Bacterial Canker - Symptoms and Some Factors Affecting its Incidence

Table of Contents

1. Introduction
2. Recognition of Bacterial Canker
   1. Fruit Mottling
   2. Wilting and Leaf Scorch
3. Time between Infection and Appearance of Symptoms
4. Sources of Inoculum
5. Survival of Bacterial Canker in the Field
6. Conclusion

Introduction

Bacterial canker, a very contagious disease in tomatoes, is caused by a bacterium called Clavibacter michiganense subspecies michiganense. Despite the great amount of effort that greenhouse tomato growers often invest in sanitizing structures between crops and in maintaining sanitation practices, some growers may still encounter this disease in their crop. This article presents information that is based on years of research carried out in Canada and the US, and it describes some of the symptoms and a few factors that relate to survival and spread of bacterial canker.

| Top of Page |

Recognition of Bacterial Canker

Symptoms vary with age, environmental conditions, and vigour of the plant. When infection occurs via the seed or through wounds into the vascular system, systemic symptoms occur and these usually consist of wilting. If infection occurs through broken hairs or through natural openings on the plant, localized symptoms such as marginal necrosis and leaflet spotting may appear first. However, such infections can also become systemic when the bacteria find their way into the vascular system. Some typical symptoms of bacterial canker in greenhouse crops follow.

Fruit Mottling

Earliest symptoms in greenhouse tomato crops differ from those reported for field tomatoes. Work at the Research Centre in Harrow in 1966 (Layne & Rainforth) established that one of the earliest symptoms of bacterial canker may be seen in the fruits. Such fruits may appear mottled and the degree of mottling depends on the size of fruit when it became infected. Moderately affected fruit show islands of green tissue surrounded by whitish areas. More severely affected fruit appear rough with white 'ridges' that run along the length of the fruit (see Fig. 1). Infected fruits tend to ripen later and are less uniform in colour. As moderately affected fruits ripen, the mottling becomes less noticeable and they appear very similar to healthy fruits. During summer conditions, such symptoms are less visible.

Figure 1 - Mottling in fruits of plant infected with Bacterial Canker

Wilting and Leaf Scorch

Under winter conditions of low light intensity and short days, systemically infected plants may start to wilt on one side of the plant and then die. On the other hand, during the summer months, plants may barely wilt. More often seen are tan to brownish, scorch-like spots on the leaves (see Fig. 2). Such damage could be mistaken for damage due to hot water or chemical burn.

Figure 2 - Tan, scorch-like spots on leaves due to Bacterial Canker

| Top of Page |

Time between Infection and Appearance of Symptoms

The time between initial infection and appearance of symptoms is quite varied. Delays of 4-6 weeks and as long as 3 months between first outbreaks and secondary spread to adjacent plants have been observed in the past. Other observations have shown that the time between infection and appearance of symptoms can vary from 10-34 days. Cooler temperatures, older plants, low levels of bacteria all increase the time between infection and appearance of symptoms, and decrease the severity of such symptoms. Optimum temperature for growth of the bacteria ranges from 24-270C.

| Top of Page |

Sources of Inoculum

It is believed that windblown contaminated dirt and debris from outside into greenhouses can cause infections in greenhouse crops. Based on work out of Harrow, McKeen observed that in 1966 and in 1970, epidemics in fall greenhouse crops were preceded by heavy rainstorms accompanied by high winds. Sand and debris blown into greenhouses likely provided the inoculum from infected fields to the greenhouse crops. Besides infected field tomatoes, other sources of canker bacteria include populations that survive and multiply on the leaf surfaces of other solanaceous crops and weeds (e.g. pepper, tobacco, potato, jimsonweed, black nightshade) and of some non-solanceous weeds (e.g. goosefoot and cocklebur).

| Top of Page |

Survival of Bacterial Canker in the Field

In the absence of a suitable host in unsterilized soil, the bacteria do not survive for more than 3-4 weeks. However, within plant debris, survival is much longer. In a study at Iowa State University reported in 1991, it was found that the bacteria survived for at least 24 months in infested debris at the soil surface, but for only 7 months in buried debris. The buried debris underwent more rapid physical breakdown than at the soil surface, thereby exposing the canker bacteria to attack by other soil microorganisms. Generally, survival of the canker bacteria outdoors is facilitated by cool temperatures and by the presence of plant tissue. Research has shown that populations of canker bacteria decline relatively slowly in frozen soil compared with warm soil.

| Top of Page |

Conclusion

To minimize infections due to bacterial canker, here are some tips:

• Prevent entry of dirt from outdoors into the greenhouse whether via shoes, clothing, equipment, hands etc.
• Bury all infected debris deeply into the soil, preferably mixed in with lots of dirt to enhance decomposition.
• Avoid having other solanaceous crops and weeds in the environs of the greenhouse.
• Ensure that vents are closed during windy days.

| Top of Page |