Transmission and movement of potential Citrus Tristeza Virus cross-protection sources in four soft citrus cultivars under greenhouse conditions

Abstract

Citrus Tristeza Virus (CTV), a member of the Closteroviridae family, is a globally distributed pathogen and is responsible for serious losses in citriculture. Infection with certain strains of the virus is associated with economically important diseases such as stem pitting, decline and a reduction in fruit yield, fruit size and quality. In countries where the disease and its effective aphid vector are endemic, cross-protection by the deliberate infection of virus-free propagation material with mild strains of the virus is the only practical method that can be applied to reduce the severe effects of the virus. The transmission efficiency, movement and effect on growth of four potential CTV cross-protection sources were evaluated in four soft citrus cultivars in a glasshouse experiment. The CTV sources were derived from sweet orange (Citrus sinensis L. Osb.) (SM46, SM47, SM48, SM49). The cultivars used were ‘Clemenluz’ and ‘Esbal’ clementines (C. clementina Hort. Ex Tan.), ‘Valley Gold’ mandarin (C. reticulata blanco) and ‘Morr’ tangor (C. reticulata Blanco X C. sinensis L. Osb.). The effects of the four CTV sources were compared to that of LMS6 (previous pre-immunising CTV source) and GFMS12 (current pre-immunising source) in each cultivar. Control plants were not pre-immunised. The results showed that the LMS6 source moved slowly, if at all, and was only detected sporadically in some plants of the clementine cultivars but not at all in the mandarin and tangor cultivars at different times during a 28-week period. These results preclude LMS6 as a pre-immunising source for soft citrus. Virus-free soft citrus plants will not easily acquire and maintain this source. Virus movement for LMS6 was shown to be slow, which will likely result in new shoots being virus-free and vulnerable to re-infections by various strains of CTV introduced by aphids in the field. The SM49 source however, moved the fastest and was detectable in all the cultivars tested. GFMS12, the present pre-immunising source for soft citrus moved slower than SM49. This may again result in virus-free shoots during active growing periods of trees which will make the introduction of severe natural strains possible. GFMS12, and to a lesser extent SM49, reduced the growth of the ‘Morr’ cultivar significantly