Management of Bacterial Canker of Tomatoes: an Attainable Goal

Severe epidemics of bacterial canker and wilt disease (caused by Clavibacter michiganensis subsp. michiganensis; Cmm) occurred in Israel between the years 2005 to 2007, resulting in substantial yield loss in many greenhouses. As there were no effective means to cope with the disease, a national research project named 'Khosen Clavibacter' ('Khosen' in Hebrew is an acronym for 'Green Agriculture and Clean Environment'), in which all aspects of disease development were studied, was initiated. It was found that the main source of initial inoculum in commercial greenhouses was asymptomatic seedlings provided by the nurseries. The primary source of inoculum in the nurseries was infested seeds. Means to decrease the secondary spread of the pathogen from the primary infected seedling to the adjacent, healthy seedling in the nursery, were developed. In the commercial greenhouses the disease spread spatially during routine production procedures employed by the farm workers. Cmm-contaminated tools spread the disease for only a limited distance (<4 plants) from infected plants. However, touching asymptomatic infected plants bearing guttation droplets prior to touching nearby plants spread the pathogen over longer distances within rows (>22 plants). Experiments to study the effect of plant age at the time of inoculation on disease development revealed that the period during which inoculations led to wilt and death of tomato plants (designated "window of vulnerability") ranged from plants bearing 3-4 leaves (transplanting) to those with 17-18 leaves. Plants inoculated after this period expressed less severe disease symptoms and did not wilt or die. These results suggested that implementation of precautionary measures to avoid touching tomato plants bearing guttation droplets during the window of vulnerability would have a season-long effect on plant mortality and minimize, or even prevent, yield losses. This information was distributed to tomato growers in southern Israel. From August 2010 tomato growers received SMS messages twice a week informing them of the likelihood that weather conditions in the coming days would be conducive to guttation formation. Data collected in commercial greenhouses revealed that implementing the new recommendations resulted in a significant decrease in the occurrence of severe bacterial canker epidemics.