Abstract
The control of Citrus tristeza virus (CTV), the causal agent of tristeza disease, is essential to guarantee a productive citrus industry. The specific features of a nursery block make the control of the natural spread of viruses transmitted by vectors more difficult. Thus, the knowledge of the epidemiology of CTV in nursery blocks is basic to the design control strategies of the disease. Two experimental nursery plots of alemow (Citrus macrophylla) were planted in open field in Moncada (Valencia, Spain) (with a high CTV prevalence, 85%), and in Alcanar (Tarragona, Spain) with 700 and 843 nursery plants, respectively. The plants were analysed by tissue print-ELISA using 3DF1 and 3CA5 CTV-specific monoclonal antibodies. The CTV prevalence estimated after one year of cultivation in both plots was of 24.37% and 3.91%, respectively. At the same time, a commercial nursery plot grown under plastic-net covers established in Alcanar with 1,200 plants was also analysed as above, being the CTV prevalence estimated of 0.00%. The aphid activity present in the nursery plots was monitored. Aphis spiraecola was the most abundant aphid species visiting the plants in the nursery plots grown in the open field, whereas the aphid activity registered in the plants grown under the plastic-net covers was practically zero. The percentage of individual aphids carrying CTV PCR-amplifiable targets detected by squash real-time RT-PCR in the open-field nursery plots was 17.5% in Moncada and 1.67% in Alcanar. No significant differences in the CTV prevalence between treated and non-treated plants with horticultural mineral oils were found in the area with the high CTV-inoculum pressure.
Highlights
Tristeza is the most important virus disease that affects citrus species (Moreno et al, 2008)
The natural Citrus tristeza virus (CTV) prevalence estimated in Fields 2 and 3 were much lower at 3.91% (33/843) in the experimental plot grown in the open field and 0.00% (0/1,200) in the plot grown under the plastic nets
The percentage of CTV-viruliferous aphid individuals of A. spiraecola (18.75%) and A. gossypii (15.00%) that landed in the experimental nursery plot located in the area with the high CTV inoculum pressure (Field 1) was much higher than that estimated in the area with the low CTV inoculum pressure (Field 2) (1.67% for both aphid species, respectively)
Summary
Tristeza is the most important virus disease that affects citrus species (Moreno et al, 2008). The main pathway of virus introduction (entry, establishment and dispersion) in new regions is the movement of infected plant material, followed by the local spread by aphid species in a semi-persistent manner. Toxoptera citricida Kirkaldy is the most efficient CTV aphid vector species (Yokomi et al, 1994), Aphis gossypii Glover is the main CTV vector in the absence of T. citricida (Cambra et al, 2000a; Yokomi & DeBorde, 2005). The epidemiology of CTV in adult trees is well known in Spain (Gottwald et al, 1996; Cambra et al, 2000a; Marroquín et al, 2004). The high density of plants (50,000-100,000 plants ha–1) and the constant presence of juvenile and succulent shoots facilitate the natural infection and spread of CTV, making the CTV control more difficult
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