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).     

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.     

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.     

Galling and Yields of Soybean Cultivars Grown in Meloidogyne arenana-Infested Soil

Field trials with 39 soybean cultivars and five breeding lines from public and private sources were conducted from 1982 through 1985 at sites infested with Meloidogyne arenaria. Nematode population densities and root-knot galling were measured for each soybean entry. All were efficient hosts for the nematode, and average juvenile numbers in the soil increased 5-50 × from planting to harvest. Differences (P < 0.05) in galling were found among entries in each year. Centennial, Cobb, Coker 368, Hutton, and Jeff cultivars, recognized for their resistance to M. incognita, were severely galled and yielded poorly. Bedford, Forrest, A7372, Bragg, Braxton, Gordon, and Kirby, also recognized for their resistance to M. incognita, were among the least galled cultivars. Yields of all entries, however, were too low to justify their planting in sites heavily infested with M. arenaria.     

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.     

Effect of Plant Age on Resistance of Alfalfa to Meloidogyne hapla

Meloidogyne hapla-resistant plants grown from cuttings and inoculated with M. hapla larvae were free of galls. However, 35 to 48% of the seedling intercross progeny of resistant genotypes that were inoculated in the germinated seed stage were galled. There was an inverse relationship between the age of plants grown from seed and the percentage of plants galled by M. hapla; the older the plants at inoculation, the greater the percentage of gall-free plants. The per cent of galled plants was significantly reduced when galled roots were removed and plants reinoculated. Reproduction of M. hapla on galled progeny of resistant plants was significantly less than that on susceptible plants. There were no differences in nematode reproduction on galled progeny of resistant plants, regardless of age at time of inoculation. An in,ease in inoculum levels from 100 to 10,000 M. hapla larvae did not affect resistance or susceptility. There was a direct correlation between galling of inoculated seedlings of resistant progeny and temperature; inoculated 8-week-old cuttings of resistant plants were galled only at 32 C.     

Relationship of Resistance to Meloidogyne chitwoodi (race 2) and M. hapla in Alfalfa

In the Pacific Northwest, alfalfa (Medicago sativa) is host to two species of root-knot nematodes, including race 2 of the Columbia root-knot nematode (Meloidogyne chitwoodi) and the northern root-knot nematode (Meloidogyne hapla). In addition to the damage caused to alfalfa itself by M. hapla, alfalfa’s host status to both species leaves large numbers of nematodes available to damage rotation crops, of which potato is the most important. A nematode-resistant alfalfa germplasm release, W12SR2W1, was challenged with both nematode species, to determine the correlation, if any, of resistance to nematode reproduction. Thirty genotypes were screened in replicated tests with M. chitwoodi race 2 or M. hapla, and the reproductive factor (RF) was calculated. The distribution of natural log-transformed RF values was skewed for both nematode species, but more particularly for M. chitwoodi race 2, where more than half the genotypes screened were non-hosts. Approximately 30 percent of genotypes were non-hosts or very poor hosts of M. hapla, but RF values for M. hapla on susceptible genotypes were generally much higher than RF values for genotypes susceptible to M. chitwoodi race 2. The Spearman rank correlation was positive (0.52) and significant (p-value = 0.003), indicating there is some relationship between resistance to these two species of root-knot nematode in alfalfa. However the relationship is not strong enough to suggest genetic loci for resistance are identical, or closely linked. Breeding for resistance or immunity will require screening with each species separately, or with different DNA markers if marker-assisted breeding is pursued. A number of genotypes were identified which are non-hosts to both species. These plants will be intercrossed to develop a non-host germplasm.     

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.     

Induced Resistance to Meloidogyne hapla by other Meloidogyne species on Tomato and Pyrethrum Plants

Advance inoculation of the tomato cv. Celebrity or the pyrethrum clone 223 with host-incompatible Meloidogyne incognita or M. javanica elicited induced resistance to host-compatible M. hapla in pot and field experiments. Induced resistance increased with the length of the time between inoculations and with the population density of the induction inoculum. Optimum interval before challenge inoculation, or population density of inoculum for inducing resistance, was 10 days, or 5,000 infective nematodes per 500-cm³ pot. The induced resistance suppressed population increase of M. hapla by 84% on potted tomato, 72% on potted pyrethrum, and 55% on field-grown pyrethrum seedlings, relative to unprotected treatments. Pyrethrum seedlings inoculated with M. javanica 10 days before infection with M. hapla were not stunted, whereas those that did not receive the advance inoculum were stunted 33% in pots and 36% in field plots. The results indicated that advance infection of plants with incompatible or mildly virulent nematode species induced resistance to normally compatible nematodes and that the induced resistance response may have potential as a biological control method for plant nematodes.     

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.     

Effects of DCPA,EPTC, and Chlorpropham on Pathogenicity of Meloidogyne hapla to Alfalfa

Treatments wilh the herbicides chlorpropham (isopropyl m-chlorocarbinilate), DCPA (dimethyl tetrachloroterephthalate), and El''TC (S-ethyl dipropylthiocarbamate), alone or in combination with Meloidogyne hapla Chilwood, significantly reduced the growth of both nemalode-resistant ''Nev Syn XX'' atttt susceptible ''Ranger" alfalfa (Medicago saliva L.) seedlings. M. hapla infection of both alfalfas was reduced by all herbicides because of fewer available infective courts in the treated plants. EPTC, however, reduced resistance to M. hapla, as indicated by increased galling of ''Nev Syn XX'' plants.     

Influence of Meloidogyne hapla on Alfalfa Yield and Host Population Dynamics

Self-thinning in alfalfa, a dynamic process involving the progressive elimination of the weakest plants, was enhanced by Meloidogyne hapla. Alfalfa stand densities decreased exponentially with time and were reduced 62% (P = 0.05) in the presence of M. hapla. As stand densities decreased over time, mean plant weights increased at a rate 2.59 times faster in the absence of M. hapla. In a stepwise multiple regression analysis, 65% of the total variation in yield could be explained by changes in stand density and 85% by average weight of individual stems. Alfalfa yields were suppressed (P = 0.05) by M. hapla, with suppression generally increasing with time and as the nematode population density increased. Yield suppression was attributable primarily to the decline in plant numbers and to suppression in individual plant weights.     

Effects of Meloidogyne spp. and Rhizoctonia solani on the Growth of Grapevine Rootings

A disease complex involving Meloidogyne incognita and Rhizoctonia solani was associated with stunting of grapevines in a field nursery. Nematode reproduction was occurring on both susceptible and resistant cultivars, and pot experiments were conducted to determine the virulence of this M. incognita population, and of M. javanica and M. hapla populations, to V. vinifera cv. Colombard (susceptible) and to V. champinii cv. Ramsey (regarded locally as highly resistant). The virulence of R. solani isolates obtained from roots of diseased grapevines also was determined both alone and in combination with M. incognita. Ramsey was susceptible to M. incognita (reproduction ratio 9.8 to 18.4 in a shadehouse and heated glasshouse, respectively) but was resistant to M. javanica and M. hapla. Colombard was susceptible to M. incognita (reproduction ratio 24.3 and 41.3, respectively) and M. javanica. Shoot growth was suppressed (by 35%) by M. incognita and, to a lesser extent, by M. hapla. Colombard roots were more severely galled than Ramsey roots by all three species, and nematode reproduction was higher on Colombard. Isolates of R. solani assigned to putative anastomosis groups 2-1 and 4, and an unidentified isolate, colonized and induced rotting of grapevine roots. Ramsey was more susceptible to root rotting than Colombard. Shoot growth was inhibited by up to 15% by several AG 4 isolates and by 20% by the AG 2-1 isolate. AG 4 isolates varied in their virulence. Root rotting was higher when grapevines were inoculated with both M. incognita and R. solani and was highest when nematode inoculation preceded the fungus. Shoot weights were lower when vines were inoculated with the nematode 13 days before the fungus compared with inoculation with both the nematode and the fungus on the same day. It was concluded that both the M. incognita population and some R. solani isolates were virulent against both Colombard and Ramsey, and that measures to prevent spread in nursery stock were therefore important.     

Evaluation of 31 Potential Biofumigant Brassicaceous Plants as Hosts for Three Meloiodogyne Species

Brassicaceous cover crops can be used for biofumigation after soil incorporation of the mowed crop. This strategy can be used to manage root-knot nematodes (Meloidogyne spp.), but the fact that many of these crops are host to root-knot nematodes can result in an undesired nematode population increase during the cultivation of the cover crop. To avoid this, cover crop cultivars that are poor or nonhosts should be selected. In this study, the host status of 31 plants in the family Brassicaceae for the three root-knot nematode species M. incognita, M. javanica, and M. hapla were evaluated, and compared with a susceptible tomato host in repeated greenhouse pot trials. The results showed that M. incognita and M. javanica responded in a similar fashion to the different cover cultivars. Indian mustard (Brassica juncea) and turnip (B. rapa) were generally good hosts, whereas most oil radish cultivars (Raphanus. sativus ssp. oleiferus) were poor hosts. However, some oil radish cultivars were among the best hosts for M. hapla. The arugula (Eruca sativa) cultivar Nemat was a poor host for all three nematode species tested. This study provides important information for chosing a cover crop with the purpose of managing root-knot nematodes.     

Interaction of Ditylenchus dipsaci and Meloidogyne hapla on Resistant and Susceptible Plant Species

Numbers ofDitylenchus dipsaci or Meloidogyne hapla invading Ranger alfalfa, Tender crop bean, Stone Improved tomato, AH-14 sugarbeet, Yellow sweet clover, and Wasatch wheat from single inoculations were not significantly different from numbers by invasion of combined inoculations. D. dipsaci was recovered only from shoot and M. hapla only from root tissue. Combined inoculations did not affect reproduction of either D. dipsaci or M. hapla. D. dipsaci suppressed shoot growth of all species at 15-30 C, and M. hapla suppressed shoot growth of tomato, sugarbeet, and sweet clover at 20, 25, and 30 C. There was a positive correlation (P < 0.05) between shoot and root growth suppression by D. dipsaci on all cultivars except wheat at 20 C and tomato at 30 C. M. hapla suppressed (P < 0.05) root growth of sugarbeet at 20-50 C and wheat at 30 C. Growth suppression was synergistic in combined inoculations of sweet clover shoot growth at 15 C and root growth at 20-30 C, wheat root growth at 15 and 20 C, and tomato root growth at 15-30 C (P < 0.05) D. dipsaci invasions caused mortality of alfalfa and sweet clover at 15-30 C and sugarbeet at 20-30 C. Mortality rates of alfalfa and sweet clover increased synergistically (P < 0.05) from combined inoculations.     

Predisposition of Broadleaf Tobacco to Fusarium Wilt by Early Infection with Globodera tabacum tabacum or Meloidogyne hapla

In greenhouse experiments, broadleaf tobacco plants were inoculated with tobacco cyst (Globodera tabacum tabacum) or root-knot (Meloidogyne hapla) nematodes 3, 2, or 1 week before or at the same time as Fusarium oxysporum. Plants infected with nematodes prior to fungal inoculation had greater Fusarium wilt incidence and severity than those simultaneously inoculated. G. t. tabacum increased wilt incidence and severity more than did M. hapla. Mechanical root wounding within 1 week of F. oxysporum inoculation increased wilt severity. In field experiments, early-season G. t. tabacum control by preplant soil application of oxamyl indirectly limited the incidence and severity of wilt. Wilt incidence was 48%, 23%, and 8% in 1989 and 64%, 60%, and 19% in 1990 for 0.0, 2.2, and 6.7 kg oxamyl/ha, respectively. Early infection of tobacco by G. t. tabacum predisposed broadleaf tobacco to wilt by F. oxysporum.     

Comparison of Methods for Assessing Resistance to Meloidogyne arenaria in Peanut

Use of resistant cultivars is a desirable approach to manage the peanut root-knot nematode (Meloidogyne arenaria). To incorporate resistance into commercially acceptable cultivars requires reliable, efficient screening methods. To optimize the resistance screening protocol, a series of greenhouse tests were done using seven genotypes with three levels of resistance to M. arenaria. The three resistance levels could be separated based on gall indices as early as two weeks after inoculation (WAI) using 8,000 eggs of M. arenaria per plant, while four or more weeks were needed when 1,000–6,000 eggs/plant were used. High inoculum densities (over 8,000 eggs/plant) were needed to separate the three resistance levels based on eggs per gram of root within eight WAI. A gall index based on percentage of galled roots could separate the three resistance levels at lower inoculum levels and earlier harvest dates than other assessment methods. The use of eggs vs. second-stage juveniles (J2) as inoculum provided similar results; however, it took three to five more days to collect J2 than to collect eggs from roots. Plant age affected gall index and nematode reproduction on peanut, especially on the susceptible genotypes AT201 and D098. The genotypes were separated into their correct resistance classes when inoculated 10 to 30 days after planting, but were not separated correctly when inoculated on day 40.     

Interactions between Meloidogyne incognita,M. hapla,and Pratylenchus brachyurus in Tobacco

In a greenhouse pot experiment on the pathogenicity and interactions of Meloidogyne incognita, M. hapla and Pratylenchus brachyurus on four cultivars o f tobacco the cultivars ''Hicks'' and ''NC 2326'' were susceptible to each nematode and "NC 95'' and ''NC 2512'' resistant only to M. incognita.Mean heights of susceptible plants were depressed but fresh weight of tops did not differ significantly. Meloidogyne spp. increased fresh weight of susceptible (but not the resistant) roots.Reproduction of M. incognita was decreased in the presence of P. brachyurus in one case. M. hapla reproduction was less with either of the other nematodes in five out of eight cases. In 12 combinations involving P. brachyurus, reproduction of this species was depressed in seven, not affected in four and increased in one.Mechanisms involved in associative interactions were not identified but appeared to be indirect and to involve individual host-nematode responses.     

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.     

Efficacy of Oxamyl Coated on Alfalfa Seed with a Polymer Sticker in Pratylenchus and Meloidogyne Infested Soils

A polymer sticker was used as a coating in which oxamyl was applied to seeds of alfalfa cultivar Saranac for the control of Pratylenchus penetrans and Meloidogyne hapla. The sticker, diluted 1:1 (sticker:water) to 1:5, delayed seedling emergence during the first 4 days after planting. By day 13, however, emergence from all sticker treatments was comparable to the control. Shoot growth of seedlings at day 21 was less than that of the control only from seeds coated with a 1:1 dilution; root growth and nodulation were not affected. Sticker-coated seeds absorbed 30-58% as much water in 3.5 hours as was absorbed by uncoated seeds. Oxamyl concentrations of 40-160 mg/ml in a 1:5 sticker : water mixture had no adverse affect on seedling emergence, growth, and nodulation over 3 weeks. Oxamyl at 160 mg/ml was more effective against P. penetrans than M. hapla. Growth of alfalfa in P. penetrans-infested soil was greater than that of the control in each sampling for 11 weeks. The reduction of number of P. penetrans in soil and roots moderated slowly over 11 weeks from 90% to 60%. Shoot and root growth of alfalfa from oxamyl-coated seed in M. hapla-infested soil were greater than those of the control for 7 and 11 weeks, respectively. The reduction in the number of M. hapla in the soil and roots changed from 80% at 7 weeks to 15% at 11 weeks.