Tristeza in Corsica and its eradication

Citrus tristeza virus (CTV) was found to infect citrus trees in Corsica on three occasions. The first instance concerned CTV strain K123 infecting two of three old line 'Marumi' kumquat trees on rough lemon imported from Morocco in 1959. The trees indexed negatively for CTV on Mexican lime in 1963. In 1981, trees T22 and T24 were found positive for CTV by ELISA. Strain K123 is now well known for producing no symptoms in Mexican lime. Orchards propagated from the three kumquat trees contained 25% to 71% CTV-infected trees. No spread of this CTV line has ever been observed. By 1995, all these kumquat orchards had been pulled out. The second instance concerns CTV-infected Calamondin (#Citrus madurensis#) trees that were discovered in the spring of 1994, South of the Bastia airport, in three calamondin nurseries by the regional plant protection services. The Calamondin line has probably been introduced from Florida, 15 years ago. Two of the nurseries had plots for the production of marcott trees; they provided also budwood for graft-propagation of calamondin trees in the third nursery. ELISA and direct tissue immunoprint were used to identify the infected trees in the nurseries, as well as the surrounding orchards. Of 8557 calamondin trees, 612 (7.1%) were found infected.. A few CTV-infected lemon trees were also detected in the nurseries( 39 of 10,512, i.e. 0.4%). The trees were in fact calamondin trees that had been topworked with lemon buds. Clementine, grapefruit and sweet orange trees of all 17 orchards near the nurseries were also analysed for CTV. Only one 30 year-old clementine orchard totalling 848 trees revealed 8 infected trees on repeated indexing in 1995, and one additional tree in 1996. This orchard was within 200m of one the infected calamondin marcott plots. Here, infection is probably through aphid transmission. The number of infected trees over the total number of trees tested was 408/11331 in 1994, 249/19019 in 1995, and 3/24282 in 1996. All infected trees have been removed. Additional surveys during the following years did not reveal any other infected plants in this area. A third tristeza focus was discovered at the end of 1997 at Ghisonaccia, located South of the oriental plain about 70 Kms from the previous spot, in two clementine orchards belonging to the same grower. The first orchard, planted in 1968, had 733 trees on sour orange of which 10 trees were infected. The second orchard had 895 trees on Trifoliate orange or citrange. Two thirds of the trees were planted in 1973 and contained 22 infected trees. In the other third, the trees were planted in 1996 and 4 were infected, suggesting insect transmission. All infected trees have been removed. In 8 nearby clementine orchards totalling 5100 trees, planted between 1966 and 1986, no infected tress were detected. (Résumé d'auteur)