Inheritance of Resistance to Pratylenchus penetrans in Alfalfa

Fifty-two alfalfa (Medicago sativa L.) clones, randomly selected from the cultivar Baker and the experimental line MNGRN-4, were evaluated for resistance (based on nematode reproduction) to Pratylenchus penetrans in growth chamber tests (25 C). Twenty-five clones, representing the range of nematodes and eggs per plant, were selected and retested. Four moderately resistant and two susceptible alfalfa clones were identified. Inheritance of resistance to P. penetrans was studied in these six clones using a diallel mating design. The S₁, Fl, and reciprocal progenies differed for numbers of nematodes and eggs per g dry root and for shoot and root weights (P < 0.05). Resistance, measured as numbers of nematodes in roots, was correlated between parental clones and their S₁ families (r = 0.94), parental clones and their half-sib families (r = 0.81), and S₁ and half-sib families (r = 0.88). General combining ability (GCA) effects were significant for nematode resistance traits. Both GCA and specific combining ability (SCA) effects were significant for plant size traits, but SCA was more important than GCA in predicting progeny plant size. Reciprocal effects were significant for both nematode resistance and plant size traits, which may slow selection progress in long-term selection programs. However, the GCA effects are large enough that breeding procedures that capitalize on additive effects should be effective in developing alfalfa cultivars with resistance to 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.     

Ultrastructural Pathology of Cells Affected by Pratylenchus penetrans in Alfalfa Roots

Cortical parenchyma cells penetrated and fed upon by Pratylenchus penetrans for 48 hours contained only cytoplasmic debris. Proximal cells had an increase in tannin deposits, degenerated mitochondria, increased numbers of ribosomes, and no internal membrane structure. Often the endodermis was collapsed and contained massive tannin deposits on the inner cell wall and cell lumen. Similar observations were made in the stele, except tannin deposits were not as prominent. Multivesicnlate structures were observed both in the endodermis and in the stele.     

Comparative Response of Alfalfa to Pratylenchus penetrans Populations

Four populations of Pratylenchus penetrans did not differ (P > 0.05) in their virulence or reproductive capability on Lahontan alfalfa. There was a negative relationship (r = -0 .7 9 ) between plant survival and nematode inocula densities at 26 ± 3 C in the greenhouse. All plants survived at an inoculum level (Pi) of 1 nematode/cm³ soil, whereas survival rates were 50 to 55% at 20 nematodes/cm³ soil. Alfalfa shoot and root weights were negatively correlated (r = - 0.87; P < 0.05) with nematode inoculum densities. Plant shoot weight reductions ranged from 13 % at Pi 1 nematode/cm³ soil to 69% for Pi 20 nematodes/cm³ soil, whereas root weight reductions ranged from 17% for Pi 1 nematode/cm³ soil to 75% for Pi 20 nematodes/cm³ soil. Maximum and minimum nematode reproduction (Pf/Pi) for the P. penetrans populations were 26.7 and 6.2 for Pi 1 and 20 nematodes/cm³ soil, respectively. There were negative correlations between nematode inoculum densities and plant survival (r = 0.84), and soil temperature and plant survival (r = -0 .7 8 ). Nematode reproduction was positively correlated to root weight (r = 0.89).     

Effects of Soil pH on Reproduction of Pratylenchus penetrans and Forage Yield of Alfalfa

''Vernal'' alfalfa was grown for 30 weeks in nematode-free soil and in soil infested with Pratylenchus penetrans. Charlottetown fine sandy loam soil was used at its pH of 4.4 and at adjusted reactions of 5.2, 6.4 and 7.3. Nematode reproduction was significantly greater at pit 5.2 and 6.4 and was not related to alfalfa root production over the full pH range studied. A significant nematode infestation X soil pit interaction on forage yield was recorded. Nematode infestation significantly decreased forage yields at ptt 5.2 and 6.4 but not at pH 4.4 and 7.3.     

Effects of Potassium Fertilization and Pratylenchus penetrans on Yield and Potassium Content of Red Clover and Alfalfa

Red clover and alfalfa were inoculated with Pratylenchus penetrans and grown in an Alberry sandy loam soil to which potassium (K⁺) was added at seeding at 0, 41.5, 83, and 166 μg/g. In one experiment with alfalfa, additional K⁺ was added after each forage cut to replace that which was removed. Nematode populations were not consistently affected by K⁺ fertilization. Nematode infection stunted red clover and alfalfa and resulted in lower yields at all K⁺ levels, except for alfalfa at the lowest K⁺ level. Nematode infection had no effect on taproot yields. However, it resulted in lower rootlet yields from red clover at all K⁺ levels, lower rootlet yields from alfalfa only at the highest K⁺ level in one experiment, and lower rootlet yields at all but the lowest K⁺ level in a second experiment, potassium fertilization enhanced yield of red clover and alfalfa. Yield increases were smaller from increased K⁺ fertilization in nematode-infested soil than in noninfested soil. Pralylenchus penetrans had little effect on the K⁺ content of red clover or alfalfa. The stunting of plants from nematode infection resulted in less K⁺ being removed from the soil.     

Effect of Ditylenchus dipsaci and Pratylenchus penetrans on Verticillium Wilt of Alfalfa

Verticillium albo-atrum wilt symptoms appeared faster and were significantly more severe in the presence of Ditylenchus dipsaci in Vernal, a wilt-susceptible cultivar, than in Marls Kabul, a wilt-resistant cultivar. Winter kill in the field was not affected by the nematode during the first winter, but 50% of plants were killed in the second winter. Forage yield from nematode-infected plants was significantly reduced the second year. Interaction with V. albo-atrum did not significantly reduce forage yields below that of D. dipsaci alone. Pratylenchus penetrans did not increase the severity of wilt symptoms in the presence of V. albo-atrum, nor did it affect forage yield in the greenhouse. It did, however, reduce alfalfa yields in presence of V. albo-atrum under field conditions. D. dipsaci and P. penetrans reproduced faster in Vernal than in Maris Kabul when the fungus was present.     

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.     

Relative Susceptibility of Selected Cultivars,of Potato to Pratylenchus penetrans

Pratylenchus penetrans suppressed the tuber yields of potato cultivars ''Katahdin'', ''Kennebec'', and ''Superior'', but did not affect yields of ''Russet Burbank''. In comparison with noninfested controls, all initial nematode densities (P_i) of P. penetrans (P_i = 38, 81, 164, 211/ 100 cm³ of soil) suppressed yields of Superior; a moderate P_i (81/100 cm³ soil) suppressed yields of Kennebec; and on Katahdin, a moderate P_i enhanced yields, but higher P_i''s caused a marked loss. In general, yields were related to the tolerance of the cultivars to nematode colonization. Highest nematode densities were found in the roots of Russet Burbank; the next highest, in succeeding order, were found in roots of Kennebec, Katahdin, and Superior. Symptoms of nematode invasion were confined to losses of tuber yield and root weight.     

Control of Pratylenchus penetrans and Meloidogyne hapla and Yield Response of Alfalfa Due to Oxamyl Seed Treatments

Alfalfa (Medicago sativa L. cv. Saranac) seed were soaked for 20 minutes in water, acetone, or methanol containing 10 or 50 mg/ml of oxamyl (Vydate L) or coated with a 2% aqueous cellulose solution containing the same amounts of oxamyl. Seed were analyzed for oxamyl by HPLC immediately after treatment and after 9 and 26 months of storage. Oxamyl content of alfalfa seed did not decline after 26 months of storage. The effects of seed treatment on growth of alfalfa and nematode control were examined using soils infested with Pratylenchus penetrans and Meloidogyne hapla. Germination was not affected by any of the seed treatments. Twenty-one days after sowing, the total growth of alfalfa seedlings grown from seed treated with 50 mg/ml of oxamyl in P. penetrans-infested soils had increased by 62% over controls. Nodulation per pot increased by as much as 267%, and the densities of P. penetrans per gram of root were reduced by as much as 73% compared to control plants. In M. hapla-infested soils, increases in plant growth (32%) and nodulation (71%) also occurred with oxamyl-treated seeds. Root gall reduction (86%) was also substantial due to oxamyl seed treatment.     

Management Options for Pratylenchus penetrans in Easter Lily

Alternatives to reduce or modify nematicide use for minimizing groundwater contamination in Easter lily were explored in two field trials. Alternatives to standard 1,3-dichloropropene (1,3-D) plus phorate injection in the first trial were: (i) delaying applications until after winter rains, (ii) removing roots from planting stock, (iii) 1,3-D via drip irrigation, (iv) a chitin-urea soil amendment, (v) the registered insecticide disulfoton, and (vi) several nonregistered nematicides. None of the treatments equaled the standard treatment. In the second trial, potential benefits of adding a systemic nematicide, oxamyl (OX), or a fungicide, metalaxyl (MX), to the standard treatment were explored. Preplant drip irrigation applications of metam sodium (MS), sodium tetrathiocarbonate (ST), and emulsifiable 1,3-D were evaluated alone and in combination with postplant applications of OX and MX. Several drip-applied treatments performed comparably to the standard treatment with respect to the most important criteria of crop quality, bulb circumference. Metam-sodium in combination with either or both OX and MX, 1,3-D plus OX and MX, and ST plus OX and MX provided the best results.     

Infection of Pratylenchus penetrans by Nematode-pathogenic fungi

Eleven fungal isolates were tested in agar dishes for pathogenicity to Pratylenchus penetrans. Of the fungi that produce adhesive conidia, Hirsutella rhossiliensis was a virulent pathogen; Verticillium balanoides, Drechmeria coniospora, and Nematoctonus sp. were weak or nonpathogens. The trapping fungi, Arthrobotrys dactyloides, A. oligospora, Monacrosporium dlipsosporum, and M. cionopagum, killed most of the P. penetrans adults and juveniles added to the fungus cultures. An isolate of Nematoctonus that forms adhesive knobs trapped only a small proportion of the nematodes. In 17-cm³ vials, soil moisture influenced survival of P. penetrans in the presence of H. rhossiliensis; nematode survival decreased with diminishing soil moisture. Hirsutella rhossiliensis and M. ellipsosporum were equally effective in reducing numbers of P. penetrans by 24-25% after 4 days in sand. After 25 days in soil artificially infested with H. rhossiliensis, numbers of P. penetrans were reduced by 28-53%.     

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.     

Pseudomonas chlororaphis Strain Sm3, Bacterial Antagonist of Pratylenchus penetrans

The interaction of Pseudomonas chlororaphis strain Sm3 and the root-lesion nematode Pratylenchus penetrans was investigated in three separate greenhouse experiments with soils from southern British Columbia, Canada. The bacteria were applied to the roots of strawberry plants and planted in unpasteurized field soils, with natural or supplemented infestation of P. penetrans. Nematode suppression in roots was evident after 6 or 10 weeks in all experiments. Root or shoot growth were increased after 10 weeks in two experiments. Population dynamics of P. chlororaphis Sm3 in the rhizosphere was followed using an antibiotic-resistant mutant of P. chlororaphis Sm3. There was no apparent correlation between bacterial density in the rhizosphere and P. penetrans suppression in strawberry roots and rhizosphere soil, although the soil with the highest nematode reduction also had the largest P. chlororaphis Sm3 population in the rhizosphere.     

Temporal Changes in the Vertical Distribution of Pratylenchus penetrans under Raspberry

Population densities of Pratylenchus penetrans and the biomass of fine roots of raspberry at depths of 0-5, 5-10, 10-20, and 20-30 cm were determined every 2 weeks for 2 years. The vertical distribution of P. penetrans varied from season to season, but the seasonal changes were not similar for the 2 years. In most seasons, the greatest population density was in the 5 to 10-cm-depth interval. Population densities of P. penetrans were not consistently correlated with the vertical distribution of raspberry roots in any season.     

Impact of Plant Nutrition on Pratylenchus penetrans Infection of Prunus avium Rootstocks

A hypothesis that cherry rootstocks grown under optimal nutrient conditions are affected less by Pratylenchus penetrans infection than those grown under deficient nutrient conditions was tested by growing four Prunus avium L. rootstocks (''Mazzard'', ''Mahaleb'', ''GI148-1'', and ''GI148-8'') at a soil pH of 7.0 over a period of 3 months under greenhouse conditions (25 ñ 2 °C). Pratylenchus penetrans was inoculated at 0 (control) or 1,500 nematodes per g fresh root weight for a total of 3,600, 4,200, 10,500, and 11,400 per plant on Mazzard, Mahaleb, GI148-1, and GI148-8, respectively, with nutrients (commercial fertilizer) applied once at planting (deficient) or twice weekly (optimal). The experiment was repeated once. The optimum nutrient regime resulted in greater soil nutrient levels and plant growth; higher leaf concentrations of N, P, K, and Mg; and fewer P. penetrans than under the deficient nutrient regime. The addition of fertilizer either may increase nematode mortality in the soil or improve rootstock resistance to nematode infection. Increases in Ca in leaves from the nutrient-deficient and nematode-infected treatments suggested the plants were physiologically stressed. The Pf/Pi ratios indicated that these rootstocks may have had resistance to P. penetrans; however, because of the dominant role of nutrition in the experimental design, the question of resistance could not be properly addressed.     

Effect of Initial Nematode Population Density on the Interaction of Pratylenchus penetrans and Verticillium dahliae on 'Russet Burbank' Potato

Four similar growth chamber experiments were conducted to test the hypothesis that the initial population density (Pi) of Pratylenchus penetrans influences the severity of interactive effects of P. penetrans and Verticillium dahliae on shoot growth, photosynthesis, and tuber yield of Russet Burbank potato. In each experiment, three population densities of P. penetrans with and without concomitant inoculation with V. dahliae were compared with nematode-free controls. The three specific Pi of JR penetrans tested varied from experiment to experiment but fell in the ranges 0.8-2.5, 1.8-3.9, 2.1-8.8, and 7.5-32.4 nematodes/cm³ soil. Inoculum of V. dahliaewas mixed into soil, and the assayed density was 5.4 propagules/gram dry soil. Plants were grown 60 to 80 days in a controlled environment. Plant growth parameters in two experiments indicated significant interactions between P. penetrans and V. dahliae. In the absence of V. dahliae, P. penetrans did not reduce plant growth and tuber yield below that of the nematode-free control or did so only at the highest one or two population densities tested. In the presence of K dahliae, the lowest population density significantly reduced shoot weight and photosynthesis in three and four experiments, respectively. Higher densities had no additional effect on shoot weight and caused additional reductions in photosynthesis in only one experiment. Population densities of 0.8 and 7.5 nematodes/cm³ soil reduced tuber yield by 51% and 45%, whereas higher densities had no effect or a 15% additional effect, respectively. These data indicate that interactive effects between P. penetrans and V. dahliae on Russet Burbank potato are manifested at P. penetrans population densities less than 1 nematode/cm³ soil and that the nematode population density must be substantially higher before additional effects are apparent.     

Interaction of Meloidogyne naasi,Pratylenchus penetrans,and Tylenchorhynchus agri on Creeping Beentgrass

The pathogenicity and interactions of Meloidogyne naasi, Pratylenchus penetrans, and Tylenchorhynchus agri on ''Toronto C-15'' creeping bentgrass, Agrostis palustris, was studied in a long-term greenhouse experiment. Based on dry weights of roots and clippings, M. naasi alone and in all combinations with P. penetrans and T. agri was highly pathogenic to creeping bentgrass. P. penetrans and T. agri alone and in combination inhibited root growth but adversely affected top growth only when the two were co-inoculated. In combination, the effects of each species on top growth were additive, with M. naasi the dominant pathogen. Creeping bentgrass was an excellent host for M. naasi and T. agri, but a poor host for P. penetrans. T. agri inhibited population increase of M. naasi, indicating nematode-nematode competition, but neither T. agr/ nor P. penetrans was affected by any of the combinations.     

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.     

Analysis of Crop Losses in Tomato Due to Pratylenchus penetrans

The effects of Pratylenchus penetrans upon yields of ''Veebrite'' tomato were studied at initial soil population densities (P_i) of 360, 2,010, 4,580, and 14,360 nematodes/kg of soil in 20-cm (i.d.) clay-tile microplots. The lowest P_i appeared to stimulate fruit production. Higher P_i''s suppressed fruit production (total weight of marketable tomatoes and numbers of intermediate- and large-sized fruits), in comparison to control yields, the highest P_i resulted in 38% fewer fruits which weighed 44% less. These losses were at least partly due to a delay in fruit ripening, caused by the nematodes, which did not become apparent until the fourth week. Nematode populations in the soil increased at all but the highest P_i; final populations were around 7,000/kg of soil. Nematode populations in roots ranged from 230-590/gm of root at the completion of the experiment. Nematode control by fumigation would definitely be warranted at soil population densities of 2,000/kg or higher; with 500-2,000/kg, the decision to fumigate would depend on soil type and economic and hiological factors.