Clavibacter michiganensis is a bacterium that causes a bacterial disease known as bacterial canker of tomato. In this technical factsheet, we will describe the identification, damage, and management of this plant disease.
Clavibacter michiganensis subsp. michiganensis is the causal agent of bacterial canker of tomato. This disease is among the most severe bacterial diseases affecting tomato, together with the other two major bacterial diseases: bacterial spot of tomato and bacterial speck of tomato.
This bacterial disease causes wilting of the infected plant, as the bacterium colonizes the vascular tissue, leading to blockage and impairment of water transport. By cutting the stem or leaf petiole longitudinally, brown discoloration of the vascular tissue can be observed.
On the stem, the disease produces longitudinal, discolored lesions, which may evolve into deep cracks.
More characteristic are the fruit symptoms caused by bacterial canker. On immature fruits, the disease induces very small necrotic spots surrounded by a distinct white halo (see image above). Another typical symptom is the one-sided wilting or desiccation of the plant, due to pathogen movement through infected xylem vessels, resulting in localized damage on the side where infection occurred. In some cases, the disease may begin with the wilting of a single leaf; however, as the disease progresses, it eventually leads to general plant collapse and death.
In some cases, fruit symptoms may be asymptomatic externally. This occurs when the bacterium enters the fruit through the vascular system and causes internal pulp degradation, which becomes mucilaginous and takes on a yellowish coloration. These symptoms can only be detected by cutting open suspect fruits.
High relative humidity and temperatures between 24 and 28 °C favor the development of Clavibacter michiganensis. The bacterium survives in the soil and enters plants through wounds and micro-injuries caused by heavy rainfall, hail, and strong winds.
At the end of the article, you will find a video of the disease with the option to enable Italian subtitles.
Prevention and management of bacterial canker of tomato
There is no effective curative treatment for this disease; management is therefore primarily preventive. Control measures include the removal and destruction of infected plants at the first appearance of symptoms, avoiding the incorporation of infected residues into the soil.
Because this bacterium persists in the soil, crop rotations of 4–5 years are essential, keeping in mind that wild species of Solanum spp. can also act as pathogen reservoirs.
Copper-based treatments are useful only for limiting bacterial spread, since the pathogen colonizes internal vascular tissues. Consequently, copper salts should be applied when bacterial exudates are produced from cankers; if left uncontrolled, these exudates increase the inoculum potential and further spread Clavibacter michiganensis. Copper salts have a bacteriostatic effect against this disease.
This bacterium is also capable of infecting the seed embryo, in addition to the external seed coat. Therefore, the use of certified, pathogen-free planting material is the cornerstone of prevention.
Currently, tomato cultivars resistant to this pathogen are available, making it possible to select nursery plants based on their susceptibility to bacterial canker.