Clavibacter is a quarantine organism in Europe, which means that strong efforts are used to prevent the bacterium from entering. When the occasional infection is found, strict measures are taken to eradicate the infection. In tomato, growers must either destroy the infected crop entirely or, if the crop is old, only the plants in the infected area, with a substantial buffer zone. Replanting can only take place after thorough disinfection has been proven to the authorities. Potato growers lose their entire crop of the affected field and potatoes from other fields on the farm, are labelled as ‘suspect’ that year. On the infected field no potatoes or other Solaneceaous crops can be planted for five years. On other fields on the same farm, no seed potatoes can be grown for one year after infection.
Life cycle and appearance of Bacterial canker of tomato
Clavibacter is seed-borne and tuber-borne and primary infection is mostly from infected propagation material. It spreads to other seedlings by touching the plants in greenhouse crops and by water in field crops, for example through splashes of rain or irrigation. Both in greenhouses and in the field, spraying may also spread the bacteria. The bacteria can also be dispersed by machinery, crates and tools. The bacteria infect the new plants by entering through wounds and then growing into the vessels of the plant. Plant growth is inhibited and the plant wilts because the xylem vessels become blocked. The bacterium can survive some time in crop residues like volunteer tubers and potato crop residues. Infected tubers can infect healthy tubers during post-harvest storage.
Clavibacter causes spots on stems, leaves and fruits. Leaves and shoots wilt. On the lower leaves, blister-like white lesions occur, which later turn brown and may merge. The leaves wilt and curl upwards, often starting at one side of the plant. They turn brown but don’t fall off. On stems and shoots light stripes form, which then tear, allowing cankers to form. On tomato leaves, water-soaked lesions occur, usually white with a brown centre.
In potato, the disease usually only occurs in the second half of the season. The first symptoms are wilting of the lower leaves and subsequent expansion of the wilt upwards. Cutting the stem at the foot shows whitish ooze with bacteria in the case of Clavibacter infection, but other than that the disease is difficult to distinguish from wilting caused by other pathogens.
On potato tubers, first discolouration of the vessel tissue appears, especially when cutting the tubers. This discolouration is round, hence the name ring rot.