Modes of action of biopesticides designed to fight Xylella fastidiosa
Olive Quick Decline Syndrome is a disease that is devastating olive trees in Apulia, Italy. It is caused by the strain De Donno of Xylella fastidiosa, a bacterium which is efficiently transmitted by the xylem-feeding insect Philaenus spumarius (xylem transports water and nutrients from roots to stems and leaves). The objective of BIOVEXO is to develop environmentally sustainable and economically viable plant protection solutions combining Xylella-targeting biopesticides (X-biopesticides) with biopesticides combatting the insect vectors transmitting the disease (V-biopesticides).
While working on the design of new substances and their integration into effective biopesticides, work has been also underway to determine as precisely as possible the modes of activity of individual outcomes, starting with testing under in vitro conditions.
Antimicrobial activity of X-biopesticides against Xylella bacterium
BIOVEXO scientists have tested so far three products with favorable track record of antimicrobial activity in diverse pathosystems or against Xylella: a vegetable extract (a by-product from food industry which is not suitable for human consumption), and two bacterial beneficial microorganisms.
Both beneficial microorganisms did not antagonize the growth of Xylella in a dual-culture assay in which both bacteria are co-cultivated. Such a result was expected as it was already known that their mechanism of pathogen control relies on the elicitation of a plant immune response. Therefore, such a result did not rule out their testing in the open field conditions. However, a new bacterial strain, not preliminarily included in BIOVEXO, has been tested and showed in vitro antagonistic activity against Xylella (Caption 1). Such microorganism will be investigated in detail in the frame of future project activities.
On the other hand, the vegetable extract does efficiently antagonize the growth of Xylella in vitro, even at low concentrations.
Caption 1: Inhibition zone of Xylella growth by a bacterial endophyte (A) and a vegetable extract (B) | © CNR
X- and V-biopesticide activity against the transmitter P. spumarius
It has been found out that not only V- biopesticides but also microbial X-biopesticides can colonise the insect vector and form populations on it. In parallel, the survival and colonisation of the V-biopesticide entomopathogenic fungus on the insect has been studied.
All tested X- and V- biopesticides are able to colonise and survive on various organs of the insects as assessed by innovative imaging techniques of assays of insects kept in cages in the field. These results are promising also in another aspect: they show that the biopesticides examined are able to survive and work even after the spraying process.
Caption 2: Bacterial cells along the cibarium (preoral cavity) of P. spumarius (A); details of bacterial cell aggregate at the distal area of the cibarium (B) | Cornara, D., Saponari, M., Zeilinger, A.R. et al., J Pest Sci 90 (2017)
In conclusion, it is encouraging to see that the BiOVEXO project is well on track, not only developing empirically active substances and solutions but also working intensively on building understanding how individual substances act against Xylella itself as well as against its dominant transmitting vector in Southern Europe, “spittlebug” P. Spumarius.
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