How to identify and treat bacterial canker

Bacterial canker: How to identify and treat bacterial canker in tomatoes

Bacterial canker (a.k.a. Corina), first described in 1910 in North America, is caused by the bacterial pathogen Clavibacter (Corynebacterium) michiganensis subsp. michiganensis.  It is a sporadic, but very important and aggressive pathogen of tomato (Lycopersicon esculentum) that can now be found in many countries throughout the world. Its occurrence tends to be erratic with several years of low reporting being followed by occasional years where the disease reaches epidemic proportions in one or other regions of the world. There is evidence for strain variation in the bacterium with some strains producing less severe symptoms than others. Whilst tomato is the primary host crop natural infections have also been reported on sweet pepper (Capsicum annuum), eggplant (Solanum melongena) and on several weed species in the Solanaceae e.g. S. nigrum, S. douglasii and S. trifolium. In addition, several weed species, including non-solanaceous hosts, have been demonstrated to act as reservoirs for epiphytic survival and spread though the full implications of this with respect to disease epidemics is not fully understood.

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How to prevent bacterial canker

The disease can be manifest in two ways: - either as a systemic vascular infection or as a superficial or local infection. The wide array of different symptoms reported depend on various factors including the crop location (outdoor or protected), the prevailing climate, the timing of infection relative to the crop growth stage, agronomic practices deployed, the virulence (pathogenicity) of the particular strain of the pathogen involved. Yet, early recognition of the disease is essential if the disease is to be contained and, because the pathogen involved is so easily spread, extreme vigilance is required. All nursery personnel need to be made fully aware of the wide range of symptoms that can be expressed. Protected crops The initial symptoms are dull green water-soaked areas on the leaf between the veins. The water- soaked areas on the leaf quickly become necrotic and, to the untrained eye, this can be mistaken for a scorch due to sunburn or chemical damage. Affected areas can quickly coalesce to produce larger necrotic areas on the foliage (Figure 1.0). Down-curling and wilting of one or more lower leaves is also indicative of bacterial canker as the systemic infection progresses through the plant (Figure 2.0).

Figure 1.0
Water soaked lesions on the leaf surface.

 


Figure 2.0
Down curling and wilting of lower leaves. 

How to identify bacterial canker

Whilst plants are susceptible at all stages of growth disease symptoms are rarely observed prior to fruit set on trusses 3-4. However, it is important to remember that the pathogen can be seed-borne and, whilst it is unusual to see early symptoms during propagation of the crop, wilting may occur especially on grafted plants and localised lesions may also form as a result of bacterial transmission under high humidity conditions. Whilst such infected seedlings may be quickly killed or produce weak stunted plants, if conditions are unfavourable for the pathogen, infected seedlings may initially develop into apparently healthy plants without obvious disease symptoms. Such symptomless or latent infections are potentially a major problem with respect to early detection on the nursery. It is also important to remember that from localised lesions the pathogen can enter the plant via natural openings (e.g. stomata and hydathodes) from where it can gain entry into the vascular system and become systemic when the crop is handled (e.g. de-leafed / trimmed). Speed of symptom expression The time taken from initial infection to the onset of visible symptoms varies considerably and delays of 4-6 weeks and as long as 3 months between first outbreaks and secondary spread to adjacent plants has been observed previously. Experimental and other observations in infected crops have shown that the time between infection and appearance of symptoms can vary by as much as 10-34 days. This is likely to be due to a differential temperature (the optimum temperature for growth of the pathogen is in the range of 24-27oC so cooler temperatures will slow down the infection progress), plant age (5 week old transplants appear to have a longer latent period prior to symptom expression than 3 week old transplants) and inoculum potential (the more bacteria present initially the quicker the symptom development).

Download the Nursery management plan to learn more about the prevention of bacterial canker and how to threat the disease.

Visual Nursery Management plan bacterial canker tomato

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