decline of Steinernema sp. (CB2B). Nearly all J3 of Steinernema sp. (CB2B) and S. carpocapsae (Agriotos) lost their cuticle within 24 hours of being in soil. Centrifugal flotation recovered the greatest number of nematodes, with a lower variance than either the live bait or Baermann funnel techniques. A strong positive linear relationship was evident between numbers of nematodes present in the soil and the numbers that established in a bait insect. Approximately 40% of Steinernema sp. (CB2B) and 30% of the S. carpocapsae (Agriotos) present in the soil established in Galleria mellonella larvae. The extraction techniques had different efficiencies and gave different relative estimates of persistence for the two species. Persistence and infectivity was best measured using a combination of live bait and flotation techniques.     

Host Suitability of the Olive Cultivars Arbequina and Picual for Plant-Parasitic Nematodes

Host suitability of olive cultivars Arbequina and Picual to several plant-parasitic nematodes was studied under controlled conditions. Arbequina and Picual were not suitable hosts for the root-lesion nematodes Pratylenchus fallax, P. thornei, and Zygotylenchus guevarai. However, the ring nematode Mesocriconema xenoplax and the spiral nematodes Helicotylenchus digonicus and H. pseudorobustus reproduced on both olive cultivars. The potential of Meloidogyne arenaria race 2, M. incognita race 1, and M. javanica, as well as P. vulnus and P. penetrans to damage olive cultivars, was also assessed. Picual planting stocks infected by root-knot nematodes showed a distinct yellowing affecting the uppermost leaves, followed by a partial defoliation. Symptoms were more severe on M. arenaria and M. javanica-infected plants than on M. incognita-infected plants. Inoculation of plants with 15,000 eggs + second-stage juveniles/pot of these Meloidogyne spp. suppressed the main height of shoot and number of nodes of Arbequina, but not Picual. Infection by each of the two lesion nematodes (5,000 nematodes/pot) or by each of the three Meloidogyne spp. suppressed (P < 0.05) the main stem diameter of both cultivars. On Arbequina, the reproduction rate of Meloidogyne spp. was higher (P < 0.05) than that of Pratylenchus spp.; on Picual, Pratylenchus spp. reproduction was higher (P < 0.05) than that of Meloidogyne spp.     

Interactions Among Pratylenchus penetrans,P. scribneri,and Verticillium dahliae in the Potato Early Dying Disease Complex

Microplots were infested with combinations of the fungus Verticillium dahliae and Pratylenchus penetrans and P. scribneri to test for individual and combined effects of these organisms on potato yield and nematode reproduction. Verticillium dahliae alone caused yield losses in all 3 years of the experiment, and the interaction between P. penetrans and V. dahliae was significant (P ≤ 0.05) in 2 years. Pratylenchus penetrans alone caused yield losses in 2 years and P. scribneri alone caused yield losses in 1 year. No two-way or three-way interaction was found involving P. scribneri. In 1987, reproduction for low densities of P. penetrans was 5 times higher when P. scribneri was also present than when it was absent, and 3.5 times higher in 1988. In nematode species mixtures, reproduction of P. scribneri was decreased by V. dahliae in 1987-88. The final population density of P. scribneri was negatively affected by V. dahliae and positively related to the initial proportion of P. scribneri to P. penetrans. In species mixtures with proportions of P. penetrans ranging from 0.1 to 0.5, reproduction of P. penetrans was negatively affected by V. dahliae and decreased linearly in relation to the increase in the initial proportion of P. penetrans in both years. The final population density of P. penetrans was affected only by V. dahliae.     

Reproduction and Parasitism of Pratylenchus neglectus on Potato

An initial density (Pi) of 1,540 Pratylenchus neglectus/kg soil suppressed shoot growth of potato, Solanum tuberosum cv. Russet Burbank, in a greenhouse test at 3 weeks. After 6 weeks, shoot weights were reduced by Pi of 662 and 1,540 nematodes/kg soil, the final soil densities of P. neglectus were twice the respective Pi, and the numbers of nematodes per gram dry root were 5,363 and 7,981. In 1986-88 field microplot experiments with the Norchip cultivar, neither shoot nor root weight was suppressed by P. neglectus. In 1986 a Pi of 115 nematodes/kg soil suppressed the total number and weight of tubers per plant. In 1987 a Pi of 186 nematodes/kg soil suppressed the marketable and total number of tubers by 19 and 25 %, respectively. In 1988 a Pi of 1,884 nematodes/ kg soil reduced total and marketable weight by 18 and 19%, respectively. In 1986 and 1987 nematode population densities in the soil increased 34-fold and 27-fold, respectively. In 1988 the Pi of 1,884 nematodes/kg soil rose to 21,890/kg at midseason, then dropped to 4,370/kg at harvest. These studies show for the first time that P. neglectus reproduces well on potato and can cause yield losses. Because of its distribution and abundance, P. neglectus may be considered an economically important parasite of potato in Ontario.     

Effect of Hirsutella rhossiliensis on Infection of Potato by Pratylenchus penetrans

We evaluated the ability of the nematode-pathogenic fungus Hirsutella rhossiliensis (Deuteromycotina: Hyphomycetes) to reduce root penetration and population increase of Pratylenchus penetrans on potato. Experiments were conducted at 24 C in a growth chamber. When nematodes were placed on the soil surface 8 cm from a 14-day-old potato cutting, the fungus decreased the number entering roots by 25%. To determine the effect of the fungus on population increase after the nematodes entered roots, we transplanted potato cuttings infected with P. penetrans into Hirsutella-infested and uninfested soil. After 60 days, the total number of nematodes (roots and soil) was 20 ± 4% lower in Hirsutella-infested than in uninfested soil.     

Morphological and Molecular Identification of Globodera pallida Associated with Potato in Idaho

The identity of a newly discovered population of pale potato cyst nematode Globodera pallida associated with potato in eastern Idaho was established by morphological and molecular methods. Morphometrics of cysts and second-stage juveniles were generally within the expected ranges for G. pallida with some variations noted. The Idaho population and paratype material from Epworth, Lincolnshire, England, both showed variations in tail shape, with bluntly rounded to finely pointed tail termini. Compared to literature values for the paratypes, second-stage juveniles of the Idaho population had a somewhat shorter mean body length, and cysts had a slightly higher mean distance from the anus to the nearest edge of the fenestra. PCR-RFLP of the rDNA ITS region, sequence-specific multiplex PCR and DNA sequence comparisons all confirmed the identity of the Idaho population as G. pallida. The ITS rDNA sequence of the Idaho isolate was identical to those from York, England, and the Netherlands. Species-specific primers that can positively identify the tobacco cyst nematode Globodera tabacum were also developed, providing a new assay for distinguishing this species from G. pallida and the golden potato cyst nematode Globodera rostochiensis.