Root Penetration by Meloidogyne incognita Juveniles Infected with Bacillus Penetrans

Bacillus penetrans inhibited penetration by Meloidogyne incognita second-stage juveniles (J2) into tomato roots in the laboratory and greenhouse. Spores from this Florida population of B. penetrans attached to J2 of M. javanica, M. incognita, and M. arenaria. A greater proportion of J2 of M. javanica were infected than were J2 of either M. incognita or M. arenaria, and a greater number of spores attached to M. incognita than to M. arenaria.     

Free Amino Acids in Roots of Infected Cotton Seedlings Resistant and Susceptible to Meloidogyne incognita

Quantities of free amino acids in segments of cotton roots resistant and susceptible to Meloidogyne incognita were compared. Following infection, the root-knot susceptible cultivar, M8, had greater percentage increases of certain individual free amino acids than the resistant cultivar, Clevewilt, but the sum total of free amino acids was greatest in the resistant cultivar. More free amino acids were present in infected than in noninfected plants of both cultivars. The overall concn of glycine declined over the I 0-day period following inoculation. The concns of the aromatic amino acids, tyrosine and phenylalanine, varied as functions of infection, cultivar, and time of harvest. Proline in susceptible M8 increased nearly 2000-fold 10 days after infection, when considerable thickening of syncytial walls is occurring.     

Anatomical Response of Grain Sorghum Roots to Meloidogyne incognita acrita

The cotton root-knot nematode, Meloidogyne incognita acrita, reproduced on the roots of grain sorghum, causing syncytia in the cortex or stele of lateral roots. Giant cells developed either singly with few nuclei or in groups with many nuclei. Giant cells that developed in groups appeared the same as those which developed singly. The pericycle and endodermis were interrupted at the site of nematode invasion. Large areas of these tissues were absent for one-third of the circumference of the stele and extended 1.5 mm longitudinally along the root. In the area where pericycle and endodernris were absent, the parenchyma of the cortex extended to the vascular elements, and abnormal xylem surrounding giant cells extended into the region of the cortex. Root-knot galls appeared on sorghum roots as elongate swellings, discrete knots, or swellings with root proliferation. Galls were not observed on brace roots.     

Peroxidase and 6-Phosphogluconate Dehydrogenase in Resistant and Susceptible Cotton Infected by Meloidogyne incognita

Assays of specific activities and electrophoretic separations of multiple forms of 6-phosphogluconate dehydrogenase and peroxidase in cotton resistant and susceptible to Meloidogyne incognita were conducted 6 days after inoculation. Specific activities were greater in infected than in uninfected roots and also were greater in the resistant cultivar, ''Clevewilt 6-3-5,'' than in the susceptible culti.var, ''M8.'' In uninfected roots, peroxidase activity was greater in Clevewilt roots than in M8 roots, but activity of 6-phosphoglueonate dehydrogenase was the same. Multiple forms of peroxidase and 6-phosphogluconate dehydrogenase were separated and resolved by polyacrylamide gel electrophoresis. These experiments demonstrated the occurrence of altered metabolism upon infection and differences in enzyme activity between resistant and susceptible cultivars.     

Isolation of Subcellular Granules from Second-Stage Juveniles of Meloidogyne incognita

Subcellular granules from the second-stage (preparasitic) juveniles of root-knot nematode Meloidogyne incognita were isolated by isopycnic centrifugation on Percoll. The granules had an apparent density of 1.13 g/cm³. The relative specific activity of acid phosphatase in the granule extract was 8.4. Acid phosphatase activity was also detected histochemically in the subventral gland granules. Electron microscopy and malate dehydrogenase activity indicated that contamination of granules by mitochondria was negligible. Electrophoresis of the granule extract in the presence of sodium dodecyl sulfate showed 15-20 major protein bands.     

Influence of Soil Temperature on Meloidogyne incognita Resistant and Susceptible Cotton,Gossypium hirsutum

The degree of resistance by a cotton plant to Meloidogyne incognita is affected by soil temperature, particularly in moderately resistant cultivars, The total number of nematodes in the resistant and moderately resistant rools at 35 C was equal to, or greater than, the number in susceptible roots at 20, 25, or 30 C. A shift in numbers to developing and egg-bearing forms of nematodes in the susceptible cultivar as tentperature increased indicates development was affected by temperature rather than by genetic resistance mechanisms. However, the nematode resistant cultivar did not support maturation of nematodes until a soil tempurature of 35 C was attained. This indicated that resistance mechanisms are partially repressed at 35 C and differences in nematode development cannot be explained in terms of accumulated heat units. The moderately resistant cultivar was significantly more sensitive to the effects of high temperature than was the resistant cultivar.     

Response of Soybean Cultivars to Nematicidal Treatments of Soil Infested with Meloidogyne incognita

A comparison of untreated and nematicide-treated soil for soybean production revealed that Meloidogyne incognita hastened crop maturity and reduced plant ht, seed wt, and yield. Reductions of yield varied from 32-90% depending on cultivar susceptibility. DBCP was more consistent in increasing crop performance than organo-phosphale or oxime carbamate nematicides. Greatest yield increases were produced by nematicidal treatment of soils planted to soybean cultivars with the lowest susceptibility.     

Evaluation of NemX,a New Cultivar of Cotton with High Resistance to Meloidogyne incognita

The level of resistance to root-knot nematode, Meloidogyne incognita, in NemX, a new cultivar of the Acala-type upland cotton, was evaluated in relation to four resistant breeding lines (N6072, N8577, N901, and N903) and four susceptible cultivars (Maxxa, SJ2, Royale, and Prema). In growth pouch tests, an average of only 4 nematode egg masses was produced on roots of NemX or the resistant lines, compared to a significantly higher average of 21 on the susceptible cultivars. In pot tests, the nematode reproduction factor (RF = Pf/Pi) in NemX and the resistant lines averaged 0.7, compared to a significantly higher average of 10 on the susceptible cultivars. Root galling in NemX or other resistant cotton averaged 15%, compared to 74% on the susceptible cultivars, in either pot or field tests. In plots with low levels of nematode infestation (Pi ≤ 150 second-stage juveniles [J2]/500 g soil), lint yield of NemX averaged 1,370 kg/ha and was less than the yield of susceptible Maxxa (1,450 k g /h a ). However, in plots with medium or high levels of nematode infestation (Pi = 151-300 or >300 J2/500 g soil, respectively), yields of NemX decreased only slightly and averaged 1,300 or 1,050 kg/ha, respectively, whereas yields of Maxxa were severely reduced to 590 or 503 kg/ha, respectively. Fusarium wih symptoms were observed on both NemX and Maxxa, and percent occurrence increased with increasing preplant nematode density. In plots with the highest nematode densities, 22% of NemX and 65% of Maxxa plants were wilted. NemX was highly effective against five M. incognita isolates and moderately effective against a sixth isolate that had been exposed to resistant cotton over several seasons. These results showed that NemX is as resistant to M. incognita as the four breeding lines, and much more resistant than the tested susceptible cultivars of cotton.     

Interaction of Vesicular-arbuscular Mycorrhizal Fungi and Phosphorus with Meloidogyne incognita on Tomato

The influence of two vesicular-arbuscular mycorrhizal fungi and phosphorus (P) nutrition on penetration, development, and reproduction by Meloidogyne incognita on Walter tomato was studied in the greenhouse. Inoculation with either Gigaspora margarita or Glomus mosseae 2 wk prior to nematode inoculation did not alter infection by M. incognita compared with nonmycorrhizal plants, regardless of soil P level (either 3 μg [low P] or 30 μg [high P] available P/g soil). At a given soil P level, nematode penetration and reproduction did not differ in mycorrhizal and nonmycorrhizal plants. However, plants grown in high P soil had greater root weights, increased nematode penetration and egg production per plant, and decreased colonization by mycorrhizal fungi, compared with plants grown in low P soil. The number of eggs per female nematode on mycorrhizal and nonmycorrhizal plants was not influenced by P treatment. Tomato plants with split root systems grown in double-compartment containers which had either low P soil in both sides or high P in one side and low P in the other, were inoculated at transplanting with G. margarita and 2 wk later one-half of the split root system of each plant was inoculated with M. incognita larvae. Although the mycoorhizal fungus increased the inorganic P content of the root to a level comparable to that in plants grown in high P soil, nematode penetration and reproduction were not altered. In a third series of experiments, the rate of nematode development was not influenced by either the presence of G. margarita or high soil P, compared with control plants grown in low P soil. These data indicate that supplemental P (30 μ/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.     

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.     

Dynamics of Concomitant Populations of Hoplolaimus columbus,Scutellonema brachyurum,and Meloidogyne incognita on Cotton

Cotton seedlings grown in a greenhouse and a growth chamber were inoculated with Scutellonema brachyurum, Hoplolaimus columbus, and Meloidogyne incognita, singly and in all possible combinations, at two initial population (Pi) levels (100 and 300/100 cm³). S. brachyurum alone was not pathogenic to cotton at these population levels. It fed primarily as an ectoparasite but matured and reproduced within the root when it penetrated. Populations of S. brachyurum increased in the presence of H. columbus but were suppressed by M. incognita. H. columbus suppressed dry shoot weights of cotton (P = 0.05) at a Pi of 300/100 cm³ soil. Simultaneous inoculation of H. columbus with either M. incognita or S. brachyurum increased H. columbus populations over treatments with H. columbus alone, both at 60 and 90 d after inoculation. M. incognita suppressed cotton shoot weights significantly (P = 0.05) at both Pi levels. Inoculation with S. brachyurum increased M. incognita populations 60 d after inoculation, while H. columbus suppressed populations of M. incognita. Most larvae of M. incognita did not develop to maturity in the presence of H. columbus. Giant cells aborted and were necrotic 20-25 d after inoculation. Since M. incognita and H. columbus feed on different tissues, the inhibition of M. incognita may have resulted from a physiological effect of H. columbus on the host.     

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).     

Histological Responses of Four Leguminous Crops Infected with Meloidogyne incognita

Histological responses to Meloidogyne incognita infection in Rhizobium nodules of clover, horsebean, lupine, and pea were investigated. The formation of giant cells in vascular bundles of nodules and roots, and the basal connection of the nodule, were usually associated with abnormal xylem and/or deformed xylem strands. However, giant cells did not disturb or prevent the development of nodular tissues. Areas in which galls formed, wall thickness of giant cells, and number of giant cells around the nematode head varied with plant species. Ranking by gall size and giant-cell wall thickness was horsebean > lupine and pea > clover. The multinucleate condition in giant cells resulted from repeated mitoses without subsequent cytokinesis. The resulting nuclei agglomerated in irregularly shaped masses in some giant cells.     

Production and Partial Characterization of Stylet Exudate from Adult Females of Meloidogyne incognita

Adult females of Meloidogyne incognita were excised from tomato roots and incubated in 0.04 M phosphate buffered saline, pH 7.4 for 18-72 hours to allow accumulation of stylet exudate. Twenty-four percent of the females produced exudate during the initial 18-hour incubation period; 70% of those females producing exudate initially produced additional exudate during the subsequent 54-hour incubation period. Analysis of exudate by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of at least nine major protein bands. Differential staining with silver and Coomassie Brilliant Blue G-250 stains indicated that three of the bands were glycoproteins. Upon acid hydrolysis, 14 amino acids were detected in the stylet exudate. The basic amino acids lysine, histidine, and arginine comprised 21.8% of the total amino acids detected. No peroxidase activity was detected in the stylet exudates. Data presented extend and generally confirm prior work on the chemical composition of stylet exudate.     

Behavior of Tethered Meloidogyne incognita

The tethered-nematode technique was adapted for use with second-stage juveniles of Meloidogyne incognita. The data demonstrate that M. incognita exhibits the same patterns of behavior as adults of the free-living nematode, Caenorhabditis elegans. The principal differences are that M. incognita is slower and less regular in its behavior than C. elegans. The frequency of normal waves is about 0.2 Hz; that of reversal waves is about 0.06 Hz. Reversal bouts last about 1 minute. In response to a change in NaCl concentration, M. incognita modulates the probability of initiating a reversal bout in the same manner as C. elegans except that it responds more slowly and is repelled instead of attracted.     

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.     

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.     

Meloidogyne incognita Surface Antigen Epitopes in Infected Arabidopsis Roots

Surface-coat epitopes of Meloidogyne incognita were detected in root tissues of Arabidopsis thaliana during migration and feeding site formation. A whole-mount root technique was used for immunolocalization of surface coat epitopes in A. thaliana, with the aid of a monoclonal antibody raised specifically against the outer surface of infective juveniles of M. incognita. The antibody, which was Meloidogyne-specific, recognized a fucosyl-bearing glycoprotein in the surface coat. During migration in host tissues the surface coat was shed, initially accumulating in the intercellular spaces next to the juvenile and later at cell junctions farther from the nematode. Upon induction of giant cell formation, the antibody bound to proximally located companion cells and sieve elements of the phloem.