Host Status of Seven Weed Species and Their Effects on Ditylenchus destructor Infestation of Peanut

The host suitability to Ditylenchus destructor of seven common weed species in peanut (Arachis hypogaea) fields in South Africa was determined. Based on the number of nematodes per root unit, white goosefoot (Chenopodium album), feathertop chloris (Chloris virgata), purple nutsedge (Cyperus rotundus), jimson weed (Datura stramonium), goose grass (Eleusine indica), khaki weed (Tagetes minuta), and cocklebur (Xanthium strumarium) were poor hosts. Ditylenchus destructor survived on all weed species; population densities increased in peanut hulls and caused severe damage to seeds of peanut grown after weeds. Roots of purple nutsedge left in the soil suppressed populations of D. destructor and root and pod development in peanut grown after the weed. However, nematode populations in peanut hulls and seeds were not suppressed. Some weed species, especially purple nutsedge which is common in peanut fields, can be used to indicate the presence of D. destructor in the absence of peanut.     

In Vitro Embryo Explant Cultures of Peanut to Evaluate Resistance to Ditylenchus destructor

Population densities of D. destructor on embryo explants of 22 peanut genotypes grown in vitro were compared with those in roots and seeds of the same genotypes grown in the greenhouse. During the first 8 weeks after inoculation, the optimum incubation period was 6 weeks for maximum reproduction of Ditylenchus destructor on embryo explants of peanut (Arachis hypogaea L. cv. Sellie) inoculated with 250 nematodes at 25 C. Nematode numbers increased 17-fold. Deletion of MnSO₄ H₂O and a higher KH₂PO₄ concentration in the medium resulted in higher nematode reproduction. Resistance or susceptibility to D. destructor was observed in seeds of several genotypes but was not matched by differences in host suitability in roots. The results indicate that the factor for resistance or susceptibility to D. destructor is synthesized in the seeds of peanut but is not translocated to the roots. Use of embryo explant cultures of peanut as a rapid method to evaluate resistance to D. destructor did not work under the conditions described.     

Reproductive and Damage Potential of Ditylenchus destructor on Peanut

The reproductive and damage potential of Ditylenchus destructor on peanut, Arachis hypogaea cv. Sellie, was determined in greenhouse tests. Final nematode population densities (Pf) in roots, hulls, and seeds increased (P = 0.01) as a function of increasing initial population (Pi). Final population densities were higher in hulls than in seeds and roots. Final densities in hulls and seeds were positively (P = 0.01) correlated. Fresh root and hull weight and number of pods and seeds per plant were not affected by D. destructor. Second generation germination and pod and seed disease severity increased (P = 0.01), whereas fresh seed weight decreased (P = 0.01) as a function of increasing Pi, and Pf in seeds and Pf in hulls. At Pi 250 and higher, 10-25% of seeds germinated into second generation seedlings before harvest. At Pi 250 and higher, fresh weight of harvested seed was suppressed 20-50%. At Pi 50 or Pf greater than 20 per seed, pod disease severity was 3-7 (on a scale of 1 to 10) and 15-80% of seeds were blemished or unsound.     

Ditylenchus destructor in Hulls and Seeds of Peanut

The potato rot nematode, Ditylenchus destructor Thorne, is reported for the first time in hulls and seeds of peanut. The populations found differed from D. dipsaei and D. myceliophagus in habitat, number of lateral incisures, shape of tail tip, and length of postvulval sac. Infected hulls had brown necrotic tissue at the point of connection with the peg, and a black discoloration appeared first along the longitudinal veins. Infected seeds were usually shrunken, and testae and embryos had a yellow to brown or black discoloration. Of 877 seed samples graded "damaged" from all major peanut producing areas of South Africa, 73% were infected.     

Histopathology of Ditylenchus destructor on Peanut

The time and mode of entry, and development of Ditylenchus destructor in peanut were studied in field and greenhouse experiments. Few nematodes were present in the cortex of the roots. At 90-120 days after planting, D. destructor was observed in the exocarp at the base of the pod near the point of connection with the peg. The peg was invaded from this primary infection site. The endocarp of the hull was usually penetrated through openings at the base of the mesocarp and sometimes at the pod apex. Numerous D. destructor were present in the testa and the vascular bundles. Nematodes were found in the embryo but not in the cotyledons. The histopathology of D. destructor closely resembles that of the peanut testa nematode, Aphelenchoides arachidis Bos.     

Effect of Host Plant Age on Population Development and Pathogenicity of Ditylenchus destructor on Peanut

The effect of inoculating peanut, Arachis hypogaea cv. Sellie, with Ditylenchus destructor at timed intervals after planting and with different initial nematode population densities (Pi) was tested in greenhouse experiments. Final nematode population densities (Pf) in hulls and seeds were greater (Pf < 0.001) in plants inoculated at or before 9 weeks after planting. Pod disease symptoms correlated positively with the Pf in the pods. The seedgrade of peanuts inoculated at or before 9 weeks after planting was reduced, whereas grade of peanuts from plants inoculated at 15 weeks or later was not reduced. Peanut plants inoculated 12 weeks after planting with a Pi of 10-100 had a lower Pf (P < 0.05) than plants with a Pi of 250 to 8,000. Seed of plants with a Pi of 250 or less could be marketed as choice edible seed, whereas those with a Pi of 500 or more were of reduced seedgrade. These results suggest that as few as 500 nematodes per plant at 12 weeks after planting can build up to injurious levels before harvest. A nematicide should therefore be active for longer than 12 weeks after planting to sufficiently suppress the population.     

Heterorhabditid Behavior in the Presence of the Cabbage Maggot,Delia radicum, and its Host Plants

The behavior of Heterorhabditis zealandica Poinar strain T327 was investigated in the presence of the cabbage maggot, Delia radicum L., and plants that are susceptible to D. radicum infestation. Newly formed puparia and freeze-killed third instar larvae were attractive to infective nematodes. Newly harvested infective nematodes did not respond to the puparia, whereas 1-month-old and 2-month-old nematodes reached the insect targets within 15 minutes. There were no significant differences in the ability of similar-sized, third instar larval D. radicum and Galleria mellonella L., the greater wax moth, to attract nematodes. There was a tendency for a greater number of insects to attract more nematodes. The roots of ball cabbage and radish were equally attractive to nematodes, but rutabaga roots neither attracted nor repelled the nematodes. Germinated seeds of radish attracted nematodes, and there was a tendency for more numerous germinated seeds to attract more nematodes.     

Minimizing damage by Ditylenchus destructor to Peanut Seed with Early Harvest

Greenhouse and microplot experiments were conducted to evaluate the damage potential of Ditylenchus destructor on four South African commercial peanut cultivars as influenced by harvest date. The cultivars Sellie and Harts should be harvested by 150 and 120 days after planting, respectively. Losses were 12-13% with early harvest, but a 15-day delay resulted in losses of 45-49%. Harvest of Natal Common and Norden at 125 and 145 days after planting, respectively, resulted in the highest seed grade. By normal harvest time (140 and 160 days, respectively) these two cultivars were downgraded to crushing seed quality. Even though seed weight increases with time, a net loss occurs if harvest is delayed.     

Interactions Among Selected Endoparasitic Nematodes and Three Pseudomonads on Alfalfa

Meloidogyne hapla, Pratylenchus penetrans, and Helicotylenchus dihystera, reduced the growth of ''Saranac AR alfalfa seedlings when applied at concentrations of 50 nematodes per plant. All except P. penetrans reduced seedling growth when applied at 25 per seedling. M. hapla reduced growth when applied at 12 per seedling. Nematodes interacted with three pseudomonads to produce greater growth reductions than were obtained with single pathogens, suggesting synergistic relationships. Ditylenchus dipsaci, applied at 25 or 50 nematodes per seedling, reduced plant weight compared with weights of control plants, but did not interact with test bacteria. All of the nematodes except D. dipsaci produced root wounds which were invaded by bacteria.     

Separation of Three Species of Ditylenchus and Some Host Races of D. dipsaci by Restriction Fragment Length Polymorphism

This study examined the ribosomal cistron of Ditylenchus destructor, D. myceliophagus and seven host races of D. dipsaci from different geographic locations. The three species showed restriction fragment length polymorphisms (RFLPs) in the ribosomal cistron, the 18S rDNA gene, and the ribosomal internal transcribed spacer (ITS). Southern blot analysis with a 7.5-kb ribosomal cistron probe differentiated the five host races of D. dipsaci examined. Polymerase chain reaction (PCR) amplification of the ITS, followed by digestion with some restriction endonucleases (but not others), produced restriction fragments diagnostic of the giant race. Because the PCR product from D. myceliophagus and the host races of D. dipsaci was about 900 base pairs and the ITS size in D. destructor populations was 1,200 base pairs, mixtures of populations could be detected by PCR amplification. ITS fragments differentiated between D. dipsaci and Aphelenchoides rhyntium in mixed populations. This study establishes the feasibility of differentiation of the host races of D. dipsaci by probing Southern blots with the whole ribosomal cistron.     

Disc-electrophoretic Patterns of Enzymes and Soluble Proteins of Ditylenchus dipsaci and D. triformis

Soluble protein, esterase and oxidative enzyme patterns of the Waynesville, North Carolina, (WNC) and Raleigh, North Carolina, (RNC) populations of Ditylenchus dipsaci were compared. Polyacrylamide gel electrophoretic patterns of soluble protein extracts of nematodes of the two populations differed. Esterase and catalase patterns, however, were identical. Peroxidatic activity of the catalase isoenzymes from nematodes of the two populations differed when catechol was used as a cosubstrate. Distinct differences were demonstrated in soluble protein and enzyme patterns between D. dipsaci and D. triiormis.     

Reproductive and Damage Potential of Ditylenchus destructor on Six Peanut Cultivars

Commercial peanut cultivars were evaluated for host suitability and sensitivity to Ditylenchus destructor. All cultivars were susceptible. Approximately 94% of the final population were in the pods. Highest Pf occurred at harvest on early maturing cultivars. Damage occurred on four of six cuttivars at Pi = 100/3 liters of soil and all six cultivars at Pi = 1,000. Norden and Selmani were the most susceptible cultivars. Sellie was the most tolerant and highest yielding cultivar. This cultivar may be the most profitable one for growers.     

The Excretory Systems of Three Ditylenchus spp.

In Ditylenchus dipsaci the morphologically different anterior and posterior regions of the terminal excretory duct are separated by a constriction. Immediately posterior to the constriction is a valve-like structure composed of dense pieces integral with the wall of the duct. The posterior region is sometimes dilated at intervals along its length. The same structures are present in D. myceliophagus and D. destructor, but the dense pieces appear less well developed. A possible mode of action for the excretory system is discussed.     

Ditylenchus dipsaci Infestation of Trifolium repens. II. Dynamics of Infestation Development

Trifolium repens (white clover) stolons were inoculated with Ditylenchus dipsaci (stem nematode), and the development of resulting infestations was monitored. Nematodes initially remained confined to superficial locations, concentrating in petiole axils near inoculation points. They were able to migrate slowly from the inidal inoculation points and infest adjacent axils, especially in regions near the stolon tip. As time progressed, in some axils, nematodes migrated through the stolon epidermis and colonized slowly expanding subepidermal pockets of host tissue (ca. 0.2-mm length of stolon/day). In these loci nematodes established exponentially increasing populations, but the rates of locus expansion remained constant, indicating that locus expansion was limited by unidentified host-dependent factors. As a result of increasing population pressure within subepidermal loci, J4 entered a "diapause" state and the rate of egg production by adults declined, thereby reducing rate of population growth to more sustainable levels. Typically, these populations peaked at ca. 10,000 individuals in ca. 160 days occupying 3-cm lengths of stolon. Thereafter, heavily infested regions of stolons started to die, leading to the formation of longitudinal splits in their epidermis. In other axils, nematodes did not migrate into the stolons but remained confined to axils. Some of these populations increased a hundred-fold in 95 days, with population growth ending when petioles started to die. Host plant stolon morphology was affected only when subepidermal stolon populations developed high population levels (>100 nematodes) within close proximity (<2 cm) to active terminal meristems. This occurred either when axillary buds became active on previously infested nodes or when nematodes established endoparasitic populations at locations near the stolon tip during winter and spring, when the rate of stolon extension was limited by low light intensity. Affected stolon tips could "escape" from the influence of such infestations when light intensity and temperature increased. Nematode activity was limited by low temperature rather than light intensity. Global warming is likely to lead to greater damage to infested plants during the winter and early spring because the predicted milder winter temperatures will enhance nematode activity but not necessarily promote stolon growth.     

Size Differences Among Root-knot Nematodes on Resistant and Susceptible Alyceclover Genotypes

The influence of plant resistance on the size of individual root-knot nematodes was determined in greenhouse experiments. Five genotypes of alyceclover were inoculated with second-stage juveniles of Meloidogyne incognita race 3 or M. arenaria race 1. Plants were harvested at selected intervals and stained for detection of the nematodes, which were dissected from the roots. Length, width, and sagittal-sectional area of each animal were measured using an image-analysis system, and areas of nematodes in all stages were compared at different times and across alyceclover lines. Nematodes feeding on roots of resistant lines were consistently smaller than those on susceptible plants, with significant differences in growth detected after the final molt. Similar results were observed with both nematode species.     

A Nematicide Seed Treatment to Control Ditylenchus dipsaci on Seedling Alfalfa

Three nematicides were evaluated as seed treatments to control the alfalfa stem nematode (Ditylenchus dipsaci) on seedling alfalfa. Alfalfa seeds were soaked for 10 hours in a 0.5% (formulated by weight) concentration of either carbofuran, phenamiphos or oxamyl in acetone with no adverse effect on seed germination. All three treatments decreased nematode damage and increased survival of ''Ranger'' (susceptible) and ''Lahontan'' (resistant) alfalfa plants, when seeds were planted in soil infested with D. dipsaci. Mean live plant counts after 6 weeks in the untreated control, acetone alone, carbofuran, phenamiphos, and oxamyl treatments, respectively, were 4.3, 6.3, 19.0, 19.8, and 19.0 for Lahontan and 4.5, 1.5, 18.5, 19.3, and 18.0 for Ranger from 20 seeds/pot. Nematicide seed treatments resulted in significantly healthier Ranger alfalfa plants 4 months after planting. The combination of seed treatment and host resistance may provide a means of establishing alfalfa in an alfalfa monocropped system where soil populations of D. dipsaci are high.     

Mass Culturing of Ditylenchus dipsaci to Yield Large Quantities of Inoculum

Methods are described for rearing large quantities of Ditylenchus dipsaci on alfalfa tissues. Nematodes and alfalfa seed were disinfected and nematodes were reared in quantities sufficient to provide a continuous supply of inoculum for our alfalfa-breeding program. Nematodes reproduced best in darkness at 20-25 C. Cultures reached maximum numbers in 3-6 wk.     

Movement of Five Nematode Species through Sand Subjected to Natural Temperature Gradient Fluctuations

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.     

Postinfection Development of Rotylenchulus reniformis on Resistant Gossypium barbadense Accessions

The reniform nematode (Rotylenchulus reniformis) causes significant cotton (Gossypium hirsutum) losses in the southeastern United States. The research objective was to describe the effects of two resistant G. barbadense lines (cultivar TX 110 and accession GB 713) on development and fecundity of reniform nematode. Nematode development and fecundity were evaluated on the resistant lines and susceptible G. hirsutum cultivar Deltapine 16 in three repeated growth chamber experiments. Nematode development on roots early and late in the infection cycle was measured at set intervals from 1 to 25 d after inoculation (DAI) and genotypes were compared based on the number of nematodes in four developmental stages (vermiform, swelling, reniform, and gravid). At 15, 20, and 25 DAI, egg production by individual females parasitizing each genotype was measured. Unique reniform nematode developmental patterns were noted on each of the cotton genotypes. During the early stages of infection, infection and development occurred 1 d faster on susceptible cotton than on the resistant genotypes. Later, progression to the reniform and gravid stages of development occurred first on the susceptible genotype, followed by G. barbadense cultivar TX 110, and finally G. barbadense accession GB 713. Egg production by individual nematodes infecting the three genotypes was similar. This study corroborates delayed development previously reported on G. barbadense cultivar TX 110 and is the first report of delayed infection and development associated with G. barbadense accession GB 713. The different developmental patterns in the resistant genotypes suggest that unique or additional loci may confer resistance in these two lines.     

Seasonal Fluctuations of Soil and Tissue Populations of Ditylenchus dipsaci and Aphelenchoides ritzemabosi in Alfalfa

Population dynamics of A. ritzemabosi and D. dipsaci were studied in two alfalfa fields in Wyoming. Symptomatic stem-bud tissue and root-zone soil from alfalfa plants exhibiting symptoms of D. dipsaci infection were collected at intervals of 3 to 4 weeks. Both nematodes were extracted from stem tissue with the Baermann funnel method and from soil with the sieving and Baermann funnel method. Soil moisture and soil temperature at 5 cm accounted for 64.8% and 61.0%, respectively, of the variability in numbers of both nematodes in soil at the Big Horn field. Also at the Big Horn field, A. ritzemabosi was found in soil on only three of the 14 collection dates, whereas D. dipsaci was found in soil on 12 dates. Aphelenchoides ritzemabosi was found in stem tissue samples on 9 of the 14 sampling dates whereas D. dipsaci was found on all dates. Populations of both nematodes in stem tissue peaked in October, and soil populations of both peaked in January, when soil moisture was greatest. Numbers of D. dipsaci in stem tissue were related to mean air temperature 3 weeks prior to tissue collection, while none of the climatic factors measured were associated with numbers of A. ritzemabosi. At the Dayton field, soil moisture plus soil temperature at 5 cm accounted for 98.2% and 91.4% of the variability in the soil populations of A. ritzemabosi and D. dipsaci, respectively. Aphelenchoides ritzemabosi was extracted from soil at two of the five collection dates, compared to extraction of D. dipsaci at three dates. Aphelenchoides ritzemabosi was collected from stem tissue at six of the seven sampling dates while D. dipsaci was found at all sampling dates. The only environmental factor that was associated with an increase in the numbers of both nematodes in alfalfa stem tissue was total precipitation 1 week prior to sampling, and this occurred only at the Dayton field. Numbers of A. ritzemabosi in stem tissue appeared to be not affected by any of the environmental factors studied, while numbers of D. dipsaci in stem tissue were associated with cumulative monthly precipitation, snow cover at time of sampling, and the mean weekly temperature 3 weeks prior to sampling. Harvesting alfalfa reduced the numbers of A. ritzemabosi at the Big Horn field and both nematodes at the Dayton field.