2.2.15
INCIDENCE OF RALSTONIA SOLANACEARUM GROUPS ON A SINGLE POTATO FIELD DETERMINED BY TRNA CONSENSUS PRIMERS

JLN MACIEL, JRP SILVEIRA , ST VAN DER SAND and V DUARTE

Departamento de Fitossanidade, Faculdade de Agronomia, UFRGS, C.P. 776, Porto Alegre, RS 90001-970, Brazil

Background and objectives
Ralstonia solanacearum causes bacterial wilt (BW), which is a widely spread disease of potato in the tropics, subtropics and temperate regions of the world. Strains of R. solanacearum comprise a complex taxonomic group which has been subdivided informally into five races on the basis of the hosts primarily affected or five biovars on the basis of the catabolism of certain sugars and sugar alcohols. Two groups of strains included in races 1 and 3, equivalent to biovars 1 and 2, are able to infect potatoes. Polymerase chain reaction amplification of DNA from R. solanacearum strains with the tRNA consensus primers (T3A and T5A) divided the species into three fingerprint groups. Group I corresponds well with biovar 2 and the group II with biovar 1 [2]. Strains of biovar 2 usually have an narrow host range, being limited almost entirely to potatoes. They can remain latent in potato tubers, making their detection difficult and greatly increasing the risk of disseminating the pathogen via tubers used for planting [1]. Therefore, biovar 2 is considered a serious threat to potato cultivation in temperate zones, such as those of the Rio Grande do Sul State (RS), South of Brazil. In this study, we used tRNA consensus primers to discover the incidence of biovars 1 and 2 of R. solanacearum on six potato cultivars in a naturally infested field of RS.

Materials and methods
Tubers of the potato cultivars Achat, Baronesa, Elvira, Macaca, Monte Bonito and Trapeira were planted in a naturally infested field in Caxias do Sul, RS. Ten tubers were planted per plot (1x3 m). The experimental design was a randomized complete block with 10 replications. Wilted plants were harvested 45 and 65 days after planting. Seven isolates of R. solanacearum were chosen randomly among isolates from each cultivar after growing on SPA medium (sucrose-peptone-agar) containing 2,3,5-triphenyltetrazolium chloride, wilting tomato plants and giving positive reaction to polyclonal antiserum against R. solanacearum. The T3A and T5A primers were used to indicate the R. solanacearum group by PCR amplification [2].

Results and conclusions
Of 42 R. solanacearum isolates from a single potato field, four (9.5%) showed a group 2 and 38 (90.5%) a group 1 pattern. It confirmed that biovar 2 is predominant in the RS conditions. Moreover, it showed also that both biovar 1 and 2 may occur in the same field. Biovar 1 was present on cvs Achat (resistant) and Macaca along with biovar 2. The ratio of biovars 1 and 2 was 2:7 in both cultivars. However, only biovar 2 was identified on cvs Baronesa, Elvira, Monte Bonito and Trapeira (susceptible) cultivars. A possible correlation among cultivar, resistance, and biovar may be worthy of further study.

References
1. Maciel JLN, van der Sand ST, Duarte V, 1997. Proceedings of 2nd International Bacterial Wilt Symposium, pp. 77.
2. Seal, SE, Jackson, LA, Daniels, MJ, 1992. Applied and Environmental Microbiology 58, 3759-3761.