The importance of plant-parasitic nematodes as yield-limiting pathogens of cotton has received increased recognition and attention in the United States in the recent past. This paper summarizes the remarks made during a symposium of the same title that was held in July 2007 at the joint meeting of the Society of Nematologists and the American Phytopathological Society in San Diego, California. Although several cultural practices, including crop rotation, can be effective in suppressing the populations of the important nematode pathogens of cotton, the economic realities of cotton production limit their use. The use of nematicides is also limited by issues of efficacy and economics. There is a need for development of chemistries that will address these limitations. Also needed are systems that would enable precise nematicide application in terms of rate and placement only in areas where nematode population densities warrant application. Substantial progress is being made in the identification, characterization and mapping of loci for resistance to Meloidogyne incognita and Rotylenchulus reniformis. These data will lead to efficient marker-assisted selection systems that will likely result in development and release of nematode-resistant cotton cultivars with superior yield potential and high fiber quality.
Nine sources of resistance to Rotylenchulus reniformis in Gossypium (cotton) were tested by measuring population density (Pf) and root-length density 0 to 122 cm deep. A Pf in the plow layer less than the autumn sample treatment threshold used by consultants was considered the minimum criterion for acceptable resistance, regardless of population density at planting (Pi). Other criteria were ample roots and a Pf lower than on the susceptible control, as in pot studies. In a Texas field in 2001 and 2002, no resistant accessions had Pf less than the control but all did in microplots into which nematodes from Louisiana were introduced. An environmental chamber experiment ruled out nematode genetic variance and implicated unknown soil factors. Pf in field experiments in Louisiana, Mississippi, and Alabama were below threshold for zero, six and four of the accessions and above threshold in the control. Gossypium arboreum A2–87 and G. barbadense GB-713 were the most resistant accessions. Results indicate that cultivars developed from these sources will suppress R. reniformis populations but less than in pots in a single season.
cotton; Gossypium; nematode; reniform; resistance; Rotylenchulus reniformis
The possible impact of Rotylenchulus reniformis below plow depth was evaluated by measuring the vertical distribution of R. reniformis and soil texture in 20 symptomatic fields on 17 farms across six states. The mean nematode population density per field, 0 to 122 cm deep, ranged from 0.4 to 63 nematodes/g soil, and in 15 fields more than half of the R. reniformis present were below 30.5 cm, which is the greatest depth usually plowed by farmers or sampled by consultants. In 11 fields measured, root density was greatest in the top 15 cm of soil; however, roots consistently penetrated 92 to 122 cm deep by midseason, and in five fields in Texas and Louisiana the ratio of nematodes to root-length density within soil increased with depth. Repeated sampling during the year in Texas indicated that up to 20% of the nematodes in soil below 60 cm in the fall survived the winter. Differences between Baermann funnel and sugar flotation extraction methods were not important when compared with field-to-field differences in nematode populations and field-specific vertical distribution patterns. The results support the interpretation that R. reniformis below plow depth can significantly impact diagnosis and treatment of cotton fields infested with R. reniformis.
cotton; Gossypium hirsutum; management; nematode; reniform; Rotylenchulus reniformis; vertical distribution
Damage to cotton by Rotylenchulus reniformis below plow depth was evaluated in a sandy clay loam soil at Weslaco, Texas. In December 1999, 14 holes on 51-cm centers were dug 91 cm deep along the planting bed and adjacent furrow and 2 ml of 1,3-dichloropropene was placed 91, 61, and 30 cm deep as each hole was refilled and packed. This technique eliminated 96%, 81%, and 74% of R. reniformis down to 107 cm at distances 0, 25, and 51 cm laterally from the point of application (P ≤ 0.05), whereas chisel fumigation at 168 liters/ha 43 cm deep reduced nematode numbers only in the top 61 cm (P ≤ 0.001). Manual placement of fumigant increased yield 92%; chisel fumigation increased yield 88% (P ≤ 0.005). A second experiment in February 2001 placed fumigant 43 or 81 cm deep, or at both 43 and 81 cm. Holes alone had no significant effect on nematode density at planting, midseason or harvest, on root length density at midseason, or on cotton lint yield. Fumigant at 43 cm reduced nematode numbers above fumigant application depth at planting 94% (P ≤ 0.02), at midseason 37% (P ≤ 0.09), and at harvest 0%, increasing yield 57% (P ≤ 0.002). Fumigant at 81 cm reduced nematode numbers above fumigant application depth at planting 86% (P ≤ 0.02), at midseason 74% (P ≤ 0.02), and at harvest 48% (P ≤ 0.01), increasing yield 53% (P ≤ 0.002). Fumigating at both 43 and 81 cm reduced nematode numbers above 90 cm 94% at planting and 79% at midseason, increased midseason root-length density 14-fold below 76 cm, and doubled yield (P ≤ 0.02 in all cases).
cotton; fumigation; Gossypium hirsutum; nematode; reniform; Rotylenchulus reniformis; vertical distribution; yield
Forty-six accessions of G. hirsutum and two of G. barbadense were examined for resistance to Meloidogyne incognita race 3 and Rotylenchulus reniformis in environmental growth chamber experiments, with the objective of finding new sources of resistance. Only the G. barbadense accessions, TX-1347 and TX-1348, supported significantly less reproduction by R. reniformis than the susceptible control, Deltapine 16 (USDA accession SA-1186). However, they were highly susceptible to M. incognita race 3. The G. hirsutum accessions TX-1174, TX-1440, TX-2076, TX-2079, and TX-2107 had levels of resistance to M. incognita race 3 as great as or greater than those of Clevewilt 6 and Wild Mexican Jack Jones, which are the primary sources of resistance to M. incognita race 3 in the most resistant breeding lines. No accession was as resistant as the highly resistant line Auburn 623 RNR (SA-1492). Resistant accessions were from the Mexican coastal states of Campeche, Quintana Roo, Tabasco, Veracruz, and Yucatan. Populations of R. reniformis from Alabama, Mississippi, Louisiana, and Texas, and of M. incognita race 3 from Mississippi, Texas, and California, had similar reproductive rates on resistant genotypes. Thus, new sources of resistance to M. incognita race 3 but not to R. reniformis were identified in wild accessions of G. hirsutum from southern Mexico.
cotton; Gossypium barbadense; Gossypium hirsutum; Meloidogyne incognita; nematode; reniform nematode; resistance; root-knot nematode; Rotylenchulus reniformis
Field experiments in 1992 and 1994 were conducted to determine the effect of Rotylenchulus reniformis, reniform nematode, on lint yield and fiber quality of 10 experimental breeding lines of cotton (Gossypium hirsutum) in untreated plots or plots fumigated with 1,3-dichloropropene. Controls were La. RN 1032, a germplasm line possessing some resistance to R. reniformis, and Stoneville 453, a cultivar that is susceptible to reniform nematode. Several breeding lines produced greater lint yields than Stoneville 453 or La. RN 1032 in both fumigated and untreated plots. Average lint yield suppression due to R. reniformis for six of the 10 breeding lines was less than half of the 52% yield reduction sustained by Stoneville 453. In growth chamber experiments, R. reniformis multiplication factors for La. RN 1032 and breeding lines N222-1-91, N320-2-91, and N419-1-91 were significantly lower than on Deltapine 16 and Stoneville 453 at 6 weeks after inoculation. R. reniformis populations increased by more than 50-fold on all entries within 10 weeks. In growth chambers, the breeding lines N220-1-92, N222-1-91, and N320-2-91 were resistant to Meloidoglyne incognita race 3; multiplication factors were ≤1.0 at both 6 weeks and 10 weeks after inoculation compared with 25.8 and 26.5 for Deltapine 16 at 6 and 10 weeks after inoculation, respectively, and 9.1 and 2.6 for Stoneville 453. Thus, the results indicate that significant advances have been made in developing improved cotton germplasm lines with the potential to produce higher yields in soils infested with R. reniformis or M. incogaita. In addition to good yield potential, germplasm lines N222-1-91 and N320-2-91 appear to possess low levels of resistance to R. reniformis and a high level of resistance to M. incognita. This germplasm combines high yield potential with significant levels of resistance to both R. reniformis and M. incognita.
cotton; Gossypium hirsutum; Meloidogyne incognita; reniform nematode; resistance; root-knot nematode; Rotylenchulus reniformis; tolerance
The responses of second-stage juveniles (J2) of Meloidogyne incognita race 3 to calcium alginate pellets containing hyphae of the nematophagous fungi Monacrosporiura cionopagum, M. ellipsosporum, and Hirsutella rhossiliensis were examined using cylinders (38-mm-diam., 40 or 72 mm long) of sand (94% <250-μm particle size). Sand was wetted with a synthetic soil solution (10% moisture, 0.06 bar water potential). A layer of 10 or 20 pellets was placed 4 or 20 mm from one end of the cylinder. After 3, 5, or 13 days, J2 were put on both ends, on one end, or in the center; J2 were extracted from 8-ram-thick sections 1 or 2 days later. All three fungal pellets were repellent; pellets without fungi were not. Aqueous extracts of all pellets and of sand in which fungal pellets had been incubated were repellent, but acetone extracts redissolved in water were not. Injection of CO₂ (20 μl/minute) into the pellet layer attracted J2 and increased fungal-induced mortality. In vials containing four randomly positioned pellets and 17 cm³ of sand or loamy sand, the three fungi suppressed the invasion of cabbage roots by M. javanica J2. Counts of healthy and parasitized nematodes observed in roots or extracted from soil indicated that, in the vial assay, the failure of J2 to penetrate roots resulted primarily from parasitism rather than repulsion. Data were similar whether fungal inoculum consisted of pelletized hyphae or fungal-colonized Steinernema glaseri. Thus, the results indicate that nematode attractants and repellents can have major or negligible effects on the biological control efficacy of pelletized nematophagous fungi. Factors that might influence the importance of substances released by the pellets include the strength, geometry, and duration of gradients; pellet degradation by soil microflora; the nematode species involved; and attractants released by roots.
alginate; behavior; biological control; chemotaxis; Hirsutella rhossiliensis; Meloidogyne incognita; Meloidogyne javanica; Monacrosporium cionopagum; Monacrosporium ellipsosporum; nematode; nematophagous fungi; Steinernema glaseri
Movement of vermiform stages of Meloidogyne incognita, Rotylenchulus reniformis, Ditylenchus phyllobius, Steinernema glaseri, and Caenorhabditis elegans in response to carbon dioxide was studied in 40- and 72-mm-long cylinders of moist sand inside 38-mm-d acrylic tubes. Meloidogyne incognita, R. reniformis, and S. glaseri were attracted to CO₂ when placed on a linear gradient of 0.2%/cm at a mean CO₂ concentration of 1.2%. When CO₂ was delivered into the sand through a syringe needle at flow rates between 2 and 130 μl/minute, the optimal flow rate for attracting M. incognita and R. reniformis was 15 μl/minute, and maximal attraction of the two species from a distance of 52 mm was achieved after 29 and 40 hours, respectively. After 24 hours, a total CO₂ volume of 20 cm³ was sufficient to induce 96% of all M. incognita introduced to move into the half of the cylinder into which CO₂ was delivered and more than 75 % to accumulate in the 9 cm³ of sand volume nearest the source. Results indicate it may be possible to use a chemical or biological source of CO₂ to attract nematodes to nematicide granules or biocontrol agents.
behavior; Caenorhabditis elegans; carbon dioxide; chemotaxis; Ditylenchus phyllobius; Meloidogyne incognita; nematode; Rotylenchulus reniformis; Steinernema glaseri
Temperature gradient fluctuations that occur naturally as a result of heating and cooling of the soil surface were reproduced within 15-cm-d, 15-cm-long acrylic tubes filled with moist sand. Sunny and rainy periods during the late summer in eastern Texas were simulated. Five ecologically different nematode species were adapted to fluctuating temperatures for 20-36 hours at a simulated depth of 12.5 cm before being injected simultaneously into the centers of tubes at that depth. When heat waves were propagated horizontally to eliminate gravitational effects, the movement of Ditylenchus phyllobius, Steinernema glaseri, and Heterorhabditis bacteriophora relative to the thermal surface was rapid and largely random. However, Rotylenchulus reniformis moved away from and Meloidogyne incognita moved toward the thermal surface. When heat waves were propagated upward or downward, responses to temperature were the same as when propagated horizontally, irrespective of gravity. The initial direction of movement 1.5 hours after introduction to 20-era-long tubes at five depths at five intervals within a 24-hour cycle indicated that M. incognita moved away from and R. reniformis moved toward the temperature to which last exposed. Differences in movement of the five species tested relative to gravity appeared related to body length, with the smallest nematodes moving downward and the largest moving upward.
behavior; Ditylenchus phyllobius; Heterorhabditis bacteriophora; Meloidogyne incognita; nematode; Rotylenchulus reniformis; Steinernema glaseri; temperature; thermotaxis
The incidence of Meloidogyne incognita and Rotylenchulus reniformis on cotton was determined in 1989-92 from 1,089 soil samples collected from 31 counties that account for nearly 60% of the 2.2 million hectares planted to cotton in Texas. Meloidogyne incognita was commonly found in the Southern High Plains and Brazos River Valley regions of Texas (57% and 34%, respectively, of samples) but was found in less than 8% of samples from the Central Blacklands, Coastal Bend, Low Plains, or the Upper Gulf Coast regions. Rotylenchulus reniformis was widely distributed in the Brazos River Valley (24% of samples) and found occasionally in the Upper Gulf Coast (8% of samples). Meloidogyne incognita was found only rarely in soils with greater than 40% clay content, whereas Rotylenchulus reniformis was frequently found in finely textured soils but was less common in soils with greater than 40% sand content. In samples infested with M. incognita or R. reniformis, population densities of these species were at least 10-fold greater than population densities of other plant-parasitic species present in the sample. Root-knot and reniform nematodes were not found together in high population densities (>100 individuals/500 cm³) in the same sample.
cotton; Gossypium hirsutum; incidence; Meloidogyne incognita; nematode; reniform nematode; root-knot nematode; Rotylenchulus reniformis; soil texture; survey
The feasibility of counting plant-parasitic nematodes in aqueous suspensions by measuring light transmittance through aqueous suspensions with an ELISA microplate reader was explored. Absorbance readings for eggs or vermiform stages of three species were linearly related (R² > 0.99) to concentrations between 0 and 10,000 nematodes/ml. Coefficients of variation ranged from 12-23%, depending on the species and developmental stage used. The method, therefore, was at least as accurate as direct counts of nematodes in aliquots on a microscope and more than 100 times as fast. The method should have direct application in research programs on plant resistance to nematodes, nematode population dynamics, and nematode behavior.
counting; Ditylenchus phyllobius; enumeration; Meloidogyne incognita; nematode; Rotylenchulus reniformis; technique
Vermiform Rotylenchulus reniformis were anesthetized in water by 10-40% CO₂ but were fully motile for 24 hours in water below 5% CO₂. When air containing 2.5% CO₂ was blown onto agar, nematodes accumulated at the point of highest CO₂ concentration. Nematodes also accumulated when chilling (0.2-1 C) of agar by the gas flow at the accumulation point was offset with heat from a fiber optic. In Baermann funnels containing R. reniformis in silt loam and sandy clay loam soils, CO₂ in funnel water increased during 24 hours from 0 to ca. 1%; more CO₂ accumulated below the soil layer than above. Bubbling air with 2.5% CO₂ into water below soil in covered funnels increased the CO₂ gradient and increased nematode extraction, whereas bubbling air without CO₂ below soil purged CO₂ from the water and decreased nematode extraction. Manipulation of CO₂ within funnels usually increased extraction by only 30% and never by more than 3-fold. Controlling temperature gradients consistently increased extraction by 2-30-fold.
Baermann funnel; carbon dioxide; nematode behavior; nematode extraction; Rotylenchulus reniformis; temperature gradient
The reniform nematode, Rotylenchulus reniformis, has been reported from all Gulf Coast states, Arkansas, Hawaii, North Carolina, and South Carolina. Experts in 11 states identified the counties or parishes where the nematode is found and provided information regarding associated soil parameters, climate, crops, and crop management. Rotylenchulus reniformis has been detected in 187 counties and parishes of the southeastern United States and is most widespread in Louisiana, Mississippi, Alabama, Florida, and Georgia. In every state except Florida and Hawaii, economically damaging soil populations were associated with continuous cotton production. Other crops considered to be damaged by R. reniformis were soybean, tobacco, several vegetables, and pineapple (Hawaii). There was no consistent relationship between the nematode's presence and soil texture, soil pH, rainfall, or irrigation regime. However, certain respondents associated damage from the nematode primarily with silty or clay soils (Texas, Hawaii, Florida, and Georgia) or with silty soils with exceptionally tow pH (Hawaii and Louisiana).
geographical distribution; reniform nematode; Rotylenchulus reniformis; soil type; survey
Baermann funnels were modified to eliminate or reverse the small temperature gradient (1-2 C/cm) across the soil layer that normally results from water evaporation. Effects of modifications on extraction efficiency were examined at various ambient temperatures and after overnight adaptation of three nematode species at 20 and 30 C. Extraction of Meloidogyne incognita from sandy loam, Tylenchulus semipenetrans from sandy clay loam, and Rotylenchulus reniformis from silt was greatly accelerated simply by covering funnels to prevent evaporation. In most cases, covering increased the nematodes extracted by 10-100 times after 5.5-48 hours. Faster and more efficient extraction of R. reniformis occurred over a wide range of ambient temperature (18-29 C). Effects of ambient temperature and temperature gradient direction on Baermann funnel extraction of R. reniformis were partly inconsistent with the behavior of R. reniformis in agar. Nematodes in agar moved toward cold at some ambient temperatures and toward heat at other temperatures. They always appeared to move toward cold on Baermann funnels. Differences were not attributable to blockage of gas exchange by covers. In agar and in funnels, the patterns of response to ambient temperature were shifted in the direction of the storage temperature.
Baermann funnel; behavior; Meloidogyne incognita; nematode extraction; Rotylenchulus reniformis; thermal adaptation; thermotaxis; Tylenchulus semipenetrans
A survey was conducted over a 22-year period to evaluate the influence of soil texture and land use on the geographical distributions of Rotylenchulus reniformis, Meloidogyne incognita, and Tylenchulus semipenetrans in the lower Rio Grande valley. The distributions of R. reniformis and M. incognita were related to soil texture, whereas T. semipenetrans occurred wherever host plants were present regardless of soil texture. The incidence of M. incognita was greatest in elevated sandy loams and moderately well-drained silts of modern flood terraces of the Rio Grande river. Rotylenchulus reniformis occurred predominantly in clay silts and clays of ancient flood terraces. Clay loams and sandy clay loams of the central, irrigated portion of the lower Rio Grande valley appeared favorable for M. incognita and R. reniformis. Differences between the geographical distributions of these two species could not be attributed to host crops.
citrus; geographical distribution; Meloidogyne incognita; population ecology; Rotylenchulus reniformis; survey; TylenchuIus semipenetrans
Soil solarization was evaluated for control of Rotylenchulus reniformis in the lower Rio Grande Valley of Texas. In field experiments, solarization significantly reduced soil nematode population densities 0-15 cm deep and increased yields of lettuce and cowpea. The length of time required for 90% mortality of nematodes in soil heated under controlled conditions in the laboratory varied from 25 hours to less than 1 hour between 41 and 47 C. Daily exposures of nematode-infested soil to lethal temperatures for sublethal time periods had a cumulative lethal effect. In water, vermiform stages required up to 10 days to recover from sublethal thermal stress. Eggs were similar to juveniles in their sensitivity to high temperatures. Lethal time-temperatures under controlled conditions were in general agreement with field results.
reniform nematode; Rotylenchulus reniformis; solarization; temperature; thermal death
Changes in the volumes of second-stage juveniles of Meloidogyne incognita were monitored in aqueous solutions of polyethylene glycol supplemented with dilute balanced salts. At key points within a 48-hour cycle of fluctuating water potential, nematodes were placed under hypoxic conditions or exposed to the respiratory inhibitor, sodium cyanide, to detect any respiration-dependent process that regulates volume. Aerobic respiratory arrest at -500 kPa induced pronounced water loss, lateral and dorsoventral collapse of the body wall, and abnormal failure to shorten longitudinally. Durations of hypoxia that were innocuous in dilute solutions were lethal during 500 kPa increases and decreases in water potential; the same water potential changes under aerobic conditions had no effect on viability. Data are consistent with the hypothesis that respiration is essential to survive water potential changes.
cyanide; hypoxia; Meloidogyne incognita; osmoregulation; oxygen; polyethylene glycol; respiratory inhibitors; volume regulation; water potential
developmental biology; ecdysis; ecology; nematode; silverleaf nightshade; Solanum elaeagnifolium
nematode volume measurement; nematode morphometrics; digital morphometrics; nematode water content
The motility and survival of Orrina phyllobia fourth-stage juveniles (J4) were examined in NaCl, sucrose, and synthetic soil solutions. Synthetic soil solutions (SSSs) contained Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, and NO₃⁻ at relative concentrations identical to those in a known agronomic soil. Nematode activity was dependent on solute composition and on water potential. In all solutions, motility ceased at a water potential of -30 × 10⁵ Pa and nematodes partially desiccated. Activity inhibition in NaCl began at -5 × 10⁵ Pa. At -15 × 10⁵ Pa, a high level of activity was sustained only in SSS. Lethal effects occurred at -15 and -60 × 10⁵ Pa in NaCl and SSS, respectively. No lethal effects were measured in sucrose solutions. Hydrogen ion concentration over the pH range 4.5-9.5 had no measurable effect on nematode activity.
osmolality; water potential; ionic solutes
The survival of desiccation by J4 Orrina phyllobia was examined at controlled relative humidities. When nematodes were transferred from water to air at 10% relative humidity (rh), 80% died within 30 minutes. When nematodes were transferred from water to air with rh at 70% or greater for ca. 15 minutes prior to being transferred to 10% rh, more than 90% of them survived desiccation. This phenomenon is referred to as preconditioning and occurred at much faster rates (2-30 minutes) than has been observed for other nematode species (24 hours). Differences in preconditioning rates may be due to technique-dependent variations in boundary layer resistance around nematodes during desiccation.
anhydrobiosis; relative humidity
The activity and survival of Orrina phyllobia fourth-stage juveniles (J4) were examined in aqueous solutions representing 96 combinations of eight predominant soil solution ions at total concentrations of 100, 200, and 1,000 meq/liter. Various water potentials were imposed by the addition of mannitol or polyethylene glycol to ionic solutions. Nematode longevity increased as water potential was decreased. Longevity was approximately doubled at a water potential of -23 × 10⁵ Pa and more than tripled at -60 × 10⁵ Pa. No combination oflons at 200 meq/liter was lethal after a 6-day exposure. Several ion combinations significantly increased longevity at -10 and -23 × 10⁵ Pa. Single cation Na⁺ solutions consistently inhibited activity and more than doubled nematode longevity.
longevity; soil solution; osmolality
The effects of oxygen and temperature on the activity and survival of infective forth-stage juveniles of Nothanguina phyllobia Thorne were examined in aqueous suspension. Rate of movement was not affected by a wide range of O₂ concentration (0.8-8.6 ppm). Activity decreased below 0.8 ppm 0 2, and at 0.15 ppm O₂ nematodes became motionless. Activity increased as a linear function of temperature up to a thermal optimum of 24 C; beyond 24 C activity decreased. Survival was greatly prolonged at low temperature. At 23 C, 50% mortality occurred within 7 d, whereas at 4 C, 70% survived after 98 d.
oxygen; temperature; activity; survival; Nothanguina phyllobia; biological control of weeds
The histopathogenesis of the foliar galls induced by Nothanguina phyllobia Thorne in Solanum elaeagnilolium Cav. was examined via serial sections prepared from plant shoots at 11 time intervals (0.5-30 days) following inoculation. Nematodes infected the blades and petioles of young leaves surrounding the shoot apex. Hypertrophy and hyperplasia of the palisade, pith, cortical, and vascular parenchyma resulted in the formation of confluent leaf, petiole, and stem galls up to 25 cm³ in volume. Externally, leaf galls were irregular, light-green, convoluted spheroid bulges distending the abaxial surface. Mature galls contained a cavity lined with parenchymogenous nutritive tissue comprising intercellular spaces and actively dividing hypertrophied cells. These cells contained granular cytoplasm, hypertrophied nuclei, and brightly stained large nucleoli. Vascular tissues were not discernibly affected during the early stages of gall development. As gall development progressed, however, vascular elements were often displaced and disoriented. The histopathology of this nematode indicates that N. phyllobia is a highly specialized parasite and, for that reason, is suitable as a biological control agent.
histopathology; nutritive tissue; hypertrophy; hyperplasia; biological control