01384naa a2200169 a 450000100080000000500110000800800410001910000160006024500980007626000090017452008850018365300130106865300230108165300250110465300150112977300700114416468531995-08-25       1977    bl ---       0--  u    #d1 aNAVARRO, L.  aCitrus virus diseases in spain in relation to plant productionbpresent and future prospects.  c1977  aThe outbreak of tristeza in Spain forced the change of the traditional sour orange rootstock for tristeza tolerant rootstocks. Then it was disclosed that almost all the Spanish citrus varieties were infected with exocortis, psorosis,xyloporosis, and other virus and virus-like diseases that restricted the use of many rootstocks. Consequently, the necessity arose for importing `virus-free' foreign varieties. In this paper we discuss the problems caused by this situation and the research program established to solve them. This program includes all the varieties commercially grown in Spain and it is directed to obtain `virus-free' old line plants by the techniques of shoot-tip grafting in vitro and thermotherapy, and nucellar plants, by the techniques of seedlingselection and nucellus plants, by the techniques of seedling selection and nucellus and ovule culture in vitro.  aIn vitro  aShoot-tip grafting  aVariety improvements  aVirus-free  tReprinted from Proc. Int. Soc. Citriculturegv.1, p.136-140, 1977