Halo blight in the bean plant

The oily stains accompanying the yellowish rings on the leaves and pods of bean plants are some of the symptoms of the disease known as “Halo blight” – highly important in temperate zones like Spain. The seeds are one of the most important sources of transmission of the pathogen, which means the detection of this bacterium in seeds is one of the most efficient control methods. Nevertheless, agricultural engineer Arantza Rico Martínez has shown, in her PhD thesis, that this blight pathogen cannot be detected in Spain using routine techniques for the certification of bean seeds.

Unexpected results

Halo blight is the common name for the disease caused by the bacteria Pseudomonas syringae pv. phaseolicola. One of the aims of this PhD was precisely to genetic characterisation of the bacteria in order to make advances in the control of the disease. To this end, 152 diseased beanstalks, obtained from commercial crop fields between 1993 and 2001.

However, the results of the analyses were not the expected ones. Contrary to prevision, the phenotypic and molecular characterisation of the stumps of P. syringae pv. phaseolicola showed that most of them did not produce an antimetabolitic toxin known as phaseolotoxin, described as specific to this patovar and, apart from not producing it, it was shown that they lacked the genes responsible for producing this toxin.

This finding is relevant because one of the most effective control methods for the illness is based on the detection of the presence of the genes responsible for the biosynthesis of phaseolotoxin in the seed. Given that neither the toxin nor the genes can be detected, it is impossible for this method to detect the bacteria responsible for halo blight.

The thesis also analysed other serological methods, which are routinely used in the detection of this bacteria and it was shown that neither were the non-toxigenic isolated beansttalks detected using this methodology.

The importance of the results is greater if analysed in an international context, given that it is the first time the existence of a non-toxigenic population of P. syringae pv. phaseolicola with epidemiological importance has been described, to the point where it is seen as the main cause of halo blight in Spain . In this sense, a molecular more exhaustive characterisation of the non-toxigenic beanstalks has enabled Arantza Rico’s research group to propose the subdivision of the patovar phaseolicola in two groups differentiated by the possession of the genes responsible for the biosynthesis of phaseolotoxin.

Origin of fireblight

Another aim of the thesis was to genetically characterise Spanish populations of the bacteria Erwinia amylovora, cause of the disease known as “Fuego bacteriano” or fireblight which affects species from the rosacea family, amongst which are fruit trees like the apple and pear and ornamental bushes. The symptoms of this disease are very characteristic: the plant has necrosed leaves, acquiring a burnt aspect.

Given the high genetic homogeneity of E. amylovora, very little is known about the epidemiological ways for the dispersion of the disease. So, with the aim of finding out how this pathogen introduces itself, the thesis used various techniques of genetic characterisation. The conclusion drawn is that the observed diversity supports the hypothesis of multiple introductions of the pathogen in Spain and in other European countries. This disease, moreover, was first detected in Gipuzkoa in 1995.