Estimation of the Number of Citrus tristeza virus-Viruliferous Aphids Landing on Individual Citrus Seedlings and Viral Incidence in Different Citrus Rootstocks in Spain

The number of aphid species landing, from January to June 2004, on alemow, Cleopatra mandarin and Carrizo citrange grown in experimental nursery blocks in the same area in the north of the Valencian Community, were estimated by counting the number of young shoots/plant and the number of aphids trapped on sticky shoots. The proportions of different aphid species represented by captured individuals (10,569) were: Aphis gossypii (34.9%), Myzus persicae (12.7%), A. spiraecola (8.6%), A. craccivora (2.4%), A. fabae (0.8%), Toxoptera aurantii (0.8%) and others (39.9%). C. macrophylla was the most visited citrus species with estimated 1,656 aphids landing/plant during the test period. May was the month with higher aphid populations. The number and percentage of aphids carrying Citrus tristeza virus (CTV) was determined by a validated nested-PCR method in a single closed tube using extracted RNA from previously trapped aphids that were squashed on paper. Viral RNA was detected in 13.3% of tested A. gossypii individuals. CTV infection rates assessed by tissue print-ELISA in experimental plots located in the same area were proportional to the number of aphids landing on each susceptible host. Average annual CTV infection incidence ranged from approximately 4% for alemow and Volkamer lemon to 2% in Cleopatra mandarins. Nevertheless, despite the number of CTV viruliferous vectors visiting Carrizo citrange, sour orange and citrumelo, CTV incidence was only 0.6%, 0.2% and 0%, respectively, suggesting some resistance to natural (aphid-vectored) infection in these particular citrus seedlings