Bacterial canker (Clavibacter michiganensis) is one of the most destructive diseases of tomatoes which is caused by Clavibacter michiganensis subsp. michiganensis (smith) formerly known as Corynebacterium michiganensis. This is a very serious disease and can survive in plant debris for long periods. The bacteria can also be carried via the seed, that is why it is so important to use seed from a reputable seed company. The disease attacks the plant at any stage of growth.

Condition for development of bacterial canker

  • Initial infestation can occur through stomata, leaf epidermis, leaf hair or root
  • Infected plant debris in the soil, weeds, other host plants and infected seed can be the source of infection.
  • Seed can carry the pathogen either on the inside or on the surface of the seed.
  • The bacteria can also spread through splashed water, contaminated implements, hands and feet of labourers and the trellising and pruning of plants.
  • Infection mainly occurs through damaged roots, transplanting and wounds from garden-shears or knifes.
  • The disease spreads rapidly in fast growing plants.

Bacterial canker symptoms

There is a large variation in symptoms or expressions that can be ascribed to the different races of the bacterial canker pathogen. Factors such as different cultivation practices, different stages of plant development, time of infection and the tolerance of the cultivar/variety.

Wilting of the leaflets is the first symptom on plants of all ages when bacterial canker sets in. On older plants the lower leaves start to wilt first, margins dry out leaves curl upwards and inwards. These leaves become brown and wither but remain on the plant. Often the disease develops on one side of the plant which will cause a single stem to die back. The disease spreads quickly thereafter. Bacterial canker infected plants may die early but they commonly survive until harvest.

The most common way to check for bacterial canker is to cut the stem lengthwise. If bacterial canker is present a creamy white, yellow or reddish brown line will be noted just under the woody tissue. As decay progresses, the pith becomes yellow and mealy in appearance and cavities are formed in the soft tissue. The discolouration of the pith and its ready separation from the woody tissue are a valuable means of identifying the bacterial canker.

In heavy infestation the bacteria will spread towards the fruits. The fruits are typically small and stunted and deformed. Fruit infected with the bacteria have dark cavities in the middle which renders the seed useless. Fruit are often infected by the bacteria through rain. These infections appear first as very small, snowy white spots that do not extend deeply into the fruit. The margins of the spots remain white and flat.. The centres become slightly raised and tan coloured and roughened by slight breaks in the surface. The spots look like birds eye’s. The spots stay the same size which is distinguishable from bacterial spot disease, however this method is not always that accurate.

The two main groups in which bacterial cancer of tomatoes can be divided.
Systemic wilting of the plant.
Downward turning of lower leaves
Marginal necrosis of leaflets
Wilting of leaflets (often unilaterally on a leaf)
Upward curling of the leaflet edges
First symptoms usually in the second or third tomato truss above the truss being harvested.
Bird's – Eye Spots:
Usually on fruit sized 2 – 5 cm although any fruit can show symptoms
Lesions with raised brown centres that are surrounded by an opaque white halo can appear on any part of the fruit but mainly on the exposed areas.
Bird's-eye spots are usually 3 – 6 mm in diameter.
In mature fruit the white halo disappears and necrosis develops.
Infected plants and fruit mostly do not reach maturity, and the fruit drop easily.
Marbling of fruit may occur in some plants and can be diagnosed early.
Vascular tissue of these fruits are associated with large areas of decay and bacterial excretions.
In serious cases even the developing seeds may appear necrotic with little cavities in the placenta
No symptoms can be seen.

Controlling backterial cancer

Bacterial Canker is a very difficult disease to control because of the ability of the organism to survive for long periods in the vicinity of the plant. It can also occur in low concentrations in plants without any symptoms. It is very difficult to trace infected plants because of the change in expression of symptoms. The bacterium has a very high ability to infect plants. The following guidelines should be followed:

  • Use only certified seed. Sealed seed doesn’t necessary mean certified seed.
  • If uncertain, treat seed in warm water (54°C) for 25 minutes.
  • Sterilize the seedling mixture, the seedling trays and the surface of the nursery.
  • When using a seedling bed, use soil that was never used for tomatoes before and sterilize the soil with methyl bromide.
  • Remove all infected plants.
  • A copper containing fungicide could be used.
  • All the infected plant debris from nurseries and tunnels should be burned or otherwise removed to a place where contamination is eliminated.
  • Sealed drums or covered garbage heaps can ease storage.
  • An infected nursery should be isolated until it is sterilized.
  • In any infected area, shoes, hands and implements should be disinfected before leaving the area.
  • Use Bacterial Canker resistant/tolerant cultivars.