It spreads thanks to the scent of flowers, thus exploiting the work of the bees. This is what researchers from a team at the universities of Bologna and Bolzano have discovered about Erwinia amylovora, the bacterium responsible for the "bacterial fire", one of the most serious and insectable infectious diseases for pear and apple trees. The affected plants undergo a modification of the scent of their flowers, thus pushing the bees to propagate the infection.
The results of the study - published in The ISME Journal - show how Erwinia amylovora, one of the main pathogens of apple and pear trees, is able to modify the fragrance of the flowers in the trees affected in
Erwinia amylovora is known to be responsible for the "Bacterial fire shot", one of the most serious and insectable infectious diseases for fruit trees that are very widespread like pear and apple. In the affected plants, the flowers are tinged with dark and the leaves wither until they seem burnt (hence the name, "Colpo di fuoco"). In a short time, the infection can progress to the death of the whole tree.
The danger of the disease is aggravated in particular by the ease with which the bacterium is able to spread by exploiting the work of pollinating insects. And it is precisely on this aspect that the research group's work has been concentrated.
Taking advantage of the natural defense reactions of the plants and the work of the bees, Erwinia amylovorariesce so to spread quickly and effectively.
Scholars have noted that, given the choice between healthy flowers and flowers affected by the bacterium, bees show a preference for healthy ones. The reason? The different aroma emitted from sick flowers. "Our research - explains Francesco Spinelli, a professor at the University of Bologna who coordinated the study - highlights how Erwinia amylovora, one of the main pathogens of apple and pear trees, is able to change the scent of the flowers of its host : a strategy that favors the passage of bacteria from sick flowers to healthy ones ".
It all comes from a natural reaction of the plant that, once hit by the bacterium, activates its defenses by issuing specific volatile compounds that bees are able to perceive. "Following the infection - continues Professor Spinelli - the trees react by producing some odorous compounds, among which there is in particular salicylic acid, a key molecule for the defense mechanisms of plants but also a repellent compound for the "bees.
The flowers of diseased trees are therefore less "attractive" to bees than healthy ones. Why then does the infection continue to spread? "The reaction of sick plants - explains the teacher - is not able to completely block the work of the bees, who are still pushed to rest on the flowers". In short, the number of bees reaching sick trees is less than those who choose healthy ones, but it is still sufficient to promote the spread of the bacterium.
"Once a bee is placed on a sick flower, it is contaminated by the bacterium and at the same time is rejected by the repellent action of the odorous compounds produced by the diseased plant. As a consequence, the next flower that will probably choose will be that of a still healthy tree: in this way the infection continues to transmit from plant to plant ".
"This pathogen - confirms Francesco Spinelli - is able to manipulate in a very refined way the symbiotic interaction between plant and pollinators in order to spread itself". A skill that can end up endangering entire crops.
The study, carried out by a research group at the universities of Bologna and Bolzano, was published in The Nature Magazine, the ISME Journal, entitled "Pathogen-induced changes in the floral scent may increase honeybee-mediated dispersal of Erwinia amylovora ".
Researchers and teachers of the Department of Agro-Food Sciences and Technologies participated for the University of Bologna: Antonio Cellini, Irene Donati, Maria T. Rodriguez-Estrada, Stefano Savioli and Francesco Spinelli.