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.     

Incidence and Distinguishing Characteristics of Meloidogyne chitwoodi and M. hapla in Potato from the Northwestern United States

From September 1980 to June 1981, a survey was conducted in the major potato growing regions of northern California, Idaho, Nevada, Oregon. and Washington to determine the distribution of Meloidogyne chitwoodi and other Meloidogyne spp. Meloidogyne chitwoodi and M. hapla were the only root-knot nematode species detected parasitizing potato in all the states surveyed. Meloidogyne chitwoodi occurred alone in 83% of the samples and M. hapla in 11%, with 6% of all samples containing both species. The greater incidence of M. chitwoodi, as compared to M. hapla, may be due to the cool growing season encountered in 1980 (which favored M. chitwoodi but not M. hapla) and to the increased acreage of small grains (which are good hosts for M. chitwoodi but not M. hapla) planted in rotation with potato. Differentiation between these two species can be determined by a differential host test, perineal patterns of mature females, and shape of the tail tip amt of the tail hypodermal terminus of L₂ juveniles.     

Effect of the Mi Gene in Tomato on Reproductive Factors of Meloidogyne chitwoodi and M. hapla

The effect of the Mi gene on the reproductive factor of Meloidogyne chitwoodi and M. hapla, major nematode pests of potato, was measured on nearly isogenic tomato lines differing in presence or absence of the Mi gene. The Mi allele controlled resistance to reproduction of race 1 of M. chitwoodi and to one of two isolates of race 2. No resistance to race 3 of M. chitwoodi or to M. hapla was found. Variability in response to isolates of race 2 may reflect diversity of virulence genotypes heretofore undetected. Resistance to race 1 of M. chitwoodi could be useful in potato if the Mi gene were functional following transferral by gene insertion technology into potato. Since the Mi gene is not superior to RMc₁ derived from Solarium bulbocastanum, the transferral by protoplast fusion appears to offer no advantage.     

Differential Response of Thor Alfalfa to Meloidogyne chitwoodi Races and M. hapla

Second-stage juveniles (J2) of races 1 and 2 of Meloidogyne chiiwoodi and M. hapla readily penetrated roots of Thor alfalfa and Columbian tomato seedlings; however, few individuals of M. chitwoodi race 1 were able to establish feeding sites and mature on alfalfa. Histopathological studies indicate that J2 of race 1 either failed to initiate feeding sites or they caused cell enlargement without typical cell wall thickening. The protoplasm of these cells coagulated, and juveniles of race 1 did not develop beyond the swollen J2 stage. A few females of race 1 fed on small giant cells and deposited a few eggs at least 20 and 30 days later than M. chitwoodi race 2 and M. hapla, respectively. Failure of race 1 to establish feeding sites was related to egression of J2 from the roots. The M. chitwoodi race 1 J2 egression from alfalfa roots was higher than egression of race 2 and M. hapla. Egression of J2 of M. chitwoodi races 1 and 2 from tomato roots was similar and higher than that of M. hapla. Thus egression plays an important role in the host-parasite relationship of M. chitwoodi and alfalfa.     

A PCR Assay to Identify and Distinguish Single Juveniles of Meloidogyne hapla and M. chitwoodi

Random amplified polymorphic DNA (RAPD) bands that distinguish Meloidogyne hapla and M. chitwoodi from each other, and from other root-knot nematode species, were identified using a series of random octamer primers. The species-specific amplified DNA fragments were cloned and sequenced, and then the sequences were used to design 20-mer primer pairs that specifically amplified a DNA fragment from each species. Using the primer pairs, successful amplifications from single juveniles were readily attained. A mixture of four primers in a single PCR reaction mixture was shown to identify single juveniles of M. hapla and M. chitwoodi. To confirm specificity, the primers were used to amplify DNA from several isolates of M. hapla that originated from different crops and locations in North America and also from isolates of M. chitwoodi that differed in host range. In characterizing the M. hapla isolates, it was noted that there was a mitochondrial DNA polymorphism among isolates for cleavage by the restriction endonuclease DraI.     

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.     

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.     

Importance of Temperature in the Pathology of Meloidogyne hapla and M. chitwoodi on Legumes

Effects of temperatures on the host-parasite relationships were studied for three legume species and four populations of root-knot nematodes from the western United States. The nematode populations were Meloidogyne hapla from California (MHCA), Utah (MHUT), and Wyoming (MHWY), and a population of M. chitwoodi from Utah (MCUT). The legumes were milkvetch (Astragalus cicer), alfalfa (Medicago sativa), and yellow sweet clover (Melilotus officinalis). All milkvetch plants survived inoculation with all nematode populations, while alfalfa and yellow sweet clover were more susceptible. On yellow sweet clover, MHCA was most pathogenic at 30 °C based on suppression of shoot growth while MHUT, MHWY, and MCUT were most pathogenic at 25 °C. All nematode populations suppressed growth of yellow sweet clover more than growth of milkvetch and alfalfa. The reproductive factor (Rf = final nematode population/initial nematode population) of MHCA was positively correlated (r = 0.83) with temperature between 15 °C and 30 °C. The greatest Rf occurred on alfalfa inoculated with MHCA at 30 °C. The Rf of MHUT, MHWY, and MCUT were positively correlated (r= 0.76, r= 0.78, and r= 0.73, respectively) with temperature between 15 °C and 25 °C. The Rf values of MHUT and MHWY were similar on all species and exceeded the Rf of MCUT at all temperatures (P < 0.05).     

Reproduction of Meloidogyne spp. on Resistant Peanut Genotypes from Three Breeding Programs

Three described species of root-knot nematode parasitize peanut (Arachis hypogaea): Meloidogyne arenaria race 1 (Ma), M. hapla (Mh), and M. javanica (Mj). Peanut cultivars with broad resistance to Meloidogyne spp. will be useful regardless of the species present in the field. The objective of this study was to determine whether peanut genotypes with resistance to M. arenaria originating from three different breeding programs were also resistant to M. hapla and M. javanica. The experiment used a factorial arrangement (completely randomized) with peanut genotype and nematode population as the factors. The five peanut genotypes were ''COAN'' and AT 0812 (highly resistant to Ma), C209-6-13 (moderately resistant to Ma), and ''Southern Runner'' and ''Georgia Green'' (susceptible to Ma). The four nematode populations were two isolates of Ma (Gibbs and Gop) and one isolate each of Mh and Mj. On COAN or AT 0812, both Ma and Mj produced <10% of the eggs produced on Georgia Green. On the peanut genotype C209-6-13, Ma and Mj produced about 50% of the eggs produced on Georgia Green. None of the resistant genotypes exhibited a high level of resistance to Mh. The lack of resistance to Mh in any cultivars or advanced germplasm is a concern because the identity of a Meloidogyne sp. in a particular peanut field is generally not known. Breeding efforts should focus on moving genes for resistance to M. hapla into advanced peanut germplasm, and combining genes for resistance to the major Meloidogyne spp. in a single cultivar.     

Size Differences Among Root-knot Nematodes on Resistant and Susceptible Alyceclover Genotypes

The influence of plant resistance on the size of individual root-knot nematodes was determined in greenhouse experiments. Five genotypes of alyceclover were inoculated with second-stage juveniles of Meloidogyne incognita race 3 or M. arenaria race 1. Plants were harvested at selected intervals and stained for detection of the nematodes, which were dissected from the roots. Length, width, and sagittal-sectional area of each animal were measured using an image-analysis system, and areas of nematodes in all stages were compared at different times and across alyceclover lines. Nematodes feeding on roots of resistant lines were consistently smaller than those on susceptible plants, with significant differences in growth detected after the final molt. Similar results were observed with both nematode species.     

Host-Parasite Relationships of Meloidogyne trifoliophila Isolates from New Zealand

Root-infecting nematodes are commonly found on white clover in New Zealand pasture where they reduce yield, nitrogen fixation, and persistence. The dominant root-knot nematode on white clover in New Zealand is confirmed in this study as Meloidogyne trifoliophila by isozyme phenotype comparison with the type population from Tennessee. Results from a host differential test differed in the host ranges of M. trifoliophila and M. hapla from New Zealand locations, with M. trifoliophila failing to reproduce on the standard host plants of the test. The size and character of white clover root galls differ between species as M. trifoliophila galls are large, elongate, and smooth compared to the M. hapla galls, which are small, round, inconspicuous, and generally have adventitious, lateral roots. Culture and identification of root-knot nematode populations from sites in the North Island of New Zealand showed that M. trifoliophila is more widespread and abundant than M. hapla. Similar differential resistant and susceptible galling responses among half-sib families of white clover from a breeding program indicated that all M. trifoliophila populations tested were of the same pathotype. This resistant material was not effective in reducing reproduction of M. hapla. Meloidogyne trifoliophila did not develop to maturity on six grasses tested, but galls were formed on some species.     

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.     

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.     

Description and SEM Observations of Meloidogyne chitwoodi n. sp. (Meloidogynidae), a Root-knot Nematode on Potato in the Pacific Northwest

Meloidogyne chitwoodi n. sp. is described and illustrated from potato (Solanum tuberosum) originally collected from Quincy, Washington, USA. This new species resembles M. hapla, but its perineal pattern is basically round to oval with distinctive and broken, curled, or twisted striae around and above the anal area. The vulva is in a sunken area devoid of striae. Vesicles or vesicle-like structures are present in the median bulb of females. The larva tail, being short and blunt with a hyaline tail terminal having little or no taper to its rounded terminus, is distinctively different from M. hapla. SEM observations revealed the nature of the perineal pattern and details of the head of larvae and males, and showed the spicules to have dentate tips ventrally. Hosts for M. chitwoodi n. sp. include potato, tomato, corn, and wheat but not strawberry, pepper, or peanut. The latter three crops are excellent hosts for M. hapla. The known distribntion of this new root-knot species presently involves certain areas of Idaho, Washington, and Oregon. The common name "Columbia root-knot nematode" is proposed for M. chitwoodi n. sp.     

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.     

Identification of Sources of Resistance to Four Species of Root-knot Nematodes in Tobacco

Resistance to the southern root-knot nematode, Meloidogyne incognita races 1 and 3, has been identified, incorporated, and deployed into commercial cultivars of tobacco, Nicotiana tabacum. Cultivars with resistance to other economically important root-knot nematode species attacking tobacco, M. arenaria, M. hapla, M. javanica, and other host-specific races of M. incognita, are not available in the United States. Twenty-eight tobacco genotypes of diverse origin and two standard cultivars, NC 2326 (susceptible) and Speight G 28 (resistant to M. incognita races 1 and 3), were screened for resistance to eight root-knot nematode populations of North Carolina origin. Based on root gall indices at 8 to 12 weeks after inoculation, all genotypes except NC 2326 and Okinawa were resistant to M. arenaria race 1, and races 1 and 3 of M. incognita. Except for slight root galling, genotypes resistant to M. arenaria race 1 responded similarly to races 1 and 3 of M. incognita. All genotypes except NC 2326, Okinawa, and Speight G 28 showed resistance to M. javanica. Okinawa, while supporting lower reproduction of M. javanica than NC 2326, was rated as moderately susceptible. Tobacco breeding lines 81-R-617A, 81-RL- 2K, SA 1213, SA 1214, SA 1223, and SA 1224 were resistant to M. arenaria race 2, and thus may be used as sources of resistance to this pathogen. No resistance to M. hapla and only moderate resistance to races 2 and 4 of M. incognita were found in any of the tobacco genotypes. Under natural field infestations of M. arenaria race 2, nematode development on resistant tobacco breeding lines 81-RL-2K, SA 1214, and SA 1215 was similar to a susceptible cultivar with some nematicide treatments; however, quantity and quality of yield were inferior compared to K 326 plus nematicides.     

Suppression of Root-knot Nematode Populations with Selected Rapeseed Cultivars as Green Manure

Meloidogyne chitwoodi races 1 and 2 and M. hapla reproduced on 12 cultivars of Brassica napus and two cultivars of B. campestris. The mean reproductive factors (Rf), Rf = Pf at 55 days ÷ 5,000, for the three nematodes were 8.3, 2.2, and 14.3, respectively. All three nematodes reproduced more efficiently (P < 0.05) on B. campestris than on B. napus. Amending M. chitwoodi-infested soil in plastic bags with chopped shoots of Jupiter rapeseed reduced the nematode population more (P < 0.05) than amendment with wheat shoots. Incorporating Jupiter shoots to soil heavily infested with M. chitwoodi in microplots reduced the nematode population more (P < 0.05) than fallow or corn shoot treatments. The greatest reduction in nematode population density was attained by cropping rapeseed for 2 months and incorporating it into the soil as a green manure.     

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).     

Resistance of Auto- and Allotetraploid Triticeae Species and Accessions to Meloidogyne chitwoodi based on Genome Composition

The Columbia root-knot nematode Meloidogyne chitwoodi parasitizes several plant species, including grasses that have been developed for semiarid environments, and substantially reduces the productivity of cereals and the longevity of perennial grasses growing under semiarid conditions throughout the intermountain region. Thirty-two auto- and allotetraploid (2n = 28) taxa in the perennial Triticeae were evaluated as possible sources of resistance to M. chitwoodi. Low levels of root galling were observed on roots of all accessions; root-gall indices ranged from 0 (no galls) to 1.95 in the grasses compared to 4.67 for the susceptible ''Ranger'' alfalfa check on a scale of 1 to 6. Even though the gall ratings were low, significant (P < 0.01) differences among accessions of the same species, among species, and among genera with different genomes were observed. Within the reproductive indices, which ranged from 0.01 to 1.20 in the grasses compared to 65.38 for the alfalfa check, there was no difference among genera with different genomes and accessions within the same species and genome; however, there was a significant (P < 0.05) difference among species with the same genomes. This variation can be traced to Thinopyrum nodosum (Jaaska-19), which was the only accession with a reproductive factor greater than 1.00. Based on the data, all auto- and allotetraploids are considered resistant to M. chitwoodi.     

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.