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.     

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).     

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.     

Optimal Levels of Meloidogyne incognita Inoculum for Infection of Tomato and Peach in Vitro

Penetration of second-stage juveniles (J2) of Meloidogyne incognita into tomato root explants and in vitro propagated peach plantlet roots were compared. Five inoculum levels were used: 25, 50, 75, 100, and 200 J2 for tomato; and 50, 100, 200, 500, and 1,000J2 for peach. The greatest root penetration into tomato was 30% at the 75 J2 level, but the maximum penetration into peach roots was only 8% at the 200 J2 level. The difference (P = 0.05) in penetration of M. incognita at all inoculum levels into these two hosts indicates that penetration versus inoculum density for in vitro studies need to be determined for different plant species.     

Survival of Paecilomyces lilacinus in Selected Carriers and Related Effects on Meloidogyne incognita on Tomato

Laboratory and microplot experiments were conducted to determine the influence of carrier and storage of Paecilomyces lilacinus on its survival and related protection of tomato against Meloidogyne incognita. Spores of P. lilacinus were prepared in five formulations: alginate pellets (pellets), diatomaceous earth granules (granules), wheat grain, soil, and soil plus chitin. Fungal viability was high in wheat and granules, intermediate in pellets, and low in soil and chitin-amended soil stored at 25 ± 2 C. In 1985 P. lilacinus in field microplots resulted in about a 25% increase in tomato yield and 25% gall suppression, compared with nematodes alone. Greatest suppression of egg development occurred in plots treated with P. lilacinus in pellets, wheat grain, and granules. In 1986 carryover protection of tomato against M. incognita resulted in about a threefold increase in tomato fruit yield and 25% suppression of gall development, compared with plants treated with nematodes alone. Higher numbers of fungus-infected egg masses occurred in plots treated with pellets (32%) than in those treated with chitin-amended soil (24%), wheat (16%), granules (12%), or soil (7%). Numbers of fungal colony-forming units per gram of soil in plots treated with pellets were 10-fold greater than initial levels estimated at planting time in 1986.     

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.     

Reproduction of Virulent Isolates of Meloidogyne incognita on Susceptible and Mi-resistant Tomato

The reproductive potential of natural and laboratory-selected Meloidogyne incognita isolates virulent against the tomato Mi resistance gene, all derived from a single egg-mass, were compared when the nematodes were inoculated on susceptible and resistant tomato. Fewer second-stage juveniles (P = 0.01) of the two virulent populations selected under laboratory conditions matured to females on the resistant tomato compared to the susceptible cultivar. In contrast, no differences were found between the number of egg masses produced on the resistant versus the susceptible tomato by the two natural virulent isolates. No clear general trends concerning the fecundity of the females could be inferred from the comparative analysis of the numbers of eggs per egg mass x tomato cultivar combination. These observations suggested that the genetic changes induced under environmentally controlled nematode growth might be different from those occurring in natural Mi-resistance breaking biotypes grown without environmental control.     

Penetration and Postinfection Development of Meloidogyne incognita on Cotton as Affected by Glomus intraradices and Phosphorus

The influence of the vesicular-arbuscular mycorrhizal fungus Glomus intraradices (Gi) and superphosphate (P) on penetration, development, and reproduction of Meloidogyne incognita (Mi) was studied on the Mi-susceptible cotton cultivar Stoneville 213 in an environmental chamber at 28 C. Plants were inoculated with Mi eggs at planting or after 28 days and destructively sampled 7, 14, 21, and 28 days after nematode inoculation. Mi penetration after 7 days was similar in all treatments at either inoculation interval. At 28 days, however, nematode numbers were least in mycorrhizal root systems and greatest in root systems grown with supplemental P. The rate of development of second-stage juveniles to ovipositing females was unaffected by Gi or P when Mi was added at planting, but was delayed in mycorrhizal root systems when Mi was added 28 days after planting. Nematode reproduction was lower in mycorrhizal than in nonmycorrhizal root systems at both Mi inoculation intervals. Nematode reproduction was stimulated by P when Mi was added at planting, but was similar to reproduction in the low P nonmycorrhizal treatment when Mi was added 28 days after planting. Eggs per female were increased by P fertility when Mi was added at planting.     

Pathological Interaction of a Combination of Heterodera schachtii and Meloidogyne hapla on Tomato

Increased culturing of a tomato population of Heterodera schachtii (UT1C) on tomato for 480 days (eight inoculation periods of 60 days each) significantly increased virulence to ''Stone Improved'' tomato. A synergistic relationship existed between Meloidogyne hapla and H. schaehtii on tomato. A combination of H. schachtii (UTIC) and M. hapla significantly reduced tomato root weights by 65, 64, and 61% below root weights of untreated controls, and single inoculations of M. hapla and H. schachtii, respectively. This corresponded to root reductions of 42, 44, and 46% from a combination of H. schachtii (UT1B) and M. hapla. Antagonism existed between H. schachtii and M. hapla with regard to infection courts and feeding sites. The root-knot galling index dropped from 6.0 with a single inoculation of M. hapla to 4.3 and 3.3 with combined inoculations of M. hapla plus UT1B and M. hapla plus UTIC cyst nematode populations. The pathological virulence of H. schachtii to sugarbeet was not lost by extended culturing on tomato; there were no differences in penetration, maturation, and reproduction between sugarbeet populations continually cultured on sugarbeet and the population continually cultured on tomato.     

Impact of Paecilomyces lilacinus Inoculum Level and Application Time on Control of Meloidogyne incognita on Tomato

Microplot experiments were conducted to evaluate the effects of inoculum level and time of application of Paecilomyces lilacinus on the protection of tomato against MeIoidogyne incognita. The best protection against M. incognita was attained with 10 and 20 g of fungus-infested wheat kernels per microplot which resulted in a threefold and fourfold increase in tomato yield, respectively, compared with tomato plants treated with this nematode alone. Greatest protection against this pathogen was attained when P. lilacinus was delivered into soil 10 days before planting and again at planting. Yield was increased twofold compared with yield in nematode-alone plots and plots with M. incognita plus the fungus. Percentages of P. lilacinus-infected egg masses were greatest in plots treated at midseason or at midseason plus an early application, compared with plots treated with the fungus 10 days before planting and (or) at planting time.     

Influence of Glomus intraradices and Soil Phosphorus on Meloidogyne incognita Infecting Cucumis melo

The interaction among Glomus intraradices, Meloidogyne incognita, and cantaloupe was studied at three soil phosphorus (P) levels in a greenhouse. All plants grew poorly in soil not amended with P, regardless of mycorrhizal or nematode status. In soil amended with 50 μg P /g soil, M. incognita suppressed the growth of nonmycorrhizal plants by 84%. In contrast, growth of mycorrhizal plants inoculated with M. incognita was retarded by only 21%. A similar trend occurred in plants grown in soil with 100 μg P /g soil. Mycorrhizal infection had no effect on the degree of root-knot gall formation and did not affect the number of nematode eggs per egg mass. Mineral levels in plant shoots generally declined as soil P levels increased and were not significantly influenced by G. intraradices or M. incognita.     

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.     

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.     

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.     

Varietal Response of Tomato to the Interaction of Salinity and Meloidogyne incognita Infection

Response of tomato (Lycopersicon esculentum) cultivars to a range of conductivity levels was tested in the presence and absence of Meloidogyne incognita. The conductivity levels were produced by appropriate adjustment of a 1:1 solution of sodium chloride and calcium chloride. The growth of M. incognita resistant (''Beefmaster'' and ''Atkinson'') and susceptible (''Hunts 2580'' and ''Ronita'') tomato plants was inversely related to soil salinity between EC_e 0 and 5 mmhos/cm. Nematode inoculation of salt-stressed plants significantly reduced plant height, fresh and dry weight, number of flowers, and fruit weight in most cultivars. In Hunts 2580, flower number and fruit weight increased; apparently flower production shifted from determinate to indeterminate, with negative implications for mechanical harvesting. Nematode reproduction on susceptible varieties also decreased with increase in salinity.     

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 Resistance Characteristics in Tomato Genotypes Developed for Processing

Nine resistant processing tomato (Lycopersicon esculentum) cultivars and advanced lines were compared with four susceptible cultivars in 1,3-dichloropropene-fumigated and nontreated plots on Meloidogyne incognita-infested sites over 3 years. Yield of all resistant genotypes grown in nontreated and nematicide-treated plots did not differ and was greater than yield of susceptible genotypes. M. incognita initial soil population densities caused 39.3-56.5% significant (P = 0.05) yield suppressions of susceptible genotypes. Nematode injury to susceptible plants usually caused both fruit soluble solids content and pH to increase significantly (P = 0.05). Only trace nematode reproduction occurred on resistant genotypes in nontreated plots, whereas large population density increases occurred on susceptible genotypes. Slightly greater nematode reproduction occurred on resistant genotypes at the southern desert location, where soil temperature exceeded 30 C, than at other locations. At two locations resistant MOX 3076 supported greater reproduction than other resistant genotypes.     

Population Development of Meloidogyne incognita on Soybean Defoliated by Pseudoplusia includens

Greenhouse studies examined population densities of Meloidogyne incognita race 4 on soybean (Glycine max ''Davis'') defoliated by larvae of soybean looper (Pseudoplusia indudens (Walker)). Plants were defoliated over a 2-week period beginning 5 weeks after seedlings were transplanted. Four groups of plants were infested with nematodes (5,000 eggs/pot) at 2-week intervals to allow harvesting of plants at 0, 2, 4, and 6 weeks postdefoliation (WPD). Plants in each group were harvested 4 weeks after nematode infestation. Root and nodule weights of defoliated plants were suppressed at 0 WPD, but differences were not detectable at 2, 4, and 6 WPD. Population densities of M. incognita were similar on defoliated and control plants at 0 WPD but were greater on defoliated plants at 4 and 6 WPD. Percentage hatching of eggs produced on the latter plants also was higher. Effects of insect-induced defoliation on development of M. incognita remained detectable even after soybean plant growth apparently returned to normal.     

Effects of Heterodera glycines and Meloidogyne incognita on Early Growth of Soybean

Greenhouse and field microplot studies were conducted to compare soybean shoot and root growth responses to root penetration by Heterodera glycines (Hg) and Meloidogyne incognita (Mi) individually and in combination. Soybean cultivars Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were selected for study. In the greenhouse, pot size and number of plants per pot had no effect on Hg or Mi penetration of Coker 237 roots; root weight was higher in the presence of either nematode species compared with the noninoculated controls. In greenhouse studies using a sand or soil medium, and in field microplot studies, each cultivar was grown with increasing initial population densities (Pi) of Hg or Mi. Interactions between Hg and Mi did not affect early plant growth or number of nematodes penetrating roots. Root penetration was the only response related to Pi. Mi penetration was higher in sand than in soil, and higher in the greenhouse than in the field, whereas Hg penetration was similar under all conditions. At 14 days after planting, more second-stage juveniles were present in roots of susceptible than in roots of resistant plants. Roots continued to lengthen in the greenhouse in the presence of either Mi or Hg regardless of host genotype, but only in the presence of Mi in microplots; otherwise, responses in field and greenhouse studies were similar and differed only in magnitude and variability.     

Interaction between Meloidogyne incognita and Hoplolaimus columbus on Davis Soybean

Greenhouse and laboratory experiments were performed to determine if an interaction exists between Meloidogyne incognita and Hoplolaimus columbus on Davis soybean. Greenhouse tests were performed with three population levels of M. incognita and H. columbus (0, 1,500, 6,000/1.5-liter pot) separately and in all combinations. Dry root weight (DRT) declined nonlinearly and dry shoot weight (DST) declined linearly with respect to increasing initial populations of M. incognita and H. columbus. When the two nematode species were added to the soil together, the amount of DRT and DST suppression by one species was dependent on the initial level of the concomitant species. The final root population of M. incognita or H. columbus declined linearly with increasing initial population density of the concomitant species. H. columbus suppressed M. incognita populations in the soil nonlinearly, but M. incognita had no effect on H. columbus.