The causes and prevention of oedema (lesions) in herbs

Coriander suffers from a condition that manifests as foliar lesions known as ‘oedema’. It appears very quickly, often emerging throughout an apparently healthy crop within 24 hours. As retailers will not accept any cosmetic damage to leaf crops, this disease can make the crop unsaleable and can cause losses in excess of £200 k per annum.

It was concluded that oedema and blue spot are separate conditions and should be classified as such in the scientific press. It does not appear that a calcium imbalance is responsible for either oedema or blue spot. Although significantly higher ion concentrations were found in blue spotted tissue, it was not clear whether this was responsible for the condition or vice versa. Neither condition is due solely to a particular meteorological condition. Instead, it seems more likely that the presence of an additional variable (unidentified), under close influence of its environment, is the cause of the symptoms. The hypothesis that the symptoms may have been produced by the bacterial pathogen Pseudomonas syringae was not upheld.

The aims of this project were to characterise the physiology and to determine the causes of oedema, in order to allow preventative measures to be investigated; e.g. selection of less susceptible varieties, use of plant protectant products and more appropriate management of irrigation schedules. Previously, almost no work had been done on the condition, particularly in herbs.

Three strategies were employed: i) gathering data and coriander samples from growers, ii) gathering meteorological data, and iii) setting up controlled experiments in the laboratory.