Changes in Vertical Distribution of Pratylenchus scribneri under Potato and Corn

The vertical distribution of Pratylenchus scribneri populations was monitored under irrigated corn and potato grown in loamy sand soil. population estimates were based on the number of nematodes recovered from 100-cm³ soil samples and the roots contained therein. Reproduction was assessed by counting the number of second-stage juveniles. An index of population maturity was computed to evaluate the age structure of populations. At no time were nematodes distributed uniformly among five soil depths from 0 to 37.5 cm deep. During the summer (June-September), changes in the total number of P. scribneri and the number of second-stage juveniles recovered were not consistent among the depths sampled. Early (April-June) and late (September-November) in the season, changes in the abundance, reproduction, and maturity of populations were similar among depths. The timing and pattern of increases in numbers of nematodes suggests that variation in the abundance of P. scribneri in the soil profile beneath potato and corn was caused primarily by reproduction rather than the movement of nematodes.     

Distribution and Morphology of Longidorus breviannulatus Norton &Hoffman, 1975 and Longidorus fragilis Thorne, 1974 (Nematoda: Longidoridae) from North America

In a survey of ecotypes for longidorids, primarily from the rhizosphere hardwood trees growing in sandy soil along stream banks, 828 soil samples were collected from 37 Arkansas counties in 1999-2001. Eight populations of Longidorus breviannulatus were identified from the Arkansas survey samples. A total of 19 populations from California, Illinois, Iowa, Kansas, Michigan, Nebraska, New Jersey, New York, and Wisconsin were identified from the collection of the second author. A few males were found in New York and Nebraska populations and are described herein. Seven populations of L. fragilis were identified in the Arkansas survey samples, and one population was found from Indiana. Four juvenile stages of L. fragilis are present, and data are given for them herein.     

Influence of Temperature on Multiplication and Egg Hatching of Longidorus africanus

Longidorus africanus multiplication on tomato was highest at 29 °C. Few nematodes were recovered after 6 weeks at soil temperatures of 35 °C or below 23 °C. The time to egg hatching was shortest and the percentage of eggs hatching was highest at 29 °C. The minimum temperature and the heat sum above this temperature required for egg development were calculated to be 14.3 °C and 94.08 degree-days, respectively. The thermal times required for egg development by L. africanus and L. elongatus were nearly identical. For both species the product of the base temperature and the heat sum was near constant, and at a temperature of 22.3 °C the rates of egg development were equal.     

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.     

Distribution of Field Corn Roots and Parasitic Nematodes in Subsoiled and Nonsubsoiled Soil

A field trial was conducted for 2 years in an Arredondo fine sand containing a tillage pan at 15-20 cm deep to determine the influence of subsoiling on the distribution of corn roots and plant-parasitic nematodes. Soil samples were taken at various depths and row positions at 30, 60, and 90 days after planting in field corn subsoiled under the row with two chisels and in non-subsoiled corn. At 30 and 60 days, in-row nematode population densities to 60 cm deep were not affected by subsoiling compared with population densities in nonsubsoiled plots. After 90 days, subsoiling had not affected total root length or root weight at the 20 depth-row position sampling combinations, but population densities of Meloidogyne incognita and Criconemella spp. had increased in subsoiled corn. Numbers of Pratylenchus zeae were not affected. Subsoiling generally resulted in a change in distribution of corn roots and nematodes in the soil profile but caused little total increase in either roots or numbers of nematodes. Corn yield was increased by subsoiling.     

Penetration and Development of Meloidogyne incognita in Roots of Resistant and Susceptible Corn Genotypes

Rates of penetration and development ofMeloidogyne incognita race 4 in roots of resistant (inbred Mp307, and S4 lines derived from the open-pollinated varieties Tebeau and Old Raccoon) and susceptible (Pioneer 3110) corn genotypes were determined. Seedlings grown in styrofoam containers were inoculated with 5,000 eggs of M. incognita. Roots were harvested at 3-day intervals starting at 3 days after inoculation (DAI) to 27 DAI and stained with acid fuchsin. Penetration of roots by second-stage juveniles (J2) at 3 DAI was similar for the four corn genotypes. Meloidogyne incognita numbers in Tebeau, Old Raccoon, Mp307, and Pioneer 3110 peaked at 12, 12, 15, and 27 DAI, respectively. Nematode development in the resistant genotypes was greatly suppressed compared to Pioneer 3110. Resistance to M. incognita in these genotypes appears to be expressed primarily as slower nematode development rather than differences in J2 penetration.     

Damage Function and Economic Threshold for Belonolaimus longicaudatus on Potato

Belonolaimus longicaudatus has long been recognized as a pathogen of potato (Solanum tuberosum). However, a damage function relating expected yield of potato to population densities of B. longicaudatus at planting has not been derived, and the economic threshold for nematicide application is unknown. The objectives of this study were to derive the damage function of B. longicaudatus on potato and to calculate the economic threshold population density. The damage function data for B. longicaudatus on potato were obtained from an ongoing field study to evaluate cropping systems and nematode management practices. Soil samples were collected from experimental field plots, and nematodes were extracted from a 130-cm³ subsample with a centrifugal-flotation method. A damage function was derived by linear regression of potato yield on nematode population density at planting. Based on this derived damage function and published potato prices, the economic threshold for nematicide application was calculated at 2 to 3 B. longicaudatus/130 cm³ of soil, which was near the detection threshold based on methodology used in this study.     

Vertical Distribution of Plant-parasitic Nematodes in Sandy Soil under Soybean

Vertical distribution of five plant-parasitic nematodes was examined in two north Florida soybean fields in 1987 and 1988. Soil samples were collected from 0-15 cm, 15-30 cm, and 30-45 cm deep at each site. Soil at the three depths consisted of approximately 96% sand. More than 50% of Belonolaimus longicaudatus population densities occurred in the upper 15-cm soil layer at planting, but the species became more evenly distributed through the other depths as the season progressed. Criconemella sphaerocephala was evenly distributed among the three depths in one field but was low (< 20% of the total density) in the upper 15 cm at a second site. Maximum population densities of Pratylenchus brachyurus were observed at 15-30 cm on most sampling dates. Vertical distributions of Meloidogyne incognita and Paratrichodorus minor were erratic and showed seasonal variation. A diagnostic sample from the upper 0-15 cm of these soybean fields revealed only a minority of the populations of most of the phytoparasitic species present.     

Relationship between Root Growth of Potato,Root Diffusate Production,and Hatching of Globodera rostochiensis

Hatching response of Globodera rostochiensis in potato root diffusate (PRD) collected by soaking individual potato, Solanum tuberosum, root systems in water for 2 hours was used to assess the relationship between root growth and PRD production. Resistant potato cultivars Hudson and Rosa were used as test plants. Maximum hatch occurred in PRD collected 3 weeks after plant emergence (AE) in the greenhouse, and declined after this time. Hatch was positively correlated with increased root weight only during the first 3 weeks AE. Hudson PRD was consistently more active than Rosa PRD in stimulating hatch, except when adjusted for root weight. Although the results indicated that cells at the root tip produced a more active PRD than cells located elsewhere, PRD appeared to be produced along the entire root. Differences in time length of the vegetative growth phase, extent of root growth, and volume of roots, rather than the production of a more active PRD per se, may explain why Hudson is more effective than Rosa in reducing G. rostochiensis population densities in soil.     

Population Trends and Vertical Distribution of Plant-Parasitic Nematodes Associated with Vitis labrusca L. in Michigan

Nematode population trends and vertical distribution were monitored in a southwest Michigan vineyard (Vitis labrusca cv. Concord) from 1976 through 1983. Shallow (20 cm) and deep (90 cm) applications of 1,3-dichloropropene applied at 281 (shallow) plus 658 or 1,122 (deep) liters/ ha provided excellent control of Xiphinema americanum, Criconemella xenoplax, and Meloidogyne hapla. Populations of X. americanum remained below detectable levels for the entire 8-year experimental period where the fumigant was applied. X. americanum and C. xenoplax populations exhibited multiyear cycling in nonfumigated plots. M. hapla was first detected in 1978 and increased in prominence from 1980 through 1982. Criconemella spp. were commonly parasitized by an endoparasitic fungus. Parasitism was monitored and reported as an indication of nematode population quality.     

Resistance in Potato to Pratylenchus penetrans

Potato clones from five different breeding populations were evaluated for their relative resistance and susceptibility to Pratylenchus penetrans. Resistance and susceptibility were distinguished by an index of susceptibility (SI) calculated from the numbers of P. penetrans (including eggs) per g of root of individual clones in relation to that of a susceptible control at 30 or 70 days after inoculation. Evaluations were carried out using 7.5-cm clay pots in a growth chamber at 24 C with 15-hour day length. In the initial evaluation, 70 days after inoculation, the SI of individual clones ranged from 0.01 to 0.75. Clones that supported the least P. penetrans were from a breeding population derived from Solanum tuberosum ssp. andigena that was originally selected for its resistance to the potato cyst nematode, Globodera pallida. In succeeding tests, these clones had a significantly low SI than did susceptible controls or cultivars that were previously reported to possess resistance to P. penetrans, except cv. Hudson. Resistance to P. penetrans from the Pallida-resistant breeding population was incorporated into potato germplasm better adapted to North American growing conditions.     

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.     

Niche Distribution of Paratrichodorus minor and Belonolaimus longicaudatus Following Fumigation on Potato and Cabbage

An experiment was conducted to determine population changes and niche variation in the soil at two depths (0 to 20 cm and 20 to 40 cm) of Paratrichodorus minor and Belonolaimus longicaudatus populations following fumigation. Eight plots each of potato (Solanum tuberosum) and cabbage (Brassica oleracea var. capitata), fumigated with 1, 3-dichloropropene or nonfumigated, were established. Eight plots of sorghum-sudangrass hybrid (Sorghum bicolor × S. arundinaceum var. sudanense) were also used to monitor depth distribution (0 to 20 cm and 20 to 40 cm) of B. longicaudatus and P. minor following each cabbage/potato season. Soil samples were taken 0 to 20 cm and 20 to 40 cm deep during the potato/cabbage, and sorghum-sudangrass growing seasons. During the 1993-94 and 1994-95 potato/cabbage seasons, P. minor was found at highest numbers at 20 to 40 cm, whereas numbers of B. longicaudatus were highest at 0 to 20 cm. During the 1994 and 1995 sorghum-sudangrass growing seasons, B. longicaudatus numbers were highest at 0 to 20 cm. Paratrichodorus minor numbers were highest at 0 to 20 cm and at 20 to 40 cm deep in the 1994 and 1995 sorghum-sudangrass growing seasons, respectively. Reduction by soil fumigation of B. longicaudatus at 0 to 20 cm deep did not affect depth distribution or cause P. minor populations to increase in potato or cabbage plots. Paratrichodorus minor numbers increased at 20 to 40 cm deep in the 1994-95 cabbage season after soil fumigation.     

Globodera rostochiensis Population Density Decline in Relation to Spatial Distribution around Resistant Potato Plants

Golden nematode, Globodera rostochiensis (GN) population decline under resistant potatoes was related to cyst distance from plants 23 cm apart in rows 92 cm apart. GN decline, determined by sampling an infested field planted to the resistant cultivar ''Yankee Chipper'', was 81.8% in cores 11.5 cm from plants within rows. Decline was 27.4% at 23 cm from plants between rows and 36.6% at 46 cm. Population decline of juveniles in cysts added to soil in bags was 90.3% for cysts 11.5 cm from plants within rows planted to the resistant cultivar ''Rosa''. Decline between rows was 83.5, 76.9, and 60.4% at 11.5, 23.0, and 46.0 cm from plants, respectively. Maximum decline within for rows 30.5, 46.0, 61.0, and 92.0 cm apart, respectively. Decline under fallow was 43.5%, signif- which peaked 7 weeks AE. There was no effect of soil depth on population decline at any sampling position. Decreasing row spacing resulted in 79.9, 74.2, 73.4, and 66.1% GN population decline for rows 30.5, 46.0, 61.0, and 92.0 cm apart, respectively. Decline under fallow was 43.5%, significantly less than under potatoes. Potato root weight between rows was negatively correlated with row spacing and positively correlated with GN population decline.     

Role of Nematodes,Nematicides, and Crop Rotation on the Productivity and Quality of Potato,Sweet Potato,Peanut, and Grain Sorghum

The objective of this experiment was to determine the effects of fenamiphos 15G and short-cycle potato (PO)-sweet potato (SP) grown continuously and in rotation with peanut (PE)-grain sorghum (GS) on yield, crop quality, and mixed nematode population densities of Meloidogyne arenaria, M. hapla, M. incognita, and Mesocriconema ornatum. Greater root-gall indices and damage by M. hapla and M. incognita occurred on potato than other crops. Most crop yields were higher and root-gall indices lower from fenamiphos-treated plots than untreated plots. The total yield of potato in the PO-SP and PO-SP-PE-GS sequences increased from 1983 to 1985 in plots infested with M. hapla or M. arenaria and M. incognita in combination and decreased in 1986 to 1987 when root-knot nematode populations shifted to M. incognita. The total yields of sweet potato in the PO-SP-PE-GS sequence were similar in 1983 and 1985, and declined each year in the PO-SP sequence as a consequence of M. incognita population density increase in the soil. Yield of peanut from soil infested with M. hapla increased 82% in fenamiphos-treated plots compared to untreated plots. Fenamiphos treatment increased yield of grain sorghum from 5% to 45% over untreated controls. The declining yields of potato and sweet potato observed with both the PO-SP and PO-SP-PE-GS sequences indicate that these crop systems should not be used longer than 3 years in soil infested with M. incognita, M. arenaria, or M. hapla. Under these conditions, these two cropping systems promote a population shift in favor of M. incognita, which is more damaging to potato and sweet potato than M. arenaria and M. hapla.