Biocontrol of Bayoud disease (Fusarium oxysporum f. sp. albedinis) in date palm (Phoenix dactylifera L.) seedlings
Abstract: Bayoud, caused by Fusarium oxysporum f. sp. albedinis (Foa), is a vascular disease of date palm responsible for 75 % yield losses in Morocco, with no effective control strategy yet available. Two bacteria, Bacillus amyloliquefaciens and Burkholderia cepacia, isolated from the rhizosphere of almond trees and three other microorganisms, two fungi genera (Penicillium and Aspergillus) and one bacterium (Pseudomonas aeruginosa), isolated from the solid waste compost of olive, were examined for their role to control this disease. All these microorganisms highly inhibited both growth (65 to 85 %) and sporulation (80 to 97 %) of Foa in vitro. Microscopic examinations revealed cytological alterations of Foa mycelia when cultured with these bacteria. The most impressive alteration was obtained with the Pseudomonas aeruginosa strain, expressed as a cell ballooning leading to the rupture of the cell membrane and the expulsion of the cytoplasmic contents. Chitinase activity could, in part, explain these Foa mycelium alterations. When Jihel cv. date palm plantlets, a susceptible cultivar, were induced with Bacillus amyloliquefaciens and Burkholderia cepacia, the size of the necrotic zone, which reflected the spreading of the pathogen, was reduced by more than 70 %, as compared with uninduced controls. To further investigate the mechanisms of such disease reduction, phenolic compounds which reflected date palm resistance to Foa were assessed. One month after inoculation, date palm defense reactions against Foa were different depending on the bacterium used, B. cepacia led to higher accumulation of constitutive caffeoylshikimic acid isomers while B. amyloliquefaciens triggered the induction of new phenolic compounds identified as hydroxycinnamic acid derivatives. These results add to the promising field of investigation in controlling bayoud disease.