Studies on the Pathogenic Potential and Life Cycle of Reniform Nematode on Broccoli

The significant reduction in the plant growth was observed at an initial inoculum level of 2 immature females of Rotylenchulus reniformis/g soil and hence this inoculum level was considered as the minimum damaging threshold level for broccoli. The population of reniform nematode increased with the increasing levels of inoculum. Though, the maximum nematode population was recorded at the highest level of inoculum, the rate of reproduction of nematode was found to be highest at the lowest level of initial inoculum. Symptoms like chlorosis, stunted growth, shedding of leaves, early sprouting of inflorescence and sparsely developed roots were found during the experimental studies. Rotylenchulus reniformis required 30 days to complete the life cycle on broccoli.     

Pathogenicity and life cycle of Meloidogyne javanica on broccoli

A pathogenicity trial conducted against root-knot nematode, Meloidogyne javanica on broccoli indicated that a gradual increase in the nematode inoculum from 500 to 8000 juveniles/kg soil was associated with a progressive decline in all the plant growth parameters and reproduction factor of the nematode. Although 8000 juveniles/kg soil showed maximum plant growth reduction and root knot index, statistical analysis of the data revealed that the population of 1000 juveniles/kg soil was associated with a significant decline in plant growth. Hence, this level was indicative of being the pathogenic level. The significant reduction in seedling emergence was recorded at and above 2000 juveniles/kg soil and it decreased further with increasing inoculum levels. Meloidogyne javanica required 27 days to complete the life cycle on broccoli at a temperature range of 28–35°C.     

Interrelationships of Rotylenchulus reniformis with Rhizoctonia solani on Cotton

The interrelationships between reniform nematode (Rotylenchulus reniformis) and the cotton (Gossypium hirsutum) seedling blight fungus (Rhizoctonia solani) were studied using three isolates of R. solani, two populations of R. reniformis at multiple inoculum levels, and the cotton cultivars Dehapine 90 (DP 90) and Dehapine 41 (DP 41). Colonization of cotton hypocotyl tissue by R. solani resulted in increases (P ≤ 0.05) in nematode population densities in soil and in eggs recovered from the root systems in both 40- and 90-day-duration experiments. Increases in soil population densities resulted mainly from increases in juveniles. Enhanced reproduction of R. reniformis in the presence of R. solani was consistent across isolates (1, 2, and 3) of R. solani and populations (1 and 2) and inoculum levels (0.5, 2, 4, and 8 individuals/g of soil) of R. reniformis, regardless of cotton cultivar (DP 90 or DP 41). Severity of seedling blight was not influenced by the nematode. Rhizoctonia solani caused reductions (P ≤ 0.05) in cotton growth in 40- and 90-day periods. Rotylenchulus reniformis reduced cotton growth at 90 days. The relationship between nematode inoculum levels and plant growth reductions was linear. At 90 days, the combined effects of these pathogens were antagonistic to plant growth.     

Impact of Soil Texture on the Reproductive and Damage Potentials of Rotylenchulus reniformis and Meloidogyne incognita on Cotton

The effects of soil type and initial inoculum density (Pi) on the reproductive and damage potentials of Meloidogyne incognita and Rotylenchulus reniformis on cotton were evaluated in microplot experiments from 1991 to 1993. The equilibrium nematode population density for R. reniformis on cotton was much greater than that of M. incognita, indicating that cotton is a better host for R. reniformis than M. incognita. Reproduction of M. incognita was greater in coarse-textured soils than in fine-textured soils, whereas R. reniformis reproduction was greatest in a Portsmouth loamy sand with intermediate percentages of clay plus silt. Population densities of M. incognita were inversely related to the percentage of silt and clay, but R. reniformis was favored by moderate levels of clay plus silt (ca. 28%). Both M. incognita races 3 and 4 and R. reniformis effected suppression of seed-cotton yield in all soil types evaluated. Cotton-yield suppression was greatest in response to R. reniformis at high Pi. Cotton maturity, measured as percentage of open bolls at different dates, was affected by the presence of nematodes in all 3 years.     

Pathogenicity of root-knot nematode,Meloidogyne incognita on Lens culinaris (Medik.)

Effects of the root-knot nematode (Meloidogyne incognita) on lentil (Lens culinaris) were studied under greenhouse conditions. The plants were inoculated with 250, 500, 1000, 2000 and 4000 J₂ per plant. Plant growth, yield, nodulation, seed weight, chlorophyll, nitrogen, phosphorus and potassium, (NPK) contents, as compared to control, were found decreased in all the nematode infected plants. The extent of reduction increased with an increase in inoculum levels. The reductions were significant at 500 J₂ and at higher inoculum levels, i.e. 1000, 2000 and 4000 J₂ per pot over the control. An increase in inoculum level caused enhancement in galling, egg mass production and nematode population. At higher inoculum levels, the population of the nematode in the root as well as in the soil increased to a greater magnitude than at lower inoculum levels. On the contrary, reproduction factor (RF) and rate of population increase (RPI) decreased with increasing inoculum levels.     

Polarographische Bestimmung von Chlorat-Rückständen im Boden und in der Pflanze

An increase in the inoculum level of root‐knot nematode, Meloidogyne incognita and the reniform nematode, Rotylenchulus reniformis resulted in a relative decrease in plant growth parameters of chickpea. Consequently water absorption capability of roots was impaired. M. incognita caused greater reduction than R. reniformis at the same inoculum level. In concomitant inoculation of M. incognita and R. reniformis there was greater suppression in plant growth of chickpea. The suppression in concomitant inoculations was less than the sum of the suppression caused by the same levels of inoculations of the individual species. The multiplication rate of the nematodes decreased as the inoculum level increased. The results also suggest competition for feeding sites between the two nematode species. The multiplication rate of one species progressively decrease with the increase in the inoculum levels of the other nematode.     

Pathogenicity of root knot nematode Meloidogyne incognita on okra and its management through botanicals

An investigation was carried out to study the pathogenicity of root knot nematode Meloidogyne incognita on okra and its management through various organic amendments. The inoculum level of 1000 juveniles per plant showed significant reduction in various plant growth parameters, which reveals that M. incognita is a potential pathogen of okra. With the increase in inoculums level of M. incognita (J₂), there was a progressive decrease in various plant growth parameters. The maximum reduction in plant growth parameters was observed at an inoculum level of 4000 juveniles per plant. The efficacy of five organic amendments viz. groundnut cake, castor cake, sunflower cake, linseed cake and sawdust was tested against root knot nematode M. incognita. Amending the soil with different oil cakes was found to be effective in reducing the nematode soil population, number of females, number of egg masses as well as root gall formation in okra. The highest increase in plant growth (13%) and maximum reduction in number of galls (54%), number of females (57%) and number of egg masses (55%) was recorded on application of groundnut cake.     

Genetic Structure of Races of Heterodera glycines and Inheritance of Ability to Reproduce on Resistant Soybeans

Four field populations of Heterodera glycines tested for ability to reproduce on three host differentials were each classified into one of the recognized races. A fifth population represented a new race. Genetic analysis indicated that the designated races are actually field populations that differ from each other primarily in the frequencies of three groups of genes (genes for parasitism) that act quantitatively and control the ability of the nematode to reproduce on resistant P.I. 88788, Pickett, and P.I. 90763 soybeans. Populations of race-3 have none of these genes for parasitism, or they have some in low frequency that results in an index of parasitism of less than 10 on any one of the resistant soybeans. Race-1 has a high frequency of one group of genes that enable it to reproduce on P.I. 88788. Race-2 has two groups of genes for parasitism in high frequency; one for P.I. 88788, and one for Pickett. Based on these findings, it was assumed that race-4 has three groups of genes for parasitism; one for P.I. 88788, one for Pickett, and one for P.I. 90763. Additional races may be recognized when new genes are identified, or when new gene combinations are discovered. The ability to reproduce on P. I. 88788 is inherited independently from the ability to reproduce on Pickett. Although the genetic structure of field populations does not provide a solid foundation for race designation, recognizing races under the present system may be useful when it clearly characterizes the behavior of field populations. Race designations, however, should be regarded as provisional since gene frequencies change with time in response to selection forces and, therefore, the race status of a population may change accordingly.     

Effect of Temperature on Infection and Survival of Rotylenchulus reniformis

From infestation of lettuce with preinfective females to egg deposition, populations of Rotylenchulus reniformis from Baton Rouge, Louisiana; Lubbock and Weslaco, Texas; and Mayaguez, Puerto Rico, required 41, 13, 7, and 7 days at 15, 20, 25, and 34 C, respectively. No nematode infection occurred at 10 C with any R. reniformis population, and the population from Puerto Rico did not reproduce at 15 C. Nematode survival was not influenced by temperature, since populations from Texas and Louisiana survived for 6 months without a host at - 5 , - 1 , 4, and 25 C. Survival of R. reniformis was substantially influenced by soil moisture. Soil moistures greater than 7% (< 1 bar) aided nematode survival at storage temperature of 25 C, whereas moisture adversely affected nematode survival below freezing. Soil moisture below 4% (> 15 bars) favored nematode survival below freezing but adversely affected nematodes in soils stored at 25 C. Soil moisture effects on nematode survival were less accentuated at 4 and 0 C.     

The Effect of Soil Texture and Irrigation on Rotylenchulus reniformis and Cotton

The reniform nematode, Rotylenchulus reniformis, is the most damaging nematode pathogen of cotton in Alabama. Soil texture is currently being explored as a basis for the development of economic thresholds and management zones within a field. Trials to determine the reproductive potential of R. reniformis as influenced by soil type were conducted in microplot and greenhouse settings during 2008 to 2010. Population density of R. reniformis was significantly influenced by soil texture and exhibited a general decrease with increasing median soil particle size (MSPS). As the MSPS of a soil increased from 0.04 mm in clay soil to > 0.30 mm in very fine sandy loam and sandy loam soils, R. reniformis numbers decreased. The R. reniformis population densities on all soil types were also greater with irrigation. Early season cotton development was significantly affected by increasing R. reniformis Pi, with plant shoot-weight-to-root-weight ratios increasing at low R. reniformis Pi and declining with increasing R. reniformis Pi. Plant height was increased by irrigation throughout the growing season. The results suggests that R. reniformis will reach higher population densities in soils with smaller MSPS; however, the reduction in yield or plant growth very well may be no greater than in a soil that is less preferential to the nematode.     

Comparison of concomitant and sequential inoculation of Steinernema sp. in the management of root-knot (Meloidogyne incognita) nematode infecting eggplant (Solanum melongena)

Culture of Steinernema sp. was maintained on Corcyra cephalonica larvae. Steinernema sp. (at 50, 500, 1000, 2500, 5000, 10,000 and 20,000 ij’s /500?g soil) was concomitantly inoculated with 500 J₂ of Meloidogyne incognita/500?g soil to the eggplant seedlings in the pots filled with 4?kg sterilised soil. The simultaneous inoculation of M. incognita with either of the inoculum levels (1000, 2500, 5000 and 10,000 J₃/500?g soil) of Steinernema sp. significantly reduced the damage caused by M. incognita in terms of plant growth parameters, viz. plant length, dry weight, number of flowers and weight of fruits. Moreover, the highest improvement in plant growth parameters, viz. plant length, dry weight, number of flowers and weight of fruits, was recorded in plants inoculated with 5000 J₃ of Steinernema sp./500?g soil followed by 2500, 1000 and 10,000 J₃/500?g soil. The highest reduction in the reproduction factor and number of galls/root system was recorded in the plants treated with 5000 J₃ Steinernema sp./500?g soil followed by 2500, 1000 and 10,000 J₃/500?g soil. Comparison of concomitant and sequential inoculations showed that the sequential inoculation (both prior and after) of Steinernema sp. at different inoculum levels (1000, 2500, 5000, 10,000 and 20,000 ij’s/500?g soil) was more effective in the management of root-knot nematode than the concomitant inoculation. Therefore, the application of Steinernema sp. might be useful for suppression of nematode pest on eggplant and may be used as an alternative for chemicals.     

Influence of Temperature on the Virulence of Two Races of Meloidogyne chitwoodi on Wheat and Barley

Races of the Columbia root-knot nematode, Meloidogyne chitzooodi, from Idaho (R1) and Utah (R2) suppressed (P < 0.05) tillering of Dusty winter wheat, Fielder spring wheat, Luther winter barley, and Steptoe spring barley at 15-30 C. Nematode inoculum density was negatively correlated with tillering (r = -0.79). Inoculum densities of both nematode races were negatively correlated with heads per plant (r = -0.83), head length (r = -0.87), and head dry weight (r = 0.73) of Fielder spring wheat and Steptoe spring barley at all temperatures; the greatest growth restrictions occurred at Pi 20 eggs/cm³ soil. Both nematode races were most damaging at 25-30 C. Fielder spring wheat and Steptoe spring barley inoculated with R2 produced fewer heads than R1 when inoculated at 15 C, whereas the same cultivars inoculated with R1 produced fewer heads than R2 at 30 C. No differences were observed between root growth of winter and spring wheat or between winter and spring barley. Nematode reproduction was positively correlated to temperature (r = 0.87) and negatively correlated with inoculum density (r = -0.86). Reproductive rates were greatest with Pi = 2 eggs/cm³ soil at 25 C and lowest with Pi = 20 eggs/cm³ soil at 15 C for both nematode races.     

A screening technique to evaluate pigeonpea for resistance to Rotylenchulus reniformis*

Rotylenchulus reniformis is one of the most important nematode pests of pigeonpea. A simple greenhouse technique has been developed to aid evaluation of pigeonpea genotypes for resistance to R. reniformis. In greenhouse pot experiments, eggsacs of R. reniformis in pigeonpea (cv. ICPL 87) roots were counted by eye and with the aid of a stereoscopic microscope at 15, 30 and 45 days after seedling emergence in soils infested with various numbers of vermiform R. reniformis. Seedlings were rated for the number of eggsacs per root system on a one (no eggsacs) to nine (more than 50 eggsacs) scale. Eggsac ratings were more uniform when roots were evaluated at 30 – 45 days than at 15 days and an inoculum of 15 to 30 individuals/cm³ soil also helped reduce variability. Eggsacs were not easily visible without the aid of a stereoscopic microscope. Of the 14 stains tested, exposure of nematode-infected roots to 0.25% trypan blue for three min was effective in staining the eggsacs blue without staining the roots. Using the stain, the assessment of infestation by R. reniformis was equally accurate with or without the aid of a stereoscopic microscope. Exposure of eggsacs to trypan blue enhanced the emergence of juveniles from the eggsacs.     

Crop Rotation and Races of Meloidogyne incognita in Cotton Root-knot Management

The influence o f various crop rotations and nematode inoculum levels on subsequent population densities of Meloidogyne incognita races 1 and 3 were studied in microplots. Ten different 3-year sequences o f cotton, corn, peanut, or soybean, all with cotton as the 3rd-year crop, were grown in microplots infested with each race. Cotton monoculture, two seasons o f corn, or cotton followed by corn resulted in high race 3 population densities and severe root galling on cotton the 3rd year. Peanut for 2 years preceding cotton most effectively decreased the race 3 population and root galls on cotton the 3rd year. Race 1 did not significantly influence cotton growth or yield at initial populations of up to 5,000 eggs/500 cm³ soil. At 5,000 eggs/500 cm³, cotton growth was suppressed by race 3 but yield was not affected.     

Influence of Nonhost Plants on Population Decline of Rotylenchulus reniformis

The influence of Chloris gayana, Crotalaria juncea, Digitaria decumbens, Tagetes patula, and a chitin-based soil amendment on Hawaiian populations of Rotylenchulus reniformis was examined. Chloris gayana was a nonhost for R. reniformis. The nematode did not penetrate the roots, and in greenhouse and field experiments, C. gayana reduced reniform nematode numbers at least as well as fallow. Tagetes patula was a poor host for reniform nematode and reduced reniform nematode numbers in soil better than did fallow. Crotalaria juncea was a poor host for R. reniformis, and only a small fraction of the nematode population penetrated the roots. Crotalaria juncea and D. decumbens reduced reniform nematode populations at least as well as fallow. A chitin-based soil amendment, applied at 2.24 t/ha to fallow soil, did not affect the population decline of reniform nematode.     

Effect of Controlled Cold Storage on Recovery of Rotylenchulus reniformis from Naturally Infested Soil

Rotylenchulus reniformis is rapidly becoming the most economically important pest associated with cotton in the southeastern United States. Incentive programs have been implemented to support sampling of production fields to determine the presence and abundance of R. reniformis. These sampling programs have dramatically increased the number of soils samples submitted to nematology laboratories during autumn. The large numbers of samples overwhelm most labs and require placement in cold storage until extraction. Therefore, the objective of this study was to examine the length of time soils infested with R. reniformis can be stored before nematode extraction without compromising the accuracy of estimates of population densities. A sandy loam and a silty loam were the two cotton production soils used in this study. Rotylenchulus reniformis numbers decreased 61%during the first 180 days of storage in both soils. Rotylenchulus reniformis numbers from the initial sampling through 180 days decreased as a linear function. The decline of R. reniformis numbers during storage was estimated as 0.28% of the population lost daily from the maximum population through 180 days. The diminution of nematode numbers from 180 through 1,080 days in storage continued, but at a slower rate. Numbers of R. reniformis declined to less than 89%, 93%, and 99% of the initial population within 360, 720, and 1,080 days, respectively, of storage. The reduction of R. reniformis numbers over 180 days can be adjusted, allowing a more accurate estimation of R. reniformis levels in soil samples stored at 4 °C.     

Vertical Distribution of Rotylenchulus reniformis in Cotton Fields

The possible impact of Rotylenchulus reniformis below plow depth was evaluated by measuring the vertical distribution of R. reniformis and soil texture in 20 symptomatic fields on 17 farms across six states. The mean nematode population density per field, 0 to 122 cm deep, ranged from 0.4 to 63 nematodes/g soil, and in 15 fields more than half of the R. reniformis present were below 30.5 cm, which is the greatest depth usually plowed by farmers or sampled by consultants. In 11 fields measured, root density was greatest in the top 15 cm of soil; however, roots consistently penetrated 92 to 122 cm deep by midseason, and in five fields in Texas and Louisiana the ratio of nematodes to root-length density within soil increased with depth. Repeated sampling during the year in Texas indicated that up to 20% of the nematodes in soil below 60 cm in the fall survived the winter. Differences between Baermann funnel and sugar flotation extraction methods were not important when compared with field-to-field differences in nematode populations and field-specific vertical distribution patterns. The results support the interpretation that R. reniformis below plow depth can significantly impact diagnosis and treatment of cotton fields infested with R. reniformis.     

Influence of Rhizoctonia solani on Egg Hatching and Infectivity of Rotylenchulus reniformis

The effects of culture filtrates of Rhizoctonia solani and root exudates of R. solani-infected cotton (Gossypium hirsutum) seedlings on hatching of eggs and infectivity of females of Rotylenchulus reniformis were evaluated in an attempt to account for the enhanced nematode reproduction observed in the presence of this fungus. Crude filtrates of R. solani cultures growing over sterile, deionized distilled water did not affect egg hatching. Exudates from roots of cotton seedlings increased hatching of R. reniformis eggs over that observed in water controls. Exudates from cotton seedling roots not infected or infected with R. solani did not differ in their effect on egg hatching. However, infection of cotton seedlings by reniform females was increased in the presence of R. solani, resulting in the augmented egg production and juvenile population densities in soil observed in greenhouse studies.     

Root-Parasitic Nematodes Enhance Soil Microbial Activities and Nitrogen Mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes (Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg^–1 soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.     

Interactions Between Calonectria crotalariae and Heterodera glycines on Soybean

The interactions of Heterodera glycines at four egg inoculum levels (0, 100, 1,000, and 10,000 per pot) and three cyst levels (0, 100, and 200 per pot) and Calonectria crotalariae at 500, 5,000, and 50,000 microsclerotia per pot were evaluated on soybean. At the two lowest nematode egg levels, the presence of C. crotalariae did not affect nematode reproduction. At 10,000 eggs per pot, however, nematode reproduction was increased significantly at each microsclerotial level. The increase in nematode reproduction was stepwise at 500 and 5,000 microsclerotia per pot but declined at 50,000 microsclerotia per pot. Similar results were obtained when cysts rather than eggs were used as nematode inoculum. The nematode x fungus interaction significantly affected 60-day plant growth parameters of both Lee 74 and Centennial soybean. The nematode x fungus interaction was antagonistic to plant roots and significantly influenced root injury ratings. The presence of C. crotalariae in tissues of stock plants or plants used as race differentials did not alter the analysis of this population as race 3.