Preferred Temperature of Meloidogyne incognita

In laboratory thermal gradients, newly hatched infective juveniles of the plant-parasitic root-knot nematode Meloidogyne incognita migrated toward a preferred temperature that was several degrees above the temperature to which they were acclimated. After shifting egg masses to a new temperature, the preferred temperature was reset in less than a day. Possible functions of this type of thermotaxis are discussed, including the use of thermal gradients around plant roots to locate hosts and to maintain a relatively straight path while ranging in the absence of other cues (a collimating stimulus).     

Behavioral Responses of Meloidogyne incognita to Small Temperature Changes

Small, rapid temperature changes were generated by incandescent radiation, and behavioral responses of Meloidogyne incognita juveniles were recorded with high time resolution by computer tracking. Temperature changes away from the preferred temperature resulted in decreases in the rate of movement and increases in the rate of change of direction, whether the changes were toward warmer or cooler temperatures. These behavioral changes lasted about 30 seconds. Temperature changes toward the preferred temperature caused the response rates to change in the opposite directions, and the behavioral changes persisted for several minutes. These results demonstrate that nematodes can respond to a purely temporal thermal stimulus in a manner consistent with efficient indirect orientation or klinokinesis. The rate of temperature change was estimated to be of the order of 10⁻⁴ C/second, suggesting that the nematodes detected a change of about 0.001 C.     

Histopathology of Root-knot Nematode (Meloidogyne incognita) Infection on White Yam (Dioscorea rotundata) Tubers

White yam tissues naturally and artificially infected with root-knot nematodes were fixed, sectioned, and examined with a microscope. Infective second-stage juveniles of Meloidogyne incognita penetrated and moved intercellularly within the tuber. Feeding sites were always in the ground tissue layer where the vascular tissues are distributed in the tubers. Giant cells were always associated with xylem tissue. They were thin walled with dense cytoplasm and multinucleated. The nuclei of the giant cells were only half the size of those found in roots of infected tomato plants. Normal nematode growth and development followed giant cell formation. Females deposited eggs into a gelatinous egg mass within the tuber, and a necrotic ring formed around the female after eggs had been produced. Second-stage juveniles hatched, migrated, and re-infected other areas of the tuber. No males were observed from the tuber.     

Scanning Electron Microscope Study of the Root-knot Nematode (Meloidogyne incognita) on Tomato Root

This study examines the types of structural information that can be gained by utilizing the scanning electron microscope (SEM) and a cryofracture technique to examine the host-parasite interaction. Roots of tomato, Lycopersicon esculentum cv. Marglobe, were cultured aseptically and inoculated with the root-knot nematode, Meloidogyne incognita. Twenty-four hours to four weeks after inoculation, developing galls were removed from the cultures and processed for SEM observation. The cryofracture technique was used to reveal internal structural features within the developing galls. The results illustrate structural details concerning penetration of the roots, differentiation of syncytia, and development of the nematodes beginning with the second-stage larvae and ending with adult egg-laying females.     

Relationship Between Egg Viability and Population Densities of Meloidogyne incognita on Cotton

Cotton seedlings were inoculated with a range of initial populations (Pi) of Meloidogyne incognita in greenhouse experiments to test the relationship between nematode population densities and egg viability. In two of three experiments, a significant (P < 0.05) negative linear relationship was detected between percentage of hatch of first generation eggs and log Pi. A similar relationship between hatch and root-gall index was observed. In two experiments numbers of eggs judged to be nonviable based on appearance were significantly greater (P < 0.05) in the highest Pi (60,000 eggs/seedling) treatments than in treatments with lower Pi (600-6,000 eggs/seedling). It was concluded that Pi affects egg viability measured as percentage of hatch and that this relationship may play a role in the density-dependent winter survival rates of Meloidogyne species.     

Influence of Initial Population Densities of Meloidogyne incognita on Three Chile Cultivars

The effects of Meloidogyne incognita on the Big Jim, Jalapeno, and New Mexico No. 6 chile (Capsicum annuum) cultivars were investigated in microplots for two growing seasons. All three cultivars were susceptible to M. incognita and reacted similarly to different initial populations of this nematode. Severe stunting and yield suppressions occurred at all initial M. incognita densities tested ranging from 385 to 4,230 eggs and larvae/500 cm³ soil. Regression analysis of the microplot data from a sandy loam soil showed yield losses of 31% for the 1978 season and 25% for the 1979 season for the three cultivars for each 10-fold increase in the initial population of M. incognita.     

Meloidogyne incognita Infested Soil Amended With Chicken Litter

The effects of chicken litter on Meloidogyne incognita in cotton, Gossypium hirsutum cv. DPL50 were determined in field microplots. Litters (manure and pine-shaving bedding) from a research facility and a commercial broiler house were used. Treatments consisted of 0.25%, 0.5%, and 1% litter by dry weight of soil for each kind of litter. Three control treatments consisted of soil not amended with litter, with and without nematodes, and one treatment to which mineral fertilizer was added at a nitrogen rate equivalent to that of the 0.5% litter rate, with nematodes. Microplots were inoculated at planting with 900 eggs/100 cm³ soil in 1993 and 1,000 eggs/100 cm³ soil in 1994. At 92 and 184 days after planting, nematode population densities decreased linearly with increasing rates of litter. Nematode numbers at midseason were larger in plots treated with mineral fertilizer than in plots treated with a rate of litter equivalent to the 0.5% rate. Fungal and bacterial population densities fluctuated throughout the growing season. Bacterial numbers had a positive linear relationship, with increasing rates of litter only in October 1993; however, significant positive relationships were observed throughout the 1994 growing season. In 1994, nematode population density at 92 days after planting decreased linearly with increasing bacterial numbers 30 days after planting. No other significant relationships between nematode densities and microbial densities were observed. Fungi and bacteria isolated from the litter and litter-amended soil were identified. Fungal genera isolated included Acremonium, Aspergillus, Eurotium, Paecilomyces, Petriella, and Scopulariopsis, whereas bacteria genera included Arthrobacter, Bacillus, and Pseudomonus.     

Interaction of Meloidogyne incognita and Water Stress in Two Cotton Cultivars

A series of controlled-environment experiments were conducted to elucidate the effects of Meloidogyne incognita on host physiology and plant-water relations of two cotton (Gossypium hirsutum) cultivars that differed in their susceptibility to nematode infection. Inoculation of M. incognita-resistant cultivar Auburn 634 did not affect growth, stomatal resistance, or components of plant-water potential relative to uninoculated controls. However, nematode infection of the susceptible cultivar Stoneville 506 greatly suppressed water flow through intact roots. This inhibition exceeded 28% on a root-length basis and was similar to that observed as a consequence of severe water stress in a high evaporative demand environment. Nematodes did not affect the components of leaf water potential, stomatal resistance, transpiration, or leaf temperature. However, these factors were affected by the interaction of M. incognita and water stress. Our results indicate that M. incognita infection may alter host-plant water balance and may be a significant factor in early-season stress on cotton seedlings.     

Histology of the Interactions of Paecilomyces lilacinus with Meloidogyne incognita on Tomato

Excised tomato roots were examined histologically for interactions of the fungus Paecilomyces lilacinus and Meloidogyne incognita race 1. Root galling and giant-cell formation were absent in tomato roots inoculated with nematode eggs infected with P. lilacinus. Few to no galls and no giant-cell formation were found in roots dipped in a spore suspension of P. lilacinus and inoculated with M. incognita. Numerous large galls and giant cells were present in roots inoculated only with M. incognita. P. lilacinus colonized the surface of epidermal cells as well as the internal cells of epidermis and cortex. The possibility of biological protection of plant surfaces with P. lilacinus against root-knot nematodes is discussed.     

Effect of Meloidogyne incognita on Plant Nutrient Concentration and Its Influence on the Physiology of Beans

Phaseolus vulgaris plants, 3, 8, 11, and 13 days old, were inoculated with 0, 2,000, 4,000, or 8,000 second-stage Meloidogyne incognita larvae and maintained under controlled conditions. The photosynthetic rate and the shoot and root concentration of K, Ca, Mn, Fe, Cu, and Zn were determined by destructive assay at 1-27-day intervals and by nondestructive assay of leaves, stems, and roots at 27 or 28 days after inoculation. In the destructive assay, the concentration of the elements in the plant tissues did not change until 1 week after inoculation. Thereafter, the trend was mostly decreasing for shoot K and Fe and increasing in the root, whereas Ca had the opposite trend in the shoots. Manganese, Cu, and Fe showed variable trends. Generally, the concentration of K and Mn increased, whereas Ca and Fe decreased, with duration of infection in all treatments. Zinc and Cu decreased in the highest nematode treatments. The overall elemental content generally decreased with level of infection from 1 week after inoculation. Photosynthetic rate based on shoot K concentration significantly decreased with level of infection. In most of the nondestructive assays, the concentrations of shoot K, Zn, and Mn decreased, whereas Ca increased with increasing nematode treatment. One of the first effects of the nematode on host physiology appears to be a change in concentration of nutrient elements in the host plant.     

Effects of Interactions among Heterodera glycines,Meloidogyne incognita,and Host Genotype on Soybean Yield and Nematode Population Densities

The effects of host genotype and initial nematode population densities (Pi) on yield of soybean and soil population densities of Heterodera glycines (Hg) race 3 and Meloidogyne incognita (Mi) race 3 were studied in a greenhouse and field microplots in 1983 and 1984. Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were planted in soil infested with 0, 31, or 124 eggs of Hg and Mi, individually and in all combinations, per 100 cm³ soil. Yield responses of the soybean cultivars to individual and combined infestations of Hg and Mi were primarily dependent on soybean resistance or susceptibility to each species separately. Yield of Centennial was stimulated or unaffected by nematode treatments, yield of Braxton was suppressed by Hg only, and yield suppressions caused by Hg and Mi were additive and dependent on Pi for Coker 237. Other plant responses to nematodes were also dependent on host resistance or susceptibility. Population densities of Mi second-stage juveniles (J2) in soil were related to Mi Pi and remained constant in the presence of Hg for all three cultivars. Population densities of Hg J2 on the two Hg-susceptible Cultivars, Braxton and Coker 237, were suppressed in the presence of Mi at low Hg Pi.     

Ultrastructural Cytochemistry of Secretory Granules of Esophageal Glands of Meloidogyne incognita

Ultrastructural cytochemical tests for several enzymes, proteins, carbohydrates, and nucleic acids were conducted on secretory granules o£ dorsal and subventral esophageal glands of preparasitic second-stage juveniles and the dorsal gland of adult females of Meloidogyne incognita. Secretory granules in the subventral glands of juveniles stained positive for acid phosphatase. Peroxidase, DNase, RNase, cellulase, and nucleic acids were not detected in these granules. Secretory granules in the dorsal gland of adult females stained positive for peroxidase (pH 7.6) in < 50% of the tests, Acid phosphatase, β-glucuronidase, DNase, RNase, polyphenoloxidase, cellulase, and carbohydrates were not detected in dorsal gland granules in adult females. Positive staining with cobalt thiocyanate, a stain for amino groups of basic proteins, occurred in secretory granules in the dorsal gland, ribosomes, and chromatin in adult females. Ribosomes, nuclei, and secretory granules of the dorsal gland of adult females intensely stained when incubated in three reagents specific for nucleic acid.     

Effect of Meloidogyne incognita and Importance of the Inoculum on the Yield of Eggplant

The relationship between population densities of race 1 of Meloidogyne incognita and yield of eggplant was studied. Microplots were infested with finely chopped nematode-infected pepper roots to give population densities of 0, 0.062, 0.125, 0.25, 0.50, 1, 2, 4, 8, 16, 32, 64, and 128 eggs and juveniles/cm³ soil. Both plant growth and yield were suppressed by the nematode. A tolerance limit of 0.054 eggs and juveniles/cm³ soil and a minimum relative yield of 0.05 at four or more eggs and juveniles/cm³ soil were derived by fitting the data with the equation y = m + (1 - m)z^P⁻T. Maximum nematode reproduction rate was 12,300. Hatch of eggs from egg masses in water or from sodium hypochlorite dissolved egg masses was similar (41% and 39%), but egg viability was significantly greater from egg masses in water (58%) than from sodium hypochlorite dissolved egg masses (12%) after 4 weeks. Greater numbers of nematodes were collected from roots of tomatoes from soil infested with entire egg masses than from tomato roots from soil infested with egg masses dissolved by sodium hypochlorite.     

Cuticular Collagenous Proteins of Second-stage Juveniles and Adult Females of Meloidogyne incognita: Isolation and Partial Characterization

Cuticles isolated from second-stage juveniles and adult females of Meloidogyne incognita were purified by treatment with 1% sodium dodecyl sulfate (SDS). The juvenile cuticle was composed of three zones differing in their solubility in β-mercaptoethanol (BME). Proteins in the cortical and median zones were partially soluble in BME, whereas the basal zone was the least soluble. The BME-soluble proteins from the juvenile cuticle were separated into 12 bands by SDS-polyacrylamide gel electrophoresis and characterized as collagenous proteins based on their sensitivity to collagenase and amino acid composition. The adult cuticle consisted of two zones which were dissolved extensively by BME. The basal zone was completely solubilized, leaving behind a network of fibers corresponding to the cortical zone. The BME-soluble proteins from the adult cuticle were separated by electrophoresis into nine bands one of which constituted > 55% of the total BME-soluble proteins. All bands were characterized as collagenous proteins. Collagenous proteins from juvenile cuticles also contained glycoproteins which were absent from the adult cuticles.     

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.     

Colored Mulches Affect Yield of Fresh-market Tomato Infected with Meloidogyne incognita

The effects of different-colored polyethylene mulches on the quantity and spectra of reflected light, earliness of fruit set, fruit yield and quality, and root-knot disease were studied in field-grown, staked tomato (Lycopersicon esculentum). White mulch reflected more photosynthetic light and a lower far-red-to-red ratio than red mulch, whereas black mulch reflected less than 5 percent of any color. Soil temperatures and fruit yields were recorded for tomato plants inoculated with Meloidogyne incognita race 3 at initial populations of 0, 1,000, 10,000, 50,000, or 100,000 eggs/plant and grown over black, white, or red plastic mulch in both spring and fall. Soil temperatures were lower under white mulch than under red or black mulch. Tomato yields declined as inoculum level increased. Plants grown over red mulch in the spring and inoculated with 50,000 eggs of M. incognita had greater early marketable yields than similarly inoculated plants grown over black or white mulch. Tomato plants inoculated with 100,000 eggs and grown over white mulch or red mulch in the spring had greater total yields per plot than similar plants grown over black mulch (7.39 kg and 7.71 kg vs. 3.65 kg, respectively).     

Effect of Ammonium Ions on Egg Hatching and Second-Stage Juveniles of Meloidogyne incognita in Axenic Tomato Root Culture

Eggs, either dispersed or in masses, and second-stage juveniles (J2) of Meloidogyne incognita were exposed to different concentrations of ammonium ions in a nutrient agar medium upon which excised tomato roots were growing. Egg hatch and J2 penetration of the roots was slowed or inhibited at high (54 and 324 mg/liter) but not at low (1.5 and 9 mg/liter) concentrations of ammonium nitrate. The effect of ammonium on J2 appeared to be temporary and reversible. High potassium nitrate concentration (1,116 mg/liter) did not modify egg hatch or J2 penetration. There was no adverse effect from the high ammonium nitrate concentrations or an equivalent potassium nitrate concentration on root dry weight. Ammonium ions influence nematodes both directly and via plant roots and do so independently of microbial organisms.     

Penetration and Development of Meloidogyne incognita in Roots of Resistant and Susceptible Corn Genotypes

Rates of penetration and development ofMeloidogyne incognita race 4 in roots of resistant (inbred Mp307, and S4 lines derived from the open-pollinated varieties Tebeau and Old Raccoon) and susceptible (Pioneer 3110) corn genotypes were determined. Seedlings grown in styrofoam containers were inoculated with 5,000 eggs of M. incognita. Roots were harvested at 3-day intervals starting at 3 days after inoculation (DAI) to 27 DAI and stained with acid fuchsin. Penetration of roots by second-stage juveniles (J2) at 3 DAI was similar for the four corn genotypes. Meloidogyne incognita numbers in Tebeau, Old Raccoon, Mp307, and Pioneer 3110 peaked at 12, 12, 15, and 27 DAI, respectively. Nematode development in the resistant genotypes was greatly suppressed compared to Pioneer 3110. Resistance to M. incognita in these genotypes appears to be expressed primarily as slower nematode development rather than differences in J2 penetration.     

Overwintering Stages of Meloidogyne incognita in Vitis vinifera

The overwintering of Meloidogyne incognita in and around Vitis vinifera cv. French Colombard roots was studied in a naturally infested vineyard at the Kearney Agricultural Center, in a growth chamber, in inoculated vines in microplots at the University of California, Davis, and in a greenhouse. Infected roots were sampled at intervals from onset of vine dormancy until plants accumulated about 800 degree days (DD - base 10 C). Embryogenesis within eggs, classified as less than or more than 16 cells and fully differentiated, and numbers of juveniles (second to fourth stage) and preovipositional and mature (egg-laying) adult stages in roots were determined. All stages were present at the onset of dormancy. Juveniles and immature females were not recovered during the dormant period. Mature females and eggs were always present in roots, although the number of mature females generally decreased with time after onset of dormancy. In contrast, in a greenhouse experiment that accumulated comparable DD without the host plant going through dormancy, the number of mature females increased. After bud break, the number of eggs per female increased and all nematode stages were found in host roots. Eggs in all stages of embryogenesis were observed at all times of sampling, indicating that females overwinter and are capable of laying eggs when conditions improve in the spring and need to be considered in nematode management decisions.     

Image Analysis of the Growth of Globodera pallida and Meloidogyne incognita on Transgenic Tomato Roots Expressing Cystatins

An approach based on image analysis that enables rapid collection and analysis of nematode size and shape during growth is reported. This technique has been applied to assess Meloidogyne incognita and Globodera pallida during their development over 35 and 42 days, respectively, on transgenic tomato roots expressing the wild-type rice cystatin Oc-I or an engineered variant, Oc-IAD86. Morphometric values were established that subdivided enlarged saccate females from other life stages. Analysis of this data subset indicates that the size of females and the frequency with which they parasitize roots expressing a cystatin are reduced. Results also demonstrate that cystatins can influence the growth of G. pallida prior to the adult stage. Similar image analysis procedures should be generally applicable to the study of host status or erivironmental factors that influence growth rates of plant-parasitic nematodes.