La verse parasitaire peut être provoquée par de nombreux parasites (pyrale, champignons, etc.). Pour l'enrayer, il faut lutter contre l'agent responsable.
Verse mécanique.
La verse mécanique peut affecter d'autres cultures que celles de céréales (cultures du colza, du tournesol). Elle peut elle aussi compromettre gravement le rendement.
Chaillou
versoir
Pièce travaillante des charrues servant à retourner la terre.
Les formes des versoirs influent sur le type de labour en fonction de la texture et de l'humidité du sol. Certains sont en forme d'hélice (versoirs hélicoïdaux) pour accompagner la bande de terre dans son retournement. Découpés dans un cylindre (versoirs cylindriques), ils ont une action d'émiettement plus intense sur une terre sèche. Ils présentent souvent une partie avant cylindrique et, à l'arrière, une surface gauche plus ou moins hélicoïdale (versoirs mixtes).
Aubineau
vert de betterave
Ensemble formé par les feuilles et les collets qui restent sur les champs après la récolte des betteraves sucrières.
Roger-Estrade
verticilliose
Maladie de nombreuses plantes, cultivées ou non, herbacées ou ligneuses, due principalement aux champignons Verticillium albo-atrum et Verticillium dahliœ.
Les Verticillium vivent dans le sol ; ils pénètrent dans les plantes par l'intermédiaire de petites blessures des racines. Ils se développent dans les vaisseaux du bois (trachéomycose) qu'ils obstruent, empêchant la circulation de la sève (en coupe transversale, les racines présentent un anneau brunâtre correspondant aux vaisseaux atteints). À partir des racines, les champignons gagnent progressivement l'ensemble des organes aériens.
Symptômes.
Les plantes atteintes présentent d'abord sur les feuilles des taches brunes ou jaunes, puis le feuillage et les rameaux se flétrissent et, enfin, se dessèchent. Chez un certain nombre de plantes (pomme de terre, tomate, melon), ce dessèchement s'extériorise souvent sur une seule moitié de la feuille ou de la foliole, avant de gagner l'ensemble du feuillage. Selon les cas, quelques plantes sont atteintes, ou bien des champs entiers (luzernières par exemple).
Lutte.
Dans la majorité des cas, il est impossible de sauver un végétal atteint de verticilliose. Les seules mesures envisageables sont d'ordre préventif. Il faut s'assurer de mettre en place, dans un sol sain ou désinfecté, des plants en bonne santé, et choisir de préférence des variétés résistantes.
Raynal
vertisol
Sol argileux dont le comportement est déterminé par la nature minéralogique de l'argile.
L'argile est gonflante ou semi-gonflante : elle a la propriété de gonfler à l'état humide, pour se rétracter fortement à la dessiccation. Ce comportement particulier (caractère vertique) entraîne une forte fissuration du sol à l'état sec créant d'importantes fentes appelées fentes de retrait ; ces fissures disparaissant lorsque le sol se réhydrate. L'ensemble des horizons de surface (Av et Sv) a une teneur en argile supérieure à 40 %. Les fentes de retrait apparaissent dès le début de la saison sèche, puis se développent en profondeur en s'élargissant. En surface, le sol se fragmente fortement, et les fins agrégats ainsi créés peuvent tomber dans les fentes : ce phénomène entraîne un mélange des différentes parties du sol (pédoturbation). À la saison des pluies, l'eau s'infiltre préférentiellement dans les fentes de retrait, ce qui provoque un gonflement localisé des argiles ; les fortes pressions qui en résultent peuvent causer des dégâts considérables aux infrastructures : effondrement des routes, de pylônes, d'habitations. Lorsqu'ils sont très humides, les vertisols peuvent devenir fluents et causer des glissements de terrain.
Si les propriétés chimiques sont très favorables, les propriétés physiques, en revanche sont souvent moins bonnes : le travail du sol est difficile, le sol s'assèche rapidement en saison sèche alors qu'en saison humide il devient asphyxiant. Mais ces propriétés défavorables sont considérablement atténuées lorsque se forme en surface un horizon structuré grumeleux (horizon dit de « self-mulching »), protecteur des horizons sous-jacents. Les vertisols comptent ainsi parmi les sols les plus fertiles des régions chaudes.
MCGirard
verveine
Plante dont on cultive certaines espèces (verveine odorante, verveine commune) pour leurs propriétés médicinales et d'autres pour leurs fleurs ornementales (genre Verbena, famille des verbénacées).
Deux espèces de verveines sont cultivées pour l'herboristerie. La verveine odorante (Verbena triphylla ou Lippia citriodora), encore appelée verveine citronnelle, est souvent cultivée dans les jardins du midi et de l'ouest de la France. Ses feuilles sont vendues pour fournir une infusion digestive. La verveine commune (Verbena officinalis), appelée autrefois herbe à tous les maux, herbe sacrée, guérit-tout, herbe aux sorciers, est aujourd'hui utilisée pour ses vertus digestives, apéritives, antirhumatismales et anti-talgiques.
Poisson
vesce
1. Légumineuse fourragère annuelle, dont les feuilles à nombreuses folioles sont terminées par une vrille simple ou ramifiée et présentent une nervure principale très nette (genre Vicia, famille des papilionacées). 2. Graine de cette plante.
Quatre espèces de vesces présentent un intérêt fourrager particulier : la vesce cultivée ou vesce commune (Vicia sativa), la vesce de Pannonie ou vesce de Hongrie (V. pannonica), la vesce de Cerdagne (V. varia) et la féverole (V. faba).
Vesce commune.
La vesce commune possède des feuilles primaires à une paire de folioles et des feuilles secondaires composées de 5 à 7 paires de folioles entières, à sommet tronqué, et dont l'axe est terminé par une vrille ramifiée. Les stipules portent des nectaires (organes producteurs de nectar) de couleur brun foncé ou noire. Les fleurs, d'un pourpre violacé, sont solitaires ou groupées par deux à l'aisselle des feuilles et portées par des pédoncules très courts. Les graines, de 3 à 3,5 mm de diamètre, sont quasisphériques, de couleur grise, brune, verte ou blanche, à ornementations gris brunâtre. Le poids de 1 000 grains est compris entre 45 et 120 g.
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