Population Dynamics of Pratylenchus penetrans,Paratylenchus sp., and Criconemella xenoplax on Western Oregon Peppermint

Endoparasitic nematode populations are usually measured separately for soil and roots without a determination of the quantitative relation between soil and root population components. In this study, Pratylenchus penetrans populations in peppermint soil, roots, and rhizomes were expressed as the density within a standardized core consisting of 500 g dry soil plus the roots and rhizomes contained therein. Populations of Paratylenchus sp. and Criconemella xenoplax in 500 g dry soil were also determined, thus measuring the total plant-parasitic nematode population associated with the plant. Mean wet root weight per standard core peaked in spring and again in late summer and was lowest early in the growing season and in early fall. Pratylenchus penetrans populations peaked 4 to 6 weeks after root weight peaks. The percentage of the total population in roots reached 70% to 90% in early April, decreased to 20% to 40% in August, and returned to higher percentages during the winter. Rhizomes never contained more than a minor proportion of the population. Mean Paratylenchus sp. populations increased through spring and peaked in late August. Mean C. xenoplax populations fluctuated, peaking in August or September. Populations of all parasitic species were lowest during winter. Evaluation using the standard core method permits assessment of the total P. penetrans population associated with the plant and of changes in root weight as well as the seasonal distribution of P. penetrans.     

Effect of Age of Alfalfa Root on Penetration by Pratylenchus penetrans

Penetration by all migratory life stages of Pratylenchus penetrans into roots of alfalfa (Medicago sativa L. cv. Du Puits) was inversely proportional to tissue age. Two-day-old tissue in the root hair zone was penetrated twice as much as 10- or 20-day-old sections of the tap root. Age-related differences were also observed in branch roots; these differences were not affected by increasing the number of nematodes from 1 to 10 per inoculation site, nor by increasing the length of the incubation period from 6 to 96 h. Age-related differences were only significant with 3-wk-old plants, not with 2- and 1-wk-old seedlings. Nematodes entered roots at temperatures from 5 to 30 C with maximum entry at 20 C and minimum at 5 C. At all temperatures, except 5 C, penetration into young tissue (2 days) was significantly greater than into medium (10 days) and old (20 days) tissue. Females and third-stage larvae entered the different-aged root sections 122% and 83%, respectively, more than did males. Two-day-old seedlings of the alfalfa cultivars Vernal, Saranac, and Du Puits were penetrated equally by P. penetrans. Perhaps the inverse relationship between penetration and age of root is, in part, responsible for the increasing resistance or tolerance of plants to nematode damage as they grow older.     

Anhydrobiosis in Pratylenchus penetrans and Tylenchorhynchus n. sp. in Cultivated Soils Cropped to Winter Rye

Anhydrobiotes of Pratylenchus penetrans were found in two cultivated soils sown to rye in southern Ontario during the growing season. Anhydrobiotes at the 0-2.5-cm depth were recovered from 9 and 6 of 11 samplings, respectively, of a Vineland silt loam and a Fox loamy sand during the dry summer of 1983. At the 2.5-15.0-cm depth, anhydrobiotes were recovered less frequently. In the summer of 1984, anhydrobiotes of P. penetrans were recovered once and anhydrobiotes of Tylenchorhynchus n. sp. twice in 11 samplings. The percentages of P. penetrans populations that were anhydrobiotes in 1983 and 1984 were closely related to soil moisture content and corresponding moisture tensions. Populations of P. penetrans were greatest in October in the lower soil depth, 2.5-15.0 cm; those of Tylenchorhynchus n. sp. were greater in the surface layer of soil, 0-2.5 cm, and peaked in August.     

Effects of Pratylenchus penetrans on the Infection of Strawberry Roots by Gnomonia comari

The fungus Gnomonia comari, causal agent of strawberry leaf blotch, was inoculated at the crown of young axenized strawberry plants growing in sterilized sand. Only the roots were colonized, and the infection was symptomless. When the fungus colonized the roots in the presence of the root lesion nematode Pratylenchus penetrans, the plants were extremely stunted and their root system was necrotic. Fungal conidiospores were found attached to the cuticle of nematodes extracted from soil inoculated with the two pathogens. These findings indicate that P. penetrans could transport conidiospores through soil.     

Infection of Red Clover Seedlings by Heterodera trifolii Goffart and Pratylenchus penetrans (Cobb)

Penetration of 3-day-old ''Kenland'' red clover seedlings by Heterodera trifolii Goffart and Pratylenchus penetrans (Cobb) was investigated in 50-mm petri dishes on 1% agat or discs of Miracloth® (Chicopee Mills, Inc., New York, N.Y. 10018). Penetration by both nematodes increased arithmetically with increased numbers in the inoculum. H. trifolii larvae slowly penetrated all root tissue but in relatively low numbers, 25-30% of inocula. Swelling of larvae and formation of syncytia within roots was detectable 96 hr after inoculation. Initially, adults of P. penetrans preferentially penetrated the region 3-10 mm behind the root tip, but subsequently they invaded all along roots. P. penetrans penetration efficiency was high, 75-90% of inocula, and penetration was relatively rapid. When introduced simultaneously neither nematode affected the invasiveness of the other. Prior infection by H. trifolii did not affect the invasiveness of P. penetrans.     

Penetration of Celery and Alfalfa Roots by Pratylenchus penetrans as Affected by Temperature

A greater percentage of females than juveniles or males of P. penetrans penetrated celery roots grown in infested soil at 5, 18, or 30 C; the difference was greatest at 5 C. The time of initial penetration of alfalfa seedlings inoculated with single nematodes on water agar varied with temperature. Females penetrated the seedlings earlier and over a wider range of temperatures than did males or juveniles. The rate of penetration was highest for females. After initial penetration, the penetration rate decreased with time. At 13-28 C, approximately 80% of roots were penetrated by females and only 25-30% by males and juveniles by the end of the experiment.     

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.     

Efficacy of Oxamyl Coated on Alfalfa Seed with a Polymer Sticker in Pratylenchus and Meloidogyne Infested Soils

A polymer sticker was used as a coating in which oxamyl was applied to seeds of alfalfa cultivar Saranac for the control of Pratylenchus penetrans and Meloidogyne hapla. The sticker, diluted 1:1 (sticker:water) to 1:5, delayed seedling emergence during the first 4 days after planting. By day 13, however, emergence from all sticker treatments was comparable to the control. Shoot growth of seedlings at day 21 was less than that of the control only from seeds coated with a 1:1 dilution; root growth and nodulation were not affected. Sticker-coated seeds absorbed 30-58% as much water in 3.5 hours as was absorbed by uncoated seeds. Oxamyl concentrations of 40-160 mg/ml in a 1:5 sticker : water mixture had no adverse affect on seedling emergence, growth, and nodulation over 3 weeks. Oxamyl at 160 mg/ml was more effective against P. penetrans than M. hapla. Growth of alfalfa in P. penetrans-infested soil was greater than that of the control in each sampling for 11 weeks. The reduction of number of P. penetrans in soil and roots moderated slowly over 11 weeks from 90% to 60%. Shoot and root growth of alfalfa from oxamyl-coated seed in M. hapla-infested soil were greater than those of the control for 7 and 11 weeks, respectively. The reduction in the number of M. hapla in the soil and roots changed from 80% at 7 weeks to 15% at 11 weeks.     

Production of the Antibiotic Phenazine-1-Carboxylic Acid by Fluorescent Pseudomonas Species in the Rhizosphere of Wheat

Pseudomonas fluorescens 2-79 and P. aureofaciens 30-84 produce the antibiotic phenazine-1-carboxylic acid and suppress take-all, an important root disease of wheat caused by Gaeumannomyces graminis var. tritici. To determine whether the antibiotic is produced in situ, wheat seeds were treated with strain 2-79 or 30-84 or with phenazine-nonproducing mutants or were left untreated and then were sown in natural or steamed soil in the field or growth chamber. The antibiotic was isolated only from roots of wheat colonized by strain 2-79 or 30-84 in both growth chamber and field studies. No antibiotic was recovered from the roots of seedlings grown from seeds treated with phenazine-nonproducing mutants or left untreated. In natural soils, comparable amounts of antibiotic (27 to 43 ng/g of root with adhering soil) were recovered from roots colonized by strain 2-79 whether or not the pathogen was present. Roots of plants grown in steamed soil yielded larger bacterial populations and more antibiotic than roots from natural soils. In steamed and natural soils, roots from which the antibiotic was recovered had significantly less disease than roots with no antibiotic, indicating that suppression of take-all is related directly to the presence of the antibiotic in the rhizosphere.     

Seasonal Populations of Pratylenchus penetrans and Meloidogyne hapla in Strawberry Roots

Strawberry roots were sampled through the year to determine the populations and distribution of Pratylenchus penetrans and Meloidogyne hapla. Three strawberry root types were sampled—structural roots; feeder roots without secondary tissues; and suberized, black perennial roots. Both lesion and root-knot nematodes primarily infected feeder roots from structural roots or healthy perennial roots. Few nematodes were recovered from soil, diseased roots, or suberized roots. Lesion nematode recovery was correlated with healthy roots. In both 1997 and 1998, P. penetrans populations peaked about day 150 (end of May) and then declined. The decline in numbers corresponded to changes in total strawberry root weight and root type distribution. The loss of nematode habitat resulted from loss of roots due to disease and the transition from structural to suberized perennial roots. Meloidogyne hapla juvenile recovery peaked around 170 days (mid June) in 1997 and at 85, 147, 229, and 308 days (late March, late May, mid August, and early November, respectively) in 1998. There appear to be at least four generations per year of M. hapla in Connecticut. Diagnostic samples from an established strawberry bed may be most reliable and useful when they include feeder roots taken in late May.     

Suppression of Meloidogyne incognita and M. javanica by Pasteuria penetrans in Field Soil

The role of Pasteuria penetrans in suppressing numbers of root-knot nematodes was investigated in a 7-year monocuhure of tobacco in a field naturally infested with a mixed population of Meloidogyne incognita race 1 and M. javanica. The suppressiveness of the soil was tested using four treatments: autoclaving (AC), microwaving (MW), air drying (DR), and untreated. The treated soil bioassays consisted of tobacco cv. Northrup King 326 (resistant to M. incognita but susceptible to M. javanica) and cv. Coker 371 Gold (susceptible to M. incognita and M. javanica) in pots inoculated with 0 or 2,000 second-stage juveniles of M. incognita race 1. Endospores of P. penetrans were killed by AC but were only slightly affected by MW, whereas most fungal propagules were destroyed or inhibited in both treatments. Root galls, egg masses, and numbers of eggs were fewer on Coker 371 Gold in MW, DR, and untreated soil than in AC-treated soil. There were fewer egg masses than root galls on both tobacco cultivars in MW, DR, and untreated soil than in the AC treatment. Because both Meloidogyne spp. were suppressed in MW soil (with few fungi present) as well as in DR and untreated soil, the reduction in root galling, as well as numbers of egg masses and eggs appeared to have resulted from infection of both nematode species by P. penetrans.     

Histopathology of Pea Roots Axenically Infected by Pratylenchus penetrans

The histological changes in pea roots axenically infected by Pratylenchus penetrans were studied and described. Roots of pea seedlings growing aseptically on the surface of nutrient agar slants were inoculated with axenized nematodes. Six hours after inoculation most of the nematodes introduced were probing the root epidermis, but none had completely entered though a few were observed with their anterior section already in the root. Most of the nematodes penetrated the roots after 12 hr inoculation. From 18 to 24 hr after inoculation the nematodes were mostly in the mid-cortex. Invaded regions of the cortex often showed orange discoloration. As incubation continued, the number of nematodes in these roots increased, and feeding and reproductive activities extended deeper into the cortex. These activities resulted in extensive breakdown of the cortex. No nematodes were observed within the stele of infected roots; however, the endodermis of infected roots stained dark-brown. Gravid female nematodes probed the root endodermis and some endodermal cells appeared to collapse after prolonged probing by the nematode. All stages in the life cycle of the nematode were observed in infected roots; the female to male ratio inside the root was about 5:1.     

Suppression of Alfalfa Growth by Concommitant Populations of Pratylenchus penetrans and two Fusarium species

Growth of alfalfa (Medicago sativa cv. Vernal) seedlings was compared after inoculation with combinations of either Pratylenchus penetrans and Fusarium soloni or P. penetrans and F. oxysporum f. sp. medicaginis. A synergistic disease interaction occurred in alfalfa when F. oxysporum and P. penetrans were added simultaneously to the soil. Alfalfa growth was suppressed at all inoculum levels of P. penetrans and F. oxysporum, but not with F. solani. Seedlings inoculated with the nematode alone gave lower yields than when inoculated with either Fusarium species alone. Fusarium oxysporum, but not F. solani, was pathogenic to alfalfa under similar experimental conditions. Fusarium oxysporum did not alter the populations of P. penetrans in alfalfa roots, whereas the presence of F. solani was associated with a diminished number of P. penetrans in the roots.     

Effect of Meloidogyne incognita on Reproduction of Pratylenchus penetrans in Red Clover and Alfalfa

Roots of seedlings of red clover and alfalfa growing on 10⁻¹ Hoagland and Arnon solution agar were inoculated with various combinations of Meloidogyne incognita and Pratylenchus penetrans. Egg-laying by P. penetrans decreased as the number of nematodes, the ratio of entrant M. incognita to entrant P. penetrans, and the priority of invasion of roots by M. incognita increased. Embryogeny and hatching of eggs of P. penetrans, and development of larvae of M. incognita, were not affected. In red clover, the greatest red uction occurred when there were 65 entrant nematodes, the ratio of M. incognita:P. penetrans was 4:1 and M. incognita was inoculated four days prior to P. penetrans. In alfalfa, the less-favorable host for both nematodes, the greatest reduction occurred when there were 45 entrant nematodes, the ratio of M. incognita:P. penetrans was 2:1, and M. incognita was inoculated 4 days prior to P. penetrans.     

Endoparasitic Nematodes in Maize Roots in the Western Transvaal as Related to Soil Texture and Rainfall

Eight endoparasitic nematode species were recovered from 170 maize root samples in western Transvaal, Republic of South Africa. Pratylenchus zeae had the highest average population density (17,454/5 g roots), followed by P. neglectus (5,827/5 g roots), P. penetrans (5,617/5 g roots), P. brachyurus (3,060/5 g roots), Meloidogyne incognita plus M. javanica (301 juveniles/5 g roots), P. crenatus (130/5 g roots), and Rotylenchutus parvus (64/5 g roots). The 17 reasonably homogeneous farming areas (RHFA) surveyed could be ranked on the basis of the incidence of the prevalent nematode species. A positive relationship was found between the incidence of P. brachyurus and R. parvus and long-term average annual rainfall. The incidence of P. penetrans and the Meloidogyne spp. was positively related to a combination of sand percentage and long-term average annual rainfall.     

Infectivity of Pratylenchus penetrans on Alfalfa

The infectivity of Pratylenchus penetrans on alfalfa seedlings cv. Du Pulls was studied. The dense root-hair zone was the preferred zone of penetration by females, males, and third-stage larvae. A lesion initially appeared as a water-soaked area at the root surface, becoming yellow and elliptical as the nematode entered the cortex, with dark-brown cells later appearing in the centre as the nematode fed. At 20 C, females penetrated roots earlier, faster, and in greater numbers than either males or third-stage larvae. Females penetrated roots at temperatures from 5 to 35 C, with maximum penetration between 10 and 30 C, while males and third-stage larvae penetrated roots only between 10 and 30 C with maximum penetration a t 20 C. Penetration of roots by females, males, and third-stage larvae increased after storage of 5 C for 35 days, but decreased after storage of 140 days or more. Combinations of the three life stages in pairs neither enhanced nor inhibited penetration of roots by individual life stages; males were not attracted to females. Increasing inoculum density up to 20 nematodes/seedling did not affect penetration.     

Pathogenicity of Pratylenchus penetrans to Navy Bean (Phaseolus vulgaris L.)

The pathogenicity of Pratylenchus penetrans (root-lesion nematode) to Phaseolus vulgaris (navy bean) was evaluated in greenhouse experiments. Shoot and root fresh weight of cv. Sanilac plants were increased 4 and 21%, respectively, by an initial population density (Pi) of 25 P. penetrans per 100 cm³ soil. Leaf area and shoot fresh and dry weights were decreased by a Pi of 50 or more P. penetrans per 100 cm³ soil. A significant positive linear relationship existed between initial soil population densities of P. penetrans and final soil and root population densities of this nematode. Three dry bean cultivars, Sanilac, Seafarer, and Tuscola, were susceptible to P. penetrans, and yields were reduced by 43-76% when plants were exposed to a Pi of 150 P. penetrans per 100 cm³ soil. P. penetrans also reproduced on bean cultivars Saginaw, Gratiot, and Kentwood, but did not decrease bean yields, suggesting that these cultivars were tolerant to this nematode.     

Influence of Soil Temperature and pH on Pratylenchus Penetrans and P. crenatus in Alfalfa and Timothy

Numbers of Pratylenchus penetrans in alfalfa and timothy, and to a lesser extent P. crenatus in timothy, increased substantially as temperature increased from about l0 C to 30 C. However, P. crenatus in alfalfa decreased in number as temperature increased. Mobility of P. crenatus in vertical soil columns decreased as temperature increased from 9.5 C to 28.5 C. Raising the soil pH from 5.0 to 6.9 in which alfalfa was grown increased the numbers of P. penetrans and greatly reduced the numbers of P. crenatus. The numbers of both nematode species in timothy were reduced significantly as soil pH was increased. The optimum soil pH for movement of P. penetrans was 6.0. Pratylenchus crenatus moved equally well over a range of pH 5.0 to 7.0.     

Post-fire seedlings of Nothofagus alpina in Southern Chile show strong dominance of a single ectomycorrhizal fungus and a vertical shift in root architecture

We investigated belowground responses of Nothofagus alpina seedlings to post-fire conditions during natural regeneration after a wildfire in Chile, focusing on mycorrhizal community and root architecture. The complete root systems of 2-year-old N. alpina seedlings were extracted from a post-fire site with natural regeneration and compared to roots of seedlings from undisturbed forest nearby. Mycorrhizal morphotype richness was determined in each seedling. Morphometric parameters of tertiary root structure and dry biomass of whole root systems were determined in 5 cm vertical intervals and in four lateral root classes. With 43.5% of colonized vital mycorrhizal root tips, the Basidiomycete Descolea antarctica was the most abundant fungal symbiont on post-fire seedlings. Tertiary root morphology of these seedlings was distinct from control plants and characterized by a deep-reaching tap root with rather evenly distributed lateral branches whereas seedlings from the undisturbed site had shallower root systems with most lateral roots concentrated in the upper soil layers. Post-fire seedlings had more mycorrhizal rootlets and mycorrhiza-bearing third order lateral roots than control plants which was expressed in a 34% higher total root number but only a 10% higher total root biomass, although both values were not statistically significant. A major part of these fine roots in seedlings from burnt forest was found in deeper soil horizons, compared to the seedlings from undisturbed forest. According to our results, post-fire conditions clearly favour Descolea antarctica as an early ectomycorrhizal colonizer of Nothofagus seedlings at the studied site. As no significant changes in soil chemistry could be observed at the burnt site, the deep-reaching tertiary root architecture of these seedlings may be interpreted as a response to other abiotic factors like reduced moisture in surface soil.     

Effects of Tabtoxinine-beta-Lactam on Nitrogen Metabolism in Avena sativa L. Roots

The effects of tabtoxinine-β-lactam (T-β-L) on nitrate uptake and glutamine synthetase (GS) and nitrate reductase (NR) activities in roots of Avena sativa seedlings were determined. Seven-day-old oat seedlings placed in a 10 mm KNO₃ and 0.5 mm T-β-L solution for 24 hours took up T-β-L and lost approximately 90% of their root GS activity. [³H]-T-β-L taken up by roots of seven-day-old oat seedlings was associated with GS immunoprecipitated from the extract of these roots. Total nitrate uptake and in vivo NR activity were decreased approximately 50% in the T-β-L treated roots. However, T-β-L uptake did not affect the induction phases of nitrate uptake or reduction, nor did it inhibit in vitro NR activity. Thus, the decrease in nitrate uptake and reduction is a secondary effect of T-β-L action. Roots of seven-day-old oat seedlings were inoculated with Pseudomonas syringae pv tabaci (Tox+) and the pathogen population in the rhizosphere was estimated by dilution plate count; 6 × 10¹³ bacteria were recovered after 3 days, as compared to the original inoculation with 7 × 10⁹ bacteria, indicating a significant growth of the pathogen in the rhizosphere. The bacteria recovered from the rhizosphere caused chlorosis in tobacco leaves and produced T-β-L in culture; 1 × 10¹⁴ bacteria were recovered from roots of seedlings inoculated with P. syringae pv tabaci (Tox−) using the same inoculation and assay procedure as for the pv tabaci (Tox+). Extracts of surface-sterilized roots previously inoculated with P. syringae pv tabaci (Tox+) did not produce viable bacterial cultures when plated out on a complete medium. Oat seedlings growing in sand culture and inoculated with P. syringae pv tabaci (Tox+) had developed chlorosis, and root GS activity had declined to less than 10% of controls after 3 days. Conversely, seedlings inoculated with P. syringae pv tabaci (Tox−) never developed chlorosis and maintained normal levels of GS activity. All oat plants inoculated with P. syringae pv tabaci (Tox+) died within 7 days after inoculation as compared to the plants inoculated with P. syringae pv tabaci (Tox−) which grew to maturity.