Polymyxa graminis f. sp. temperata and P. graminis f. sp. tepida are distinguished on the basis of their specific ribosomal DNA sequences. In order to evaluate whether or not host specialization is associated with the special form, the occurrence of infection of both forms on barley and wheat was studied. P. graminis inocula were obtained from soils collected in Belgium and France. Their ribotypes were characterized using molecular tools specific to P. graminis f. sp. temperata or P. graminis f. sp. tepida such as restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction (PCR)-amplified rDNA, nested and multiplex PCR. Both special forms were found in each country and coexisted in some soils. The host specificity of P. graminis special forms for barley and wheat was studied from two soils collected at Gembloux (Belgium) and Chambon-sur-Cisse (France), each infested by bymo- and furoviruses. P. graminis f. sp. temperata is more frequent on barley and P. graminis f. sp. tepida on wheat. Furthermore, the quantification of each form on barley and wheat by two separated real-time quantitative PCR assays confirms the observations on the vector specialization. These results suggest a certain but not exclusive host specificity of P. graminis special forms.

In order to assess the occurrence of Wheat spindle streak mosaic virus (WSSMV) in Belgium, a reverse-transcription polymerase chain reaction (RT-PCR) was developed, targeting WSSMV isolates from Canada, France, Germany, Italy, and the United States. The primers also were designed for virus quantification by real-time RT-PCR with SYBR-Green. No cross-reaction with soilborne cereal viruses such as Barley mild mosaic virus, Barley yellow mosaic virus, Soilborne cereal mosaic virus, and Soil-borne wheat mosaic virus was observed. The RT-PCR and real-time quantitative RT-PCR allowed a more sensitive detection of WSSMV than enzymelinked immunosorbent assay. The incidence of WSSMV in Belgium was evaluated using a bioassay with wheat cvs. Cezanne and Savannah and rye cv. Halo, grown in 104 Belgian soils. The presence of WSSMV was detected from plants grown in 32% of the soils. The RT-PCR methods developed here, combined with large sampling, allowed WSSMV to be detected for the first time in Belgium. The real-time quantitative RT-PCR was developed as a tool for evaluating the resistance to WSSMV by quantifying the virus concentration in wheat cultivars.

No abstract

Résumé -La formation d'un tissu de bois de tension est induite artificiellement au moyen d'un stimulus gravitationnel obtenu en inclinant de jeunes pousses de l'année de Peuplier euraméricain cv 'Ghoy' à diverses intensités de 0 à 30° durant la saison de végétation. Au terme de l'expé-rimentation, il s'avère que l'allongement des pousses ainsi que l'angle de l'arc de bois de tension, ne sont pas affectés par l'intensité du stimulus. Au contraire, le redressement de l'axe, la quantité de fibres gélatineuses, la croissance secondaire sur écorce, la surface de la section et du xylème secondaire, les différents rayons caractéristiques, les diamètres sous écorce et l'excentricité de la moelle, répondent quantitativement à l'inclinaison. La quantité de bois de tension est corrélée au redressement, à la croissance secondaire sur écorce et à la croissance radiale sous écorce. Par ailleurs, la formation du bois de tension et l'excentricité réagissent au stimulus gravitationnel, sans pour autant être liés entre eux par une relation de cause à effet. Enfin, il existe une position naturelle d'équilibre des tiges, différente de la verticale, vers laquelle les pousses qui ont été déplacées, tentent de revenir en formant un tissu de bois de tension. stimulus gravitationnel / bois de tension / croissance / Populus euramericana cv 'Ghoy' Abstract -Effect of intensity of gravitational stimulus artificially induced on growth and tension wood formation in young shoots of poplar (P. euramericana cv 'Ghoy'). The formation of tension wood tissue is artificially induced by means of a gravitational stimulus obtained by inclining young shoots of poplar at different intensities, from 0 to 30°, during a growing season. At the end of the experiment, it appears that shoot elongation as well as the angle of tension wood arc are not affected by the stimulus intensity. In contrast, the upright reorientation of the axis, the amount of gelatinous fibres, the secondary growth over bark, the cross-section area of the stem and of the secondary xylem, the different rays, the diameter under bark and the pith eccentricity, react quantitatively to the lean intensity. The amount of tension wood is correlated with the upright reorientation, the secondary growth over bark and the radial growth under bark. Tension wood formation and eccentricity react to the gravitational stimulus but they are not causally linked. Finally, it exists a natural equilibrium position, different from the vertical, to which moved shoots try to return via the formation of a tension wood tissue. gravitational stimulus / tension wood / growth / Populus euramericana cv 'Ghoy'