Bacillus sp. strains, an effective tool to face out the bacterial canker of kiwifruit

In the last decade, bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), provoked heavy economical losses since its pandemic spread in 2008. The control of disease is achieved through integrated methods, by combining appropriate cultural practices and chemical treatments as cupric salts/antibiotics, and biocontrol agents (BCA). In this study, two strains QST713 andD747 of Bacillus sp., active principle of commercial biofungicides, were investigated for their ability to reduce the in vitro Psa growth. Both bacterial strains were assayed in planta for their capacity in directly reducing the severity of bacterial canker on green- and yellow-fleshed kiwifruit plants under greenhouse and open field conditions. Moreover, D747 strain was studied for its ability to induce resistance against Psa in yellow-fleshed kiwifruit plants after its application at the roots by irrigation with a suspension of the antagonist. In in vitro experiments the strains QST713 and D747 resulted similar in reducing Psa growth. Under greenhouse conditions, when applied on kiwifruit plants 48 h before the Psa inoculation, only the strain D747 resulted able to significantly reduce the disease severity, and in open field it prevented the natural Psa infection and lowered the disease severity. D747 was also able to reduce the disease severity by inducing plant resistance after its application at the roots. These are the first results which concern the ability of a BCA in directly reducing the pathogen growth and its disease severity, as well as its indirect capacity to induce resistance in Actinidia spp. plants towards Psa.