Crown gall in organically grown UK tomato caused by tumorigenic strains of Agrobacterium radiobacter

Crown gall is caused by Agrobacterium strains harbouring a Ti-plasmid (pTi). Disease symptoms are initiated by the transfer of a DNA fragment (T-DNA) located on pTi into the genome of recipient plant cells. In transformed plant tissues the T-DNA encodes genes that induce gall formation at the infection site.

In June 2005 individual plants in an organic tomato crop (cv. Dometica) exhibited characteristic crown gall symptoms. Plants had been grafted onto a rootstock (cv. Beaufort). Galls were around 3-5 cm in diameter and occurred at the graft but also above and below this union (Fig. 1), with some occurring up to 30 cm up the stem. Tomato is known to be experimentally susceptible to crown gall, however to the best of our knowledge there have never been any reports of crown gall in commercial UK tomato crops. Enquiries in Australia and the USA also suggest natural crown gall infections in tomato are extremely rare and in the most recent APS Compendium of Tomato Diseases (Jones et al., 1991), crown gall is not listed as a disease. In major culture collections one tumorigenic Agrobacterium strain (the French CFBP 296 strain) is reported to have been isolated from tomato.

Agrobacterium strains were isolated from galls using semi-selective Schroth's medium and identified by fatty acid profiling (Stead et al., 1992) as Agrobacterium biovar 1 strains (A. radiobacter). A PCR assay (Pulawska & Sobiczewski, 2005) showed these strains possessed pTi. Two of these strains (CSL 5794 and 5801) were suspended in phosphate buffer to an approximate concentration of 10⁸ cfu per ml and inoculated into tomato plants (cv. Moneymaker), by injecting each suspension into wounds made on the stem. A negative control plant (buffer only) was also prepared in the same manner. Typical galls formed at the inoculation sites on both Agrobacterium inoculated plants within two weeks of inoculation (Fig. 2), and pTi-harbouring A. radiobacter strains were re-isolated from these galls.

The affected tomato plants were located in a glasshouse previously affected with another Agrobacterium-induced disease, known as root mat and caused by strains harbouring a root-inducing Ri-plasmid (Weller et al., 2000). Between-season amendment of soil with composted green waste eradicated root mat in this glasshouse. Ri-plasmid harbouring strains were not detected in the crown gall affected crop.