Influence of Alfalfa Plant Growth on the Multiplication Rates and Ceiling Population Density of Meloidogyne hapla

The rates of reproduction and multiplication of Meloidogyne hapla decreased as a result of self-regulatory, density-dependent processes with time and nematode population increase in the soil and roots of Medicago sativa cv. Cuf 101. Juvenile, egg, and mature female population densities increased at a maximum rate until damage to the host resulted in alfalfa yield reductions. Temporal differences in multiplication and reproduction rates of M. hapla were observed to be a function of initial population density (Pi), host damage, and root biomass, indicating increased levels of competition for a constant but limited number of feeding sites. Over time, a log linear relationship emerged between multiplication rate of M. hapla and Pi. Slopes of -0.90953 for combined eggs and juveniles and -0.71349 for mature females indicated a gradual approach to ceiling densities. Reproductive rates decreased exponentially from an initial maximal value of 200 to a relatively constant rate of 53 eggs per female.     

Penetration and Development of Meloidogyne hapla in Resistant and Susceptible Alfalfa Under Differing Temperatures

Studies were conducted to examine under differing temperatures (12, 16, 20, 24, 28, and 32 C) the penetration anti development of Meloidogyne hapla in resistant lines ''298'' and ''Nev. Syn XX'', and susceptible ''Lahontan'' and ''Ranger'' hardy-type alfalfas. The results indicated that resistance to M. hapla was similar to that previously described for M. incognita in nonhardy alfalfa. Although initial penetration in resistant seedlings was similar to that of susceptible seedlings, nematode larvae failed to establish and develop in root tissues and nematode numbers subsequently declined. In susceptible seedlings, nematode development proceeded rapidly, and egg production began after 5 weeks. Temperature had little influence on the nematode development except to slow the response at the lower temperatures. Other studies were conducted to verify a previously reported immune (no penetration) reaction to M. hapla by the ''Vernal'' selection ''M-4''. When compared to the resistant (penetration without nematode development) Vernal selection ''M-9'' under differing temperatures (20, 24, 28, and 32 C), each selection was equally penetrated by M. hapla but at a lower level than in susceptible Ranger cuttings. Generally, no root galling was observed in either M-4 or M-9; however, very slight galling was found 35 days after inoculation on about 50% of these cuttings when grown at 32 C.     

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.     

Differential Response of Thor Alfalfa to Meloidogyne chitwoodi Races and M. hapla

Second-stage juveniles (J2) of races 1 and 2 of Meloidogyne chiiwoodi and M. hapla readily penetrated roots of Thor alfalfa and Columbian tomato seedlings; however, few individuals of M. chitwoodi race 1 were able to establish feeding sites and mature on alfalfa. Histopathological studies indicate that J2 of race 1 either failed to initiate feeding sites or they caused cell enlargement without typical cell wall thickening. The protoplasm of these cells coagulated, and juveniles of race 1 did not develop beyond the swollen J2 stage. A few females of race 1 fed on small giant cells and deposited a few eggs at least 20 and 30 days later than M. chitwoodi race 2 and M. hapla, respectively. Failure of race 1 to establish feeding sites was related to egression of J2 from the roots. The M. chitwoodi race 1 J2 egression from alfalfa roots was higher than egression of race 2 and M. hapla. Egression of J2 of M. chitwoodi races 1 and 2 from tomato roots was similar and higher than that of M. hapla. Thus egression plays an important role in the host-parasite relationship of M. chitwoodi and alfalfa.     

Pathogenicity of Meloidogyne hapla to Lettuce as Affected by Inoculum Level, Plant Age at Inoculation and Temperature

Pathogenicity of Meloidogyne hapla to lettuce was influenced by inoculum level, age of plant at inoculation and temperature. Top weight of ''Minetto'' lettuce was reduced 32% when 2-week-old lettuce plants were each inoculated with five egg masses. Higher inoculum levels did not further decrease top weight significantly. Inoculation at seeding reduced top growth more than inoculation of 1-, 2- or 3-week-old seedlings. M. hapla reduced growth more at the intermediate (21.1 C night and 26.7 C day), than at the low (15.5 C night and 21.1 C day) or high (26.7 C night and 32.2 C day), temperature regimes.     

Host Tests to Differentiate Meloidogyne chitwoodi Races 1 and 2 and M. hapla

The reproductive factor (R = final egg density at 55 days ÷ 5,000, initial egg density) of Meloidogyne chitwoodi race 2 (alfalfa race) on 46 crop cultivars ranged from 0 to 130. The reproductive efficiency of M. chitwoodi race 1 (non-alfalfa race) and M. chitwoodi race 2 was compared on selected crop cultivars. The basic difference between the two races lay in their differential reproduction on Thor alfalfa and Red Cored Chantenay carrot. M. chitwoodi race 2 reproduced on alfalfa but not on carrot. Conversely, alfalfa was a poor host and carrots were suitable for M. chitwoodi race 1. Based on host responses to M. chitwoodi races and M. hapla, a new differential host test was proposed to distinguish the common root-knot nematode species of the Pacific Northwest.     

Differential Reaction of Alfalfa Cultivars to Meloidogyne hapla and M. chitwoodi Populations

Meloidogyne hapla reproduced and suppressed growth (P < 0.05) of susceptible Lahontan and Moapa alfalfa at 15, 20, and 25 C. At 30 C, resistant Nevada Syn XX lost resistance to M. hapla. M. hapla invaded and reproduced on Rhizobium meliloti nodules of Lahontan and Moapa, inducing giant cell formation and structural disorder of vascular bundles of nodules without disrupting bacteroids. At 15, 20, and 25 C a M. chitwoodi population from Utah reproduced on Lahontan, Moapa, and Nevada Syn XX alfalfa, suppressing growth (P < 0.05). Final densities of the Utah M. chitwoodi population were greater (P < 0.05) than those of Idaho and Washington State populations on Lahontan at 15 and 25 C and on Nevada Syn XX at 15 C, but were less consistent and smaller (P < 0.05) than those of M. hapla on Lahontan and Moapa at 20 and 25 C. Inconsistent reproduction of the Utah M. chitwoodi population on alfalfa suggests the possible existence of nematode strains revealed by variability in alfalfa resistance. No reproduction or inconsistent final nematode population densities with no damage were observed on Lahontan, Moapa, and Nevada Syn XX plants grown in soil infested with Idaho and Washington State M. chitwoodi populations.     

Pathogenicity of Two Populations of Meloidogyne hapla Chitwood on Alfalfa and Sainfoin

The pathogenicity of two populations of the northern root-knot nematode, Meloidogyne hapla Chitwood, population 1 (P1) from alfalfa and population 2 (P2) from sainfoin, was studied on both alfalfa and sainfoin for 25 weeks. Alfalfa and sainfoin plants inoculated with P2 had significantly (P ≤ 0.05) higher mortality than plants inoculated with P1. Plant stands over all weeks for the uninoculated control, P1, and P2 were 90.5, 78.5, and 64.0% for alfalfa and 84.5, 51.0, and 41.0% for sainfoin, respectively. The increased virulence of P2 was again shown when means of plant species were combined (inoculation × week of count interaction). Plants inoculated with P2 had significantly higher mortality than either those inoculated with P1 or the uninoculated control beginning at week 7 and continuing through week 25. Plant stands over species at 25 weeks for the uninoculated control, P1, and P2 were 82.5, 29.0, and 18.0%, respectively. Sainfoin was significantly more susceptible to either population than alfalfa (plant species × week of count interaction). Separation between species first occurred after week 7 and continued until week 25. Percentages of plants remaining for alfalfa and sainfoin were 61.5 and 25.0 after 25 weeks. Significantly higher reproduction occurred in the alfalfa plants remaining after 25 weeks in P2 than in P1. Mean number of eggs per root system were 60,371 for P1 and 104,438 for P2, a difference of 42%. The results of this study indicate a need for breeders to adequately sample nematode populations present in the intended area of cultivar use and to design screening procedures to account for population pathogenicity variability.     

Effects of Population Densities of Meloidogyne hapla on Growth and Yield of Tomato

Growth and yield of ''Veebrite'' tomato were studied in 20-cm (i.d.) clay-tile microplots containing initially 260, 1,840, 6,120, or 27,950 Meloidogyne hapla larvae/kg of soil. Low nematode numbers stimulated, and the highest nematode population suppressed, vegetative plant growth. More tomatoes, with a higher total weight, were harvested from plants infested with 260 and 1,840 nematode larvae at planting than from those with initial densities of 6,120 and 27,950 larvae. At the two highest densities, the cumulative fruit production (weight) was suppressed by 10% and 40%, respectively. The increase in growth and yield at the lower densities appeared to be due to an increase in the size of the root systent. However, at the higher densities, yield was no longer directly related to root weight. The reproduction factor of M. hapla was negatively correlated with initial density; for the lowest and highest initial densities, it was 96X and 7X at midseason, and 354X and 3X at harvest, respectively. The equilibrium density was 63,000 larvae/kg of soil; initial densities larger than 2,000 larvae/kg of soil may require control.     

Effect of Temperature on Growth, Development and Reproduction of Meloidogyne hapla in Lettuce

Temperature was an important factor in growth, development and reproduction of Meloidogyne hapla in lettuce. Growth, as measured by increase in diameter of females, was not appreciably different at the intermediate (21.1 C night and 26.7 C day) and high (26.7 C night and 32.2 C day) temperature regimes, but was considerably less at the low temperature regime (15.5 C night and 21.1 C day) than at the two higher temperature regimes. Second-stage female larvae developed into adults 14 days after inoculation at the high, 18 days at the intermediate and 34 days at the low temperature regime. Eggs were observed 20 days after inoculation at the high, 26 days at the intermediate and 54 days at the low temperature regime. Number of eggs and larvae after 6 weeks was greater at the high than at the intermediate temperature regime and no eggs or larvae occurred at the low temperature regime during the observed 6 weeks.     

Host-parasite Relationship of Carrot Cultivars and Meloidogyne chitwoodi Races and M. hapla

Most of the 15 carrot cultivars tested were moderate to good hosts to Meloidogyne chitwoodi race 1, whereas all except Orlando Gold were nonhosts or poor hosts for M. chitwoodi race 2. All carrot cultivars were good hosts for M. hapla. The plant weights of the carrot cultivars Red Cored Chantenay and Orlando Gold infected with either race of M. chitwoodi were significantly less than uninoculated checks in pots. Under field microplot conditions, however, detrimental effects on quality were rarely observed. M. hapla was pathogenic to both cultivars in the greenhouse and the field. The tolerance level of Orlando Gold to M. hapla was lower than Red Cored Chantenay.     

Biological Relationship of Meloidogyne hapla Populations to Alfalfa Cultivars

Greenhouse and growth chamber studies were established to determine if there are pathological and physiological differences among Meloidogyne hapla populations from California (CA), Nevada (NV), Utah (UT), and Wyoming (WY) on alfalfa cultivars classified as resistant or susceptible to root-knot nematodes. In the greenhouse, plant survival was not consistent with resistance classifications. While all highly resistant Nevada Synthetic germplasm (Nev Syn XX) plants survived inoculation with all nematode populations, two cultivars classified as moderately resistant (''Chief'' and ''Kingstar'') survived (P ≤ 0.05) inoculation with M. hapla populations better than did ''Lobo'' cultivar, which is classified as resistant. Plant growth of Nev Syn XX was suppressed by only the CA population, whereas growth of the other alfalfa cultivars classified as M. hapla resistant or moderately resistant was suppressed by all nematode populations. Excluding Nev Syn XX, all alfalfa cultivars were severely galled and susceptible to all nematode populations. Except for Nev Syn XX, reproduction did not differ among the nematode populations on alfalfa cultivars. Nev Syn XX was not as favorable a host to CA as were the other cultivars; but, it was a good host (reproductive factor [Rf] = 37). Temperature affected plant resistance; the UT and WY populations were more pathogenic at 15-25 C, and CA was more pathogenic at 30 C. Nev Syn XX was susceptible to all nematode populations, except for CA, at only 30 C, and all other alfalfa cultivars were susceptible to all nematode populations at all temperatures.     

Interrelationship of Heterodera schachtii and Meloidogyne hapla on Tomato

Invasion of tomato (Lycopersicon esculentum L.) roots by combined and sequential inoculations of Meloidogyne hapla and a tomato population of Heterodera schachtii was affected more by soil temperature than by nematode competition. Maximum invasion of tomato roots, by M. hapla and H. schachtii occurred at 30 and 26 C, respectively. Female development and nematode reproduction (eggs per plant) of M. hapla was adversely affected by H. schachtii in combined inoculations of the two nematode species. Inhibition of M. hapla development and reproduction on tomato roots from combined nematode inoculations was more pronounced as soil temperature was increased over a range of 18-30 C and with prior inoculation of tomato with H. schachtii. M. hapla minimally affected H. schachtii female development, but there was significant reduction in the buildup of H. schachtii when M. hapla inoculation preceded that of H. schachtii by 20 days.     

Damage Threshold of Meloidogyne hapla to Lettuce in Organic Soil

Lettuce was seeded in pots in the greenhouse and in field microplots in 1991 and 1992. Pots and microplots were filled with untreated or fumigated organic soil infested with Meloidogyne hapla at seven initial population densities (Pi) (0 to 32 eggs/cm³ soil). Lettuce weight, severity of root galling, and number of eggs per root system (Pf) were determined after 8 weeks. At the highest Pi, M. hapla caused yield losses up to 64% in the microplots and plant death in the greenhouse tests. The Seinhorst equation was used to describe the relation between lettuce weight and Pi (r² = 0.73 - 0.98) and to calculate the damage threshold density (T). Values of T were 7 and 8 eggs/cm³ soil in the greenhouse tests of 1991 and 1992, respectively. In the microplot tests, T was 1 egg/cm³ soil in 1991 and 2 eggs/cm³ soil in 1992. The damage threshold was the same in untreated and fumigated soils. At low Pi, root galling was more severe in the pots than in the microplots. Pf increased with increasing Pi of M. hapla in both tests, but declined at Pi > T in the greenhouse tests. The reproduction rate (Pf/Pi) of M. hapla was highest at the lowest Pi.     

Biological Control of Meloidogyne hapla on Alfalfa and Tomato with the Fungus Meria coniospora

This study was to determine whether Arthrobotrys flagrans, A. oligospora, and Meria coniospora would control the root-knot nematode Meloidogyne hapla on alfalfa and tomato. Alfalfa seeds were coated with a fungus-rye powder in 2% cellulose and were planted in infested soil. Three-week-old seedlings from seed treated with M. coniospora had 60% and 58% fewer galls in two experiments than did seedlings from untreated seeds. Numbers of J2 in the soil were not reduced. Plant growth did not improve. When seed of tomato were coated with M. coniospora and planted in M. hapla-infested soil, roots had 34% fewer galls and 47% fewer J2 in the soil at 28 days. After 56 days there was no reduction in J2 numbers. Plant growth did not improve. When roots of tomato transplants were dusted with M. coniospora fungus-rye powder or sprayed with a spore suspension before planting in M. hapla-infested soil, 42% and 35%, respectively, fewer galls developed in 28 days on treated roots than on roots not treated with fungus. The numbers of J2 extracted from roots or recovered from soil were not reduced, however, and plant growth did not improve.     

Host Suitability of Twelve Leguminosae Species to Populations of Meloidogyne hapla and M. chitwoodi

Legumes of the genera Astragalus (milkvetch), Coronilla (crownvetch), Lathyrus (pea vine), Lotus (birdsfoot trefoil), Medicago (alfalfa), Melilotus (clover), Trifolium (clover), and Vicia (common vetch) were inoculated with a population of Melaidogyne chitwoodi from Utah or with one of three M. hapla populations from California, Utah, and Wyoming.Thirty-nine percent to 86% of alfalfa (M. scutellata) and 10% to 55% of red clover (T. pratense) plants survived inoculation with the nematode populations at a greenhouse temperature of 24 ± 3°C. All plants of the other legume species survived all nematode populations, except 4% of the white clover (T. repens) plants inoculated with the California M. hapla population. Entries were usually more susceptible to the M. hapla populations than to M. chitwoodi. Galling of host roots differed between nematode populations and species. Root-galling indices (1 = none, 6 = severely galled) ranged from 1 on pea vine inoculated with the California population of M. hapla to 6 on yellow sweet clover inoculated with the Wyoming population of M. hapla. The nematode reproductive factor (Rf = final nematode population/initial nematode population) ranged from 0 for all nematode populations on pea vine to 35 for the Wyoming population of M. hapla on alfalfa (M. sativa).     

Interaction of Vesicular-Arbuscular Mycorrhizae and Cultivars of Alfalfa Susceptible and Resistant to Meloidogyne hapla

The interaction between vesicular-arbuscular mycorrhizal (VAM) fungi and the root-knot nematode (Meloidogyne hapla) was investigated using both nematode-susceptible (Grasslands Wairau) and nematode-resistant (Nevada Synthetic XX) cultivars of alfalfa (Medicago sativa) at four levels of applied phosphate. Mycorrhizal inoculation improved plant growth and reduced nematode numbers and adult development in roots in dually infected cultures of the susceptible cultivar. The tolerance of plants to nematode infection and development when preinfected with mycorrhizal fungi was no greater than when they were inoculated with nematodes and mycorrhizal fungi simultaneously. Growth of plants of the resistant cultivar was unaffected by nematode inoculation but was improved by mycorrhizal inoculation. Numbers of nematode juveniles were lower in the roots of the resistant than of the susceptible cultivar and were further reduced by mycorrhizal inoculation, although no adult nematodes developed in any resistant cultivar treatment. Inoculation of alfalfa with VAM fungi increased the tolerance and resistance of a cultivar susceptible to M. hapla and improved the resistance of a resistant cultivar.     

Correlation of Tobacco Yield,Value, and Root-Knot Index with Early-to-Midseason,and Postharvest Meloidogyne Population Densities

Certain nematicidal treatments for the control of root knot of tobacco in six field experiments in North Carolina were used to determine early, midseason, and postharvest densities of eggs and larvae of Meloidogyne spp., postharvest root-knot indices, and crop yield and value. Various statistical correlations showed that population densities determined 6-8 wk after transplanting were more indicative of treatment effectiveness than postharvest densities. Logarithmic transformation of early and midseason population data stabilized the variance.     

Influence of Planting Date on Population Dynamics and Damage Potential of Pratylenchus brachyurus on Soybean

Planting date was used as a variable to determine the effects of time and different environmental conditions on the population dynamics and damage potential of Pratylenchus brachyurus on soybean at two locations in North Carolina. An initial population slightly less than the damage threshold (275 nematodes/500 cm³ soil) was used to minimize the influence of host damage on this nematode''s population dynamics and to gain greater precision in characterizing factors which influence the damage potential of P. brachyurus to soybean. Equivalent nematode numbers generally resulted in greater yield suppression of soybean in early plantings. Early planting of soybean also resulted in greater (P = 0.01) population densities of P. brachyurus at midseason which often persisted until soybean harvest. Length of time for reproduction and intraspecific competition occurring when soybeans were stunted by the nematode were the most important factors influencing the population dynamics of P. brachyurus.