Instituto Valenciano de Investigaciones Agrarias (IVIA). Apartado Oficial, Moncada, VALENCIA. 46113 SPAIN

Background and objectives
Systemic infections of Agrpbacterium tumefaciens have been described in grapevine, chrysanthemum and some herbaceous hosts. Recently, they have been reported in rose plants and this suggests that such infections may occur in other hosts. Systemic movement and latent infections could increase the difficulties in controlling this disease and could favour its dissemination. The objectives of this work are to develop sensitive techniques for detecting small bacterial populations in plants and to compare the migration of several strains of A. ;tumefaciens in different hosts.

Materials and methods
Grapevine, chrysanthemum, rose, cherry and peach X almond hybrid GF677 plants were inoculated in the crown with four strains of A. ;tumefaciens isolated from different hosts (biovars 1, 2 and A. ;vitis). After inoculation, the plants were wounded at different sites in the stem to monitor the appearance of secondary tumours. Several techniques were used to detect pathogenic Agrobacterium in the stems outside the tumours. Direct isolation and PCR were performed with the comminuted samples before and after selective enrichment in appropriate selective media. Precautions were taken after inoculations and when performing isolations to avoid external contaminations and detect only endophytic bacteria. The same techniques were applied to detect systemic migration of Agrobacterium in naturally infected walnut plants.

Results and conclusions
Detection of systemic and latent infections of A. ;tumefaciens in plant material can be achieved by using isolation and PCR with or without selective enrichment. The most sensitive technique was DNA amplification, especially after an enrichment step.

Comparative migration in the different hosts, studied by the secondary tumors appeared in the wounds, showed that they appeared more frequently in rose, followed by grapevine, hybrid GF677, chrysanthemum and cherry. The comparative migration of Agrobacterium in the hosts studied showed no clear differences among strains.

The detection of pathogenic Agrobacterium in naturally infected walnut plants confirmed the existence of migration in many of the plants analysed using the techniques described above. These results enlarge the number of plant species where the movement of Agrobacterium through the xylem has been demonstrated and suggests that this bacterium could also move inside other hosts. The translocation of A. ;tumefaciens must be taken into consideration when making a sanitary selection in an integrated control of the disease.