Grapevine fanleaf virus (GFLV) is transmitted specifically from grapevine to grapevine by the ectoparasitic root-feeding nematode Xiphinema index. Limited information is available on the survival of X. index in vineyard soil and on the retention of GFLV by X. index over extended periods of time. We addressed these two issues by quantifying the numbers of living X. index recovered from soil samples that were collected in three naturally GFLV-infected vineyards in France and subsequently stored at 7 or 20 degrees C in the absence of host plants. Our data indicated a two- to eightfold decrease in X. index numbers but the recovery of 8 to 10 living fourth-stage juveniles (J4) and adults per kilogram of soil after 4 years of storage regardless of temperature. In addition, GFLV was detected readily in all groups of 20 isolated X. index adults and J4 (except for J4 that were kept 4 years at 20 degrees C) by reverse transcription-polymerase chain reaction using total nematode RNAs and a primer set located in conserved regions at the 3' end of viral genomic RNA 2. Our findings on the long-term survival of viruliferous X. index under adverse conditions emphasize the need for new control strategies against GFLV.

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The inheritance of resistance of the self-incompatible Myrobalan plum Prunus cerasifera to the root-knot nematode Meloidogyne arenaria was studied using first a diallel cross between five parents of variable host suitability (including two highly resistant clones P.1079 and P.2175, a moderate host P.2032, a good host P.2646 and an excellent host P.16.5), followed by the G2 crosses P.16.5 × (P.2646 × P.1079) and P.2646 × (P.16.5 × P.1079). A total of 355 G1 and 72 G2 clones obtained from hard-wood cuttings sampled from trees in the field experimental design, then rooted in the nursery and inoculated individually in containers (5-10 replicates per clone) under greenhouse conditions, were evaluated for their host suitability based on a 0-5 gall-index rating under a high and durable inoculum pressure of the nematode. In the crosses involving the resistant P.1079 and P.2175 and the hosts P.2646 and P.16.5: (1) all of the G1 crosses of P.1079 were resistant while the G2 crosses segregated 1 resistant to 1 host, (2) the G1 crosses between P.2175 and either P.2646 or P.16.5 segregated 1 resistant to 1 host, and (3) all of the G1 progeny between P.2646 and P.16.5 were host. These results indicate that resistance is conferred by a single major dominant resistance gene (homozygous) in P.1079, and the same, or an allelic or a different, major dominant gene (heterozygous) in P.2175, and that P.2646 and P.16.5 are recessive for this (these) major resistance gene(s). As expected according to the hypothesis of a recessive genotype for P.2032, all of its hybrids with P.1079 were resistant, all of its hybrids with P.2646 and P.16.5 were host, and its hybrids with P.2175 segregated for resistance. Nevertheless, the 3∶2 segregation ratio of these latter hybrids suggests that clones bearing the P.2175 gene would have a selective advantage. Both resistance genes are completely dominant and confer a non-host behaviour that totally prevents the multiplication of the nematode. This is the first reported evidence of major nematode resistance genes towards M. arenaria in a species of the subgenus Prunophora in the genus Prunus. The symbols Ma1 for the P.2175 gene and Ma2 for the P.1079 gene are proposed.

Resistance variability was evaluated for five rootstock: three Myrobalan plum (Prunus cerasifera Ehr.) genotypes (P.1079, P.2175, and P.2032) grown from in vitro plantlets, one peach (P. persica (L.) Batsch `GF 305') grown from seeds, and one peach-almond hybrid (P. persica × P. amygdalus Batsch `GF 557') grown from rooted cuttings. Twenty-two root-knot nematode populations from different origins were used: Meloidogyne arenaria (Neal) Chitwood (six populations), M. incognita (Kofoid and White) Chitwood (eight populations), M.javanica (Treub) (four populations), M. hispanica Hirschmann (one population), M. hapla Chitwood (two populations), and an unclassified root-knot species (one population). The study was conducted under greenhouse conditions for 1 and 2 months. No galling or nematode reproduction was observed in P.1079 and P.2175, which should be considered immune; P.2032 showed the highest galling and nematode counts when inoculated with M. hispanica and M. javanica. In P.2032, a high proportion of males was recovered in populations that had a limited development. Because the populations of the first four Meloidogyne species reproduce by obligatory mitotic parthenogenesis, high sex ratio maybe the expression of a late form of resistance. Host suitability of `GF 305' was highly variable among M. arenaria and M. incognita populations. A lower relative variation was observed in M. javanica. `GF 557' was resistant to M. arenaria and M. incognita except for one population of M. arenaria that was weakly aggressive and susceptible to M. javanica. Consequently, resistances specific to the genus Meloidogyne for the Myrobalan plum genotypes P.1079 and P.2175, specific to the nematode species for `GF 557', and specific to the nematode population for `GF 305', were evidenced. This study indicates that, in rootstock selection procedures, it is important to test resistance to several populations within the same nematode species.

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The Myrobalan plum Prunus cerasifera clones ‘P,2175’ and ‘P,1079’ carry single major genes (Ma1 and Ma2. respectively) for resistance to the predominant root–knot nematode (RKN) species Meloidogyne arenaria (MA), Meloidogyne incognita (MI) and Meloidogyne jaranica(MJ). The Myrobalan plum clone ‘P.2980’ is another complete–spec–trum source bearing favourable agronomic features. The genetics of its resistance to MA, MI, MJ and to the population Meloidogyne sp. Florida has been investigated from G1 crosses with the host Myrobalan plum clone ‘P.16.5’ (recessive for both Ma genes), the resistant clone ‘P.2175’ (heterozygous for Mal) and the ‘Nemared’ peach (homozygous for resistance to MA, MI and MJ but a host for M. sp. Florida). The segregation of the Gl hybrids from the intraspecific crosses into two significantly separated resistant and host classes, independent of the RKN species, indicates that resistance to all four species in ‘P.2980’ is also controlled by a single major dominant (heterozygous) gene. This gene is designated Ma3 because it shares the same spectrum and efficiency as Ma1 and Ma2, and its relationship with these last two genes is discussed. The first hybrids between ‘P.2980’ and ‘Nemared’ tested only segregated for resistance to M. sp. Florida. These results illustrate the possibility of cumulating (pyramiding) Ma3 and the ‘Nemared’ peach resistance gene(s) into new interspecific rootstocks.

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.