Acquisition of an Agrobacterium Ri Plasmid and Pathogenicity by Other -Proteobacteria in Cucumber and Tomato Crops Affected by Root Mat

Root mat of cucumbers and tomatoes has previously been shown to be caused by Agrobacterium radiobacter strains harboring a root-inducing Ri plasmid (pRi). Nine other pRi-harboring -Proteobacteria have subse-quently been isolated from root mat-infected crops. Fatty acid profiling and partial 16S rRNA sequence analysis identified three of these strains as being in the genus Ochrobactrum, five as being in the genus Rhizobium, and one as being in the genus Sinorhizobium. An in vitro pathogenicity test involving inoculation of cucumber cotyledons was developed. All pRi-harboring -Proteobacteria induced typical root mat symptoms from the cotyledons. Average transformation rates for rhizogenic Ochrobactrum (46%) and Rhizobium (44%) strains were lower than those observed for rhizogenic A. radiobacter strains (64%). However, individual strains from these three genera all had transformation rates comparable to those observed from cotyledons inoculated with a rhizogenic Sinorhizobium strain (75%). Since the early 1990s, hydroponic cucumber and tomato crops in the United Kingdom and some other European coun-tries have been affected by a disorder known as root mat. In affected crops symptoms are expressed as extensive root pro-liferation within rockwool cubes and slabs with, in severe cases, losses in marketable yield. An earlier outbreak of root mat in the 1970s, in soil and straw bed cucumber crops, had been associated with the presence of Agrobacterium biovar 1 strains (Agrobacterium radiobacter). In the late 1990s, surveys of af-fected crops and subsequent host tests with Agrobacterium strains isolated during the surveys showed that the disease in cucumbers, and a similar disorder in tomatoes, was caused by A. radiobacter strains harboring a root-inducing Ri plasmi