Detection of pathogenic agrobacteria by PCR in different soils.

Agrobacteria are able to live as saprophytes in natural and cultivated soils. Isolation of non pathogenic agrobacteria is easily performed by dilution plating method while this technique is not adequate to detect pathogenic forms. Survival and persistence of pathogenic agrobacteria in soil is poorly documented since a reliable detection procedure is not available so far. A rapid and specific procedure to detect pathogenic agrobacteria in soil was used in this study. A high incidence of crown gall disease on stone fruit rootstocks was observed in a nursery field not far from Naples, Italy. After plant uprooting, Eruca sativa, Zea mais and GF677 rootstocks were transplanted in that field in order to study the survival and the dinamic of natural pathogenic agrobacteria in rhizospheric and nonrizospheric soil of host and non-host plants. Soil samples were repeatedly collected during the cultural cycle of the three plant species and were processed both by dilution plating method and by PCR after a rapid DNA extraction procedure. A total of 3000 agrobacterium-like colonies were isolated with selective media from rhizosphere and non rhizosphere soils. All of them were inoculated into tomato plants and only two resulted pathogenic. However, all soil samples were positively amplified by PCR. These results show that the procedure based on PCR may be very useful to detect the presence of virulent agrobacteria potentially dangerous in the soil. PCR analysis was successful and sensitive both in sandy and clay soils.