Evaluation of the protective capacity of new mild Citrus tristeza virus (CTV) isolates selected for a preimmunization program

Abstract

The use of tolerant rootstocks and preimmunization has satisfactorily controlled losses associated with the Citrus tristeza virus (CTV). Several researchers have shown that CTV mild isolates that are selected in the same region where they are used are superior to isolates obtained from other areas. Thus, budwoods of 20 outstanding citrus trees were collected in north and northwestern Paraná state (Brazil) citrus-producing areas and established to be used in a preimmunization program. These budwoods were tested to evaluate the potential protection of the inherently present viral complex. Based on biological indexing and molecular characterization of the capsid protein gene by RFLP (restriction fragment length polymorphism, which indicated that the plants were infected with mild isolates of CTV), some of the selected plants could be used in a preimmunization program. These potentially mild and protective isolates were challenged with severe 'Rolândia' isolate inoculations by grafting and by the brown citrus aphid (Toxoptera citricida Kirkaldy) vector, which was faster in transmitting the virus. Some isolates had a better protective value than others, particularly when challenged with the severe CTV isolate. The SSCP (single strand conformational polymorphism) molecular analysis was an excellent complementary tool for monitoring the performance of the experiments and the stability of the viral complex present in the plants.Isolate number 1, collected in the municipality of Cruzeiro do Sul (CS-1), was the most promising for protecting commercial Pêra sweet orange (C. sinensis L.) orchards in northern and northwestern Paraná. The Rolândia severe CTV isolate was stable and had a high genetic divergence among the severe isolates used as a control (Capão Bonito and Barão B) and all of the isolates tested.

Stability of Citrus tristeza virus protective isolate 'Pêra IAC' according to SSCP analysis of old and new lines of three sweet orange varieties

“…Random and simultaneous inoculation of different isolates into different parts of a tree would prevent predominance of only one isolate, leading to a stable complex with a new molecular pattern, which could be different to that in the first tree. This was observed by Souza et al (18) and Zanutto et al (22) and suggests competition between the different haplotypes of the isolates that infected the plant. The electrophoretic profile of the isolates obtained from STG&PR in the field (plants 11 and 12) were identical to those observed in the mild control 'Pêra IAC' budwood trees (Fig.…”

“…However, only Pêra sweet orange showed poor performance in relation to tristeza due to its greater susceptibility to CTV. Evaluating the protective capacity of new mild CTV isolates used in Paraná State, Zanutto et al (22) found that SSCP molecular analysis was an excellent complementary tool for monitoring the performance of the experiments and the stability of the viral complex present in the plants. In Paraná State, tristeza symptoms and SSCP analysis of Pêra Vacinada 58B and 59B, grown from clonal propagations of 'IAC Pêra' immunized material from São Paulo, suggested that the 'PIAC' protective complex in Pêra orange has been kept stable for over six years under field conditions (21).…”

Agronomic performance of Pera and related sweet orange accessions naturally infected with Citrus tristeza virus in northern Paraná State, Brazil

“…In the State of Paraná, Brazil, changes have been identified in the CTV protective complex in use and also the presence of severe strains of the virus in different accessions of Pera sweet orange (Carraro et al, 2003;Costa et al, 2010;Corazza et al, 2012;Zanutto et al, 2013). Therefore, accessions of cross-protected mother trees introduced from other regions of Brazil or naturally infected selected trees may not perform well in the presence of the citrus tristeza disease in Paraná (Corazza et al, 2012;Zanutto et al, 2013).…”

“…Therefore, accessions of cross-protected mother trees introduced from other regions of Brazil or naturally infected selected trees may not perform well in the presence of the citrus tristeza disease in Paraná (Corazza et al, 2012;Zanutto et al, 2013).…”

Citrus tristeza disease severity and agronomic performance of ‘Pêra’ sweet orange cultivars and selections in the North and Northwest regions of the State of Paraná, Brazil

“…Changes in the CTV protective complex and the presence of severe strains of the virus have been reported in the State of Paraná, Brazil (CaRRaRO et al, 2003;COSTa et al, 2010;CORaZZa et al, 2012;ZaNuTTO et al, 2013). Consequently, the use of a single mild strain in a cross protection program for 'Pêra' sweet orange may represent a high risk (mÜlleR et al, 1999).…”