Root-knot Nematode Species Research Articles

Heritability and gain from selection for field resistance against multiple root-knot nematode species (Meloidogyne incognita race 1 and M. javanica) in carrot

Heritability and gain from selection of traits associated with field resistance to multiple root-knot nematode species were estimated in carrot (Daucus carota L.). Experimental plots were uniformly and simultaneously infested with Meloidogyne incognita race 1 and M. javanica. Forty-seven half-sib families derived from the cv. ‘Brasilia’ were evaluated for: (1) percentage of commercial roots with non-galling symptoms (%RNG); (2) percentage of commercial roots with gall symptoms (%RGS) and (3) percentage of non-commercial roots (%NCR). The cultivars ‘Brasilia’ (resistant), ‘Carandai’ (intermediate) and ‘Nova Kuroda’ (susceptible) were employed as standards. Broad-sense heritability estimates were 61.9 % for %RNG; 30.6 % for %RGS and 67.9 % for %NCR. However, the gains from selection were very small for all traits. The genotypic correlation between %RGS and %NCR was –0.38 and the correlation between %RNG and %RGS was 0.13. Selection on increasing %RNG resulted in a negative correlated response on %NCR (genotypic correlation = –0.99) indicating repulsion phase linkage(s) between the genetic factor(s) controlling these traits. The phenotypic correlation of –0.88 between %RNG and %NCR suggested that incomplete penetrance and dosage effects might be involved in the expression of these traits. Selection of superior genotypes (i.e. high %RNG, low %RGS, and low %NCR) can be achieved. ‘Brasilia’ was among the genotypes selected for the following cycle of recombination reinforcing the notion that this cultivar is one of the most promising sources of stable, wide-spectrum field resistance to Meloidogyne species in D. carota.

The heat-stable root-knot nematode resistance gene Mi-9 from Lycopersicon peruvianum is localized on the short arm of chromosome 6.

The tomato gene Mi-1 confers resistance to three species of root-knot nematodes, Meloidogyne spp. However, the resistance mediated by Mi-1 is inactive at soil temperatures above 28 degrees C. Previously, we identified and mapped a novel heat-stable nematode resistance gene from the wild species Lycopersicon peruvianum accession LA2157 on to chromosome 6. Here we report further characterization of this heat-stable resistance against three Mi-1-avirulent biotypes of Meloidogyne javanica, Meloidogyne arenaria and Meloidogyne incognita. Screening segregating F(2) and F(3) progenies, derived from an intraspecific cross between susceptible LA392 and resistant LA2157, for nematode resistance at 25 degrees C and 32 degrees C, revealed a simple dominant monogenic inheritance with all the biotypes tested. We designate this gene as Mi-9. As a first step towards cloning of Mi-9, we constructed a linkage map around this gene. A total of 216 F(2) progeny from the cross between LA392 and LA2157 were screened with M. javanica at 32 degrees C and with CT119 and Aps-1, markers that flank the genetic interval that contains the Mi-1 gene. DNA marker analysis indicated that these markers also flank Mi-9. Further mapping of recombinants with both RFLP and PCR-based markers localized Mi-9 to the short arm of chromosome 6 and within the same genetic interval that spans the Mi-1 region.

PARASITIC NEMATODES ASSOCIATED WITH OLIVE IN COUNTRIES BORDERING THE MEDITERRANEAN SEA

PARASITIC NEMATODES ASSOCIATED WITH OLIVE IN COUNTRIES BORDERING THE MEDITERRANEAN SEA

Evaluation of a Core of the U.S. Capsicum Germplasm Collection for Reaction to the Northern Root-knot Nematode

Several species of root-knot nematodes [Meloidogyne incognita (Kofoid & White) Chitwood, M. arenaria (Neal) Chitwood, M. javanica (Treub) Chitwood, and M. hapla Chitwood] are major pests of peppers (Capsicum spp.) in the United States and worldwide. Resistance to M. incognita, M. arenaria, and M. javanica has been identified in several Capsicum accessions, but there are few reports of resistance to M. hapla. Therefore, we selected a 10% core (440 accessions) of the 14 available Capsicum spp. in the Capsicum germplasm collection (3,731 accessions) maintained by the U.S. Dept. of Agriculture (USDA), and evaluated this core for resistance to M. hapla in unreplicated greenhouse tests. The 11 best (most resistant) and the 3 worst (most susceptible) accessions identified in these unreplicated tests were re-evaluated in a replicated greenhouse test. Seven of these 11 “best” accessions (PI 357613, PI 357503, PI 439381, PI 297493, PI 430490, PI 267729, and PI 441676) exhibited root gall severity indices <5.0 (1 = no galls; 9 = more than 80% of the root system covered with galls) in the replicated test, and each of these indices was significantly lower than the indices of the “worst” accessions and susceptible controls. Although a gall index <5.0 indicates a moderate level of resistance, more than 3000 M. hapla eggs were extracted per gram of fresh root tissue and the reproductive index was >1.0 for each of these accessions. These observations suggest that the most resistant accessions tested are somewhat susceptible to M. hapla. The results of our evaluation of a core of the USDA Capsicum germplasm collection demonstrates clearly that there is significant genetic variability within the overall collection for M. hapla resistance. Additionally, these results identify portions of the collection where future evaluations for M. hapla resistance should be focused. For example, the origin of the two most promising C. annuum accessions (PI 357613 and PI 357503) in the core was Yugoslavia. Thus, additional accessions from this temperate region of the world should receive priority attention in any effort to identify better sources of resistance in C. annuum to M. hapla.

Characterization of Root-Knot Nematode Populations Associated with Vegetables in New York State.

The northern root-knot nematode (Meloidogyne hapla) causes significant damage on vegetables in New York State. This study was conducted to determine if other species of Meloidogyne are also present in New York vegetable fields and to better characterize the root-knot populations in order to facilitate the development of effective rotations for nematode management. Populations of Meloidogyne were collected from 18 vegetable fields across New York and maintained in the greenhouse on tomato cv. Rutgers as both field populations and single-female populations. For the purposes of species identification, nematodes from each population were examined using perineal pattern analysis, scanning electron microscopy, and cytological analysis. All techniques identified the presence of only Meloidogyne hapla in the collected samples. The host range of the populations of Meloidogyne that were collected was investigated using a modified form of the North Carolina Differential Host Test. While host ranges were variable among the collected populations, the host range of all populations closely corresponded to that of M. hapla. The internal transcribed spacer region (ITS1) of the collected populations was amplified and sequenced. All sequences from the collected populations corresponded to the published sequence of M. hapla. Based on these results, it was concluded that M. hapla is the only root-knot nematode species present in New York vegetable fields. Crop rotational strategies for management of the root-knot nematode on vegetables in New York should be devised to reflect this fact.

Susceptibility of Eight Herbs to Common Root-Knot Nematodes

Abstract Eight herb species were inoculated with two common species of root-knot nematode and grown for 2 months in a greenhouse. Root systems were examined for galls and egg mass production. All herb species were susceptible but developed fewer galls and had lower gall indices than Rutgers tomato. Burnet, chives, valerian and winter savory had few galls or low gall indices. Eggs were produced on all. Chamomile had a high gall index. Chicory, parsley, and sorrel had intermediate indices. Herbs were equally susceptible to the southern (Meloidogyne incognita) and peanut root-knot nematodes (M. arenaria), particularly at the highest inoculum densities. Mean dry weights of inoculated herbs were not always significantly less than the non-inoculated plants, suggesting that some herbs may be tolerant to root-knot nematodes.

Efficient Evaluation of Resistance to Three Root-knot Nematode Species in Selected Sweetpotato Cultivars

Five sweetpotato [Ipomoea batatas (L.) Lam.] cultivars (`Beauregard', `Excel', `Jewel', `Hernandez', and `Porto Rico') were evaluated for resistance to three root-knot nematode species: Meloidogyne arenaria (Neal) Chitwood (race 2), M. incognita (Kofoid & White) Chitwood (race 3), and M. javanica (Treub) Chitwood. Resistance screening efficiency was assessed in both 400-cm3 square pots and 150-cm3 Conetainers™. Nematode infection was assessed as the percentage of root system galled, percentage of root system necrosis, and the number of nematode eggs produced per gram of root tissue. Means of these dependent variables were not different (P ≤ 0.05) between container types, with Conetainers™ being more efficient to use. Root necrosis was not related to nematode infection, but was significant among cultivars (P = 0.0005). The resistance responses of the cultivars differed depending on the nematode species. All five cultivars were resistant to M. arenaria race 2. `Hernandez', `Excel', and `Jewel' were also resistant to M. incognita race 3 and M. javanica.

First Record of Meloidogyne partityla on Pecan in Georgia.

In July 2000, tree decline was observed in a commercial pecan (Carya illinoensis (Wang.) K. Koch) orchard in Crisp County, GA. Most affected trees exhibited dead branches in the upper canopy, stunted growth, and feeder roots with small galls and associated egg masses typical of root-knot nematode infection. All declining trees that were examined had root systems infected with a Meloidogyne sp. Efforts to culture the nematode on tomato (Lycopersicon esculentum Mill. 'Rutgers') were unsuccessful. Identification of the nematode was determined by two laboratory procedures in March 2001. Female nematodes were teased from fresh pecan root galls of declining trees in Georgia, and identified by determining the esterase phenotype from replicate samples of single females compared with standard root-knot nematode species, including a population of M. partityla (3). Galled roots also were sent to New Mexico State University in Las Cruces, where mitochondrial DNA from specimens was extracted and compared with that from standard root-knot nematode species and known populations of M. partityla (2). Specimens had esterase phenotypes and DNA patterns consistent with M. partityla. Esterase phentoypes were inconsistent with M. incognita and M. arenaria, and DNA patterns were inconsistent with M. incognita, M. javanica, and M. hapla. Specimens at both locations were identified as M. partityla Kleynhans (1). To our knowledge, this is the first report of M. partityla from Georgia and the third report of this nematode outside of South Africa. The first and second report of M. partityla from pecan in the United States occurred in Texas and New Mexico in 1996 and 2001, respectively (3,4). Our inability to culture the M. partityla-GA isolate on tomato substantiates previous experience with this nematode in the United States (3) and is not surprising, since this species has a host range limited to the Juglandaceae. Furthermore, M. partityla may be endemic to North America and not South Africa. It is believed this nematode entered South Africa on pecan seedling roots imported from the United States between 1912 and 1940 (1). The unusually narrow host range may explain why M. partityla has gone unrecognized for so long in the United States compared with the more common Meloidogyne spp. (i.e., M. incognita and M. arenaria) with wider host ranges found in pecan orchards. Determining the distribution of M. partityla within the major pecan-growing regions of Georgia and throughout North America is warranted.

Identification of Root-Knot Nematode Species Occurring on Tomatoes in Tanzania and Resistant Lines for Their Control.

This was a 5-year study that included surveys during the first 2 years, followed by field evaluations of improved tomato lines over the next 3-year period. Surveys were conducted in 12 regions of Tanzania from 1993 to 1995 to identify the causal nematodes of tomato root-knots. Eighty-seven samples were collected and assayed for presence of Meloidogyne spp. M. hapla, M. incognita, and M. javanica were detected at incidences of 1, 19, and 89%, respectively. Species mixtures were common within samples. Results indicated the presence of races 1 and 2 of M. incognita in tomato-growing regions of Tanzania. Reactions of eight advanced tomato breeding lines to the three species in greenhouse and field tests indicated that three lines, namely ARP 365-2, 367-1, and 367-2, were resistant to M. incognita races 1 and 2 and to M. javanica, but all lines were susceptible to M. hapla.

Incompatibility in diploid and tetraploid crosses of Cucumis sativus and Cucumis metuliferus

The African horned cucumber (Cucumis metuliferus Naud.; 2x = 2n = 24) contains genes that can confer resistance to many important cucumber (C. sativus L.; 2x = 2n = 14) pests [e.g., root-knotnematode, Meloidogyne incognita (Kofoid & White) Chitwood]. Cucumber is highly susceptible to this root-knot nematode species, and a recent screening of C. sativus accessions in the U.S. National Plant Germplasm collection did not identify sources of resistance. Thus,autotetraploids of Cucumis sativus and C. metuliferus were created to recover fertile resistant interspecific progeny. Autotetraploids were obtained at the highest rate when seeds were immersed in 0.5% colchicine for a period of 6 to 8 hrs. Treatment durations less than 6 hrs produced few tetraploids, and durations of 10 hrs or more were lethal to seeds or developing seedlings. Crosses between C. sativus and C. metuliferus were made using diploid and tetraploid lines in all possible combinations, including reciprocals. Fruit development occurred in crosses when diploid and tetraploid C. sativus were used as the female parent. However, seeds developed only in fruit of C. sativus (4n) ×C. metuliferus (2n) crossings. Seeds from these crosses, however,were flat and not viable. No fruit development occurred in crosses whereC. metuliferus was used as the female parent.

Improved preparation of perineal patterns of root-knot nematodes for Scanning Electron Microscopy

The perinealpatternisperhapsthemost importantcharacter for the identie cation of species of root-knot nematodes (Meloidogyne spp.). The morphology is usually examined with the light microscope (LM) which has the advantage of revealing internal structures, but resolution and depth of focus are extremely limited. The scanning electron microscope (SEM) has a greater depth of focus and more resolving power,but it is limited to imaging only the surface morphology. Several techniques for preparing perineal patterns for SEM have been described: Mulvey et al. (1975), Spaull

Identification of root-knot nematode species occurring in South Africa using the SCAR-PCR technique

Species of root-knot nematodes occurring in South Africa (Meloidogyne fallax, M. chitwoodi, M. javanica, M. incognita, M. arenaria and M. hapla) were identified and differentiated by the sequence characterised amplified region - polymerase chain reaction (SCAR-PCR) technique. Multiplex internal transcribed spacer (ITS)-PCR amplified a fragment in an unknown species for which no SCAR-PCR marker was available. M. fallax, a quarantine organism in Europe, was recorded for the first time in South Africa. The geographical distribution of M. chitwoodi is extended to four localities in South Africa. The SCAR-PCR technique reliably determined species composition, detecting mixed populations of M. hapla, M. fallax and M. chitwoodi on groundnut and of M. arenaria and M. incognita on Impatiens spp. In contrast, single species populations of M. javanica and M. incognita and the unknown species were detected in glasshouses.

Lack of correlation between (a)virulence and phylogenetic relationships in root-knot nematodes (Meloidogyne spp.)

In tomato, the Mi resistance gene controls the major root-knot nematode species Meloidogyne arenaria, M. incognita and M. javanica. However, resistance-breaking (i.e. virulent) biotypes have been reported from most of the tomato-growing areas in the world. In this study, we assembled a collection of 17 isolates belonging to these three species, either avirulent or virulent against the Mi gene, in order to analyze their genetic diversity and phylogenetic relationships. According to parsimony analysis, genomic fingerprints based on RAPD or AFLP markers were independently used to compute trees describing the relationships between the isolates. In both dendrograms, isolates belonging either to the M. incognita or M. javanica species appeared clustered together, which tends to indicate a monophyletic origin for each of these two species. Conversely, M. arenaria isolates were not included in one single clade, which raised questions about the taxonomic status of the currently designated species M. arenaria. Moreover, our data showed that clustering of isolates was not associated with their (a)virulence against the tomato Mi resistance gene. This lack of correlation between groups determined by molecular markers and virulence indicates that most of the observed DNA polymorphism is independent of virulence, which is presumably under host selection.

Induced Soil Suppressiveness to a Root-Knot Nematode Species by a Nematicide

Investigations were conducted to determine whether the nematicide DiTera, could induce soil suppressiveness to the root-knot nematode Meloidogyne incognita. To determine the effects of DiTera on soil microorganisms with antagonistic potential to nematodes, DiTera was added to autoclaved and nonautoclaved soils, and at 10, 30, and 60 days after treatment (DAT), alginate screens with M. incognita eggs were buried in the soil for 70 h, removed, and incubated at 24°C for 5 days to determine percentages of parasitized eggs and juveniles. At 30 and 60 DAT, the nonautoclaved soil treated with DiTera had higher percentages of parasitized eggs and juveniles than did the control, whereas no effects of DiTera were observed in the autoclaved soil. To further investigate the effects of DiTera on soil microbial communities, populations of specific groups of microorganisms from the soil and rhizosphere of tomato plants were quantified after DiTera addition. An increase in the population level of several groups of microorganisms from the soil and rhizosphere after DiTera treatment was observed 30 and 60 DAT compared with the control. Changes in traits of microbial physiology that confer enhanced survival or antagonism toward nematodes were also investigated; however, the application of DiTera was not correlated with a change in frequency of any one specific physiological trait. To investigate functional changes of the microbial community, activities of several soil enzymes including fluorescein diacetate hydrolysis, catalase, chitobiase, protease, and urease were measured along with pH and electrical conductivity from autoclaved or nonautoclaved soil treated with DiTera at 0, 2.5, or 5.0 g/kg soil after 1, 10, or 43 DAT. More enzymatic activity was measured as early as 1 DAT in the treatments with DiTera compared with the control. Our results demonstrate that application of DiTera to soil led to enhanced antagonism of root-knot nematodes and that this was associated with structural and functional changes of the rhizosphere bacterial community.

Inheritance of Resistance to Root-knot Nematodes (Meloidogyne sp.) in Prunus Rootstocks

Two F1 hybrid Prunus rootstocks, K62-68 and P101-41, developed from a cross of `Lovell' [susceptible to both Meloidogyne incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood] and `Nemared' (resistant to both root-knot nematode species), were selfed to produce two F2 seedling populations. Vegetative propagation by herbaceous stem cuttings was used to produce four or eight self-rooted plants of each F2 seedling for treatment replications. Eggs of M. incognita and M. javanica were inoculated into the potted media where plants were transplanted, and plants were harvested and roots examined for signs and symptoms associated with root-knot nematode infection ≈120 days later. Segregation ratios in both F2 families suggested that resistance to M. incognita in `Nemared' is controlled by two dominant genes (Mi and Mij) and that to M. javanica by a single dominant gene (Mij). Thus, Mij conveys resistance to both M. incognita and M. javanica.

Rapid identification of cyst (Heterodera spp., Globodera spp.) and root-knot (Meloidogyne spp.) nematodes on the basis of ITS2 sequence variation detected by PCR-single-strand conformational polymorphism (PCR-SSCP) in cultures and field samples.

Cyst and root-knot nematodes show high levels of gross morphological similarity. This presents difficulties for the study of their ecology in natural ecosystems. In this study, cyst and root-knot nematode species, as well as some ectoparasitic nematode species, were identified using the second internal transcribed spacer (ITS2) sequence variation detected by polymerase chain reaction-single-strand conformational polymorphism (PCR-SSCP). The ITS2 region was sufficiently variable within the taxa investigated to allow species to be separated on the basis of minor sequence variation. The PCR primers used in this study were effective for 12 species with three genera within the Heteroderinae (Globodera pallida, G. rostochiensis, Heterodera arenaria/avenae, H. ciceri, H. daverti, H. hordecalis, H. mani, H. schachtii, H. trifolii, Meloidogyne ardenensis, M. duytsi and M. maritima). However, pathotypes of Globodera pallida and G. rostochiensis could not be distinguished. The method was tested at two coastal dune locations in The Netherlands (one in the lime-poor dunes of the north and one in calcareous dunes of the south) to determine the population structure of cyst nematodes. At each site, cyst nematodes were associated with three plant species: two plant species on the foredune (Elymus farctus and Ammophila arenaria) and one plant species occurring further inland (Calamagrostis epigejos). Two species of cyst nematodes, H. arenaria and H. hordecalis, were found. H. arenaria associated with vigorous A. arenaria and H. hordecalis in association with degenerating A. arenaria and C. epigejos. The field survey demonstrated that in coastal dunes abiotic factors may be the important affecting the distribution of cyst nematodes.

Evaluation of Sweetpotato Cultivars to Root-knot Nematodes

This study was conducted to determine whether the type of pot used for the evaluation affected the resistance response of the sweetpotato plants, and to assess the resistance response to different root-knot nematode species. Five sweetpotato [Ipomoea batatas (L.) Lam] cultivars, `Beauregard', `Exce'l, `Jewel', `Hernandez', and `Porto Rico', were screened for M. incognita (race 3), Meloidogyne arenaria (race 2), and M. javanica, in both 10-cm-side, square pots and 4-cm-diameter, cone pots. Gall index, necrosis index, and number of nematode eggs per gram of root were used to estimate nematode-resistance reaction. Mean of all indices between the 2 pot types were not significantly different (α = 0.05). Gall and necrosis indices were not correlated in any of the cultivars. Resistance response depended on cultivars and nematode species for all variables analyzed. `Beauregard' was the most susceptible to Meloidogyne. `Hernandez' and `Excel' were found to be the most resistant cultivars to the Meloidogyne species.

RAPD linkage map of the genomic region encompassing the root-knot nematode (Meloidogyne javanica) resistance locus in carrot

Inheritance studies have indicated that resistance to the root-knot nematode (Meloidogyne javanica) in carrot inbred line ’Brasilia-1252’ is controlled by the action of one or two (duplicated) dominant gene(s) located at a single genomic region (designated the Mj-1 locus). A systematic search for randomly amplified polymorphic DNA (RAPD) markers linked to Mj-1 was carried out using bulked segregant analysis (BSA). Altogether 1000 ten-mer primers were screened with 69.1% displaying scorable amplicons. A total of approximately 2400 RAPD bands were examined. Four reproducible markers (OP-C21700, OP-Q6500, OP-U12700, and OP-AL15500) were identified, in coupling-phase linkage, flanking the Mj-1 region. The genetic distances between RAPD markers and the Mj-1 locus, estimated using an F2 progeny of 412 individuals from ’Brasilia 1252’×’B6274’, ranged from 0.8 to 5.7 cM . The two closest flanking markers (OP-Q6500 and OP-AL15500) encompassed a region of 2.7 cM . The frequency of these RAPD loci was evaluated in 121 accessions of a broad-based carrot germplasm collection. Only five entries (all resistant to M. javanica and genetically related to ’Brasilia 1252’) exhibited the simultaneous presence of all four markers. An advanced line derived from the same cross, susceptible to M. javanica but relatively resistant to another root-knot nematode species (M. incognita), did not share three of the closest markers. These results suggest that at least some genes controlling resistance to M. incognita and M. javanica in ’Brasilia 1252’ reside at distinct loci. The low number of markers suggests a reduced amount of genetic divergence between the parental lines at the region surrounding the target locus. Nevertheless, the low rate of recombination indicated these markers could be useful landmarks for positional cloning of the resistance gene(s). These RAPD markers could also be used to increase the Mj-1 frequency during recurrent selection cycles and in backcrossing programs to minimize ’linkage drag’ in elite lines employed for the development of resistant F1 hybrids.

Virulence and molecular diversity of parthenogenetic root-knot nematodes, Meloidogyne spp.

Root-knot nematodes (RKN) are sedentary endoparasites causing severe damage to a wide variety of crops, including tomato. Among them, the parthenogenetic species Meloidogyne arenaria, M. incognita and M. javanica are of particular economic importance. The genetic diversity and relationships of 17 populations belonging to these three major species, either avirulent or virulent against the tomato Mi resistance gene, were investigated in order to determine whether (a)virulence of the nematodes could be related to their molecular fingerprints. Genomic polymorphisms between populations were assessed by using amplified fragment length polymorphism (AFLP) markers, and data were treated by means of a multiple correspondence analysis. A total of 1550 polymorphic amplified DNA fragments were identified and used to compute the relationships between the populations. As expected, the three RKN species were clearly distributed into distinct groups, but combination of data for virulence phenotypes and DNA markers showed that clustering of populations was not associated with their (a)virulence against the tomato Mi resistance gene. Such a lack of correlation indicates that most of the observed DNA polymorphism is independent of virulence, which is presumably under host selection. This result demonstrates that virulent populations do not share a common origin, and strongly suggests that they might have appeared late after the establishment of these clonal lineages, as the result of independent mutational events.

Resistance Response of the Ma genes from `Myrobalan' Plum to Meloidogyne hapla and M. mayaguensis

In `Myrobalan' plum (Prunus cerasifera Ehr.), Ma1 and Ma2 are single major dominant genes that control the resistance to the predominant root-knot nematode (RKN) species Meloidogyne arenaria (Neal) Chitwood, M. incognita (Kofoid & White) Chitwood, and M. javanica (Treub). These genes were evaluated for activity to the northern RKN M. hapla Chitwood and the tropical RKN M. mayaguensis Rammah & Hirschmann, neither of which is controlled by the Mi gene from tomato. This study was conducted under greenhouse conditions using a resistance screening based on high and durable inoculum pressure by the nematodes. Tests were conducted simultaneously for: M. arenaria (as a reference for the Ma genes); M. hapla and M. mayaguensis from crosses segregating for either Ma2 alone or Ma2 and Ma1 and involving the resistant parental clones P.2175 (heterozygous for Ma1) and P.1079 (homozygous for Ma2); and the host parental clone P.2646 (recessive for both Ma genes). Each parental clone and each individual of the segregating progenies reacted in a similar way to M. arenaria and M. mayaguensis, indicating that the Ma genes also control resistance to M. mayaguensis. By contrast, all parental clones and progenies were completely resistant to M. hapla, and, despite high inoculum pressure, no effect of the Ma genes on this species could be established.