A Barley mild mosaic virus (BaMMV) isolate from France (BaMMV-Sil) capable of overcoming rym5-controlled resistance was inoculated to barley genotypes carrying various genes for resistance to the barley mosaic viruses. BaMMV-Sil was unable to infect genotypes carrying rym1, rym4, rym8, rym9, or rym11 but genotypes carrying rym3, rym5, rym6 or no known bymovirus resistance gene were susceptible. Plants carrying rym7 or rym10 showed partial resistance with delayed virus accumulation. The two genomic RNAs of BaMMV-Sil were sequenced and compared to published sequences and those of a further common strain isolate from the UK. Four amino acid differences were observed between BaMMV-Sil and European common strain isolates in the polypeptide encoded by RNA1, the RNA species which determines pathogenicity on the rym5 genotypes. Only two of these differences are likely to be functionally important (His rather than Gln at position1217 in the VPg cistron; His rather than Asp at position 1776 in the NIb cistron). Comparisons with related viruses in the genera Bymovirus and Potyvirus suggest that the change in the VPg, which occurs within a motif conserved amongst all viruses within the family Potyviridae, is the more likely cause of rym5 resistance-breaking.

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Breeding for resistant cultivars is the only way to prevent high yield loss in barley caused by the soil-borne barley mild mosaic virus (BaMMV) complex. We have characterized the BaMMV resistance of barley cv. Chikurin Ibaraki 1. Doubled haploid lines were obtained from the F(1) between the susceptible six-rowed winter barley cultivar, Plaisant, and Chikurin Ibaraki 1. Each line was tested for reaction to BaMMV by mechanical inoculation followed by DAS-ELISA. Of 44 microsatellites that covered the genome, 22 polymorphic markers were tested on one susceptible and one resistant bulk, each comprising 30 lines. Differential markers and additional microsatellite markers in the same region were then tested on the whole population. A bootstrap analysis was used to compute confidence intervals of distances and to test the orders of the resistance gene and the closest markers. A segregation of 84 resistant/98 susceptible lines fitted a 1:1 ratio (chi(2)=1.08, P=0.30), which corresponds to a single gene in this DH lines population. The resistance gene was flanked by two markers near the centromeric region of chromosome 6HS-Bmag0173, at 0.6+/-1.2 cM, and EBmac0874, at 5.8 +/- 3.4 cM. We propose to name this new resistance gene rym15. This resistance gene and associated markers will increase the possibilities to breed efficiently for new cultivars resistant to the barley mosaic disease.

Soft winter wheat cultivars were infected in the field or in a growth chamber with an inoculum consisting of soilborne wheat mosaic virus (SBWMV) and wheat yellow mosaic virus (WYMV). Only SBWMV was screened by ELISA in the leaves and roots of plants. Among 33 wheat cultivars studied in the field, 9 (Capitole, Cargo, Fortuna, Moulin, Rescler, Albatros, Fidel, Rempart, Tarasque) were resistant to the SBWMV-WYMV complex. However, the first five cultivars mentioned above showed transitory mosaic symptoms. Cultivar Pernel, which is susceptible to WYMV and resistant to SBWMV in the field did not show a breakdown of resistance to SBWMV. This might be accounted for by the spread of SBWMV in the plant shoots. In the cultivars resistant to SBWMV, virus could not be detected in the leaves, but was clearly identified in the roots. The virus could be detected only during a part of the life cycle of the plant. Upon completion of the tillering stage, virus concentrations were equally high in the resistant and in the susceptible cultivars. SBWMV associated with all the resistant cultivars studied could be carried to healthy plants by zoospores of Polymyxa graminis attacking the roots at least during the tillering stage. However, the level of inoculum present in the plants of each cultivar was not evaluated. At the end of the life cycle of wheat, resting spores of P. graminis do occur in all wheat cultivars resistant to the SBWMV-WYMV complex. In a growth chamber after inoculation by a soil carrying SBWMV and WYMV, root multiplication of SBWMV has been tested in eleven field-resistant cultivars : Albatros, Apexal, Avalon, Cocagne, Capitole, Fleuron, Fortuna, Moulitt, Pernel, Sabre, Tarasque. Sabre and Avalon showed the same virus concentration as Hardi the susceptible cultivar. In contrast, Albatros and Cocagne showed a lower virus concentration in their roots (the concentration was 10 to 200 times lower in Cocagne than in Hardi). The other cultivars showed an intermediate behavior. The genetic origin of this resistance is discussed.Additional key words : ELISA, Polymyxa graminis, resistance. RÉSUMÉMultiplication du virus de la mosaïque du blé (VMB = SBWMV) dans les racines de blé infectées par un sol portant le VMB et le virus de la mosaïque jaune du blé (VMJB = WYMV). Des blés tendres d'hiver ont été soumis au champ ou en enceinte climatique à un inoculum constitué par le complexe virus de la mosaïque du blé (VMB), virus de la mosaïque jaune du blé (VMJB). Seul le VMB a été recherché à l'aide d'un test immunoenzymatique dans les feuilles et les racines des plantes. Au champ, sur 33 cultivars étudiés, 9 d'entre eux : « Capitole, Cargo, Fortuna, Moulin, Rescler, Albatros, Fidel, Rempart, Tarasque » sont symptomatologiquement résistants au complexe VMB-VMJB. Cependant les 5 premiers blés cités présentent des symptômes fugaces de mosaïque. Pour le cultivar « Pernel », sensible au VMJB et résistant au VMB au champ, on n'a pas observé de rupture de résistance qui se serait exprimée par une multiplication du VMB dans la partie aérien...