Biocontrol of cypress canker by the phenazine producer Pseudomonas chlororaphis subsp. aureofaciens strain M71

Pseudomonas chlororaphis subsp. aureofaciens strain M71 was tested for its efficacy in controlling Seiridium cardinale, the fungus responsible for bark canker of common cypress (Cupressus sempervirens). The bacterium was able to completely inhibit the mycelial growth and conidium germination of the fungus in vitro and prevented canker induction in field trials. Strain M71 produced two phenazine compounds, phenazine-1-carboxyilic acid and 2-hydroxyphenazine. They were extracted from the bacterial culture, purified, identified and tested for their activity against S. cardinale and three other fungi responsible for cypress canker, viz. Diplodia cupressi, Seiridium cupressi and Seiridium unicorne. Phenazine-1-carboxylic acid was the sole compound active against the four fungi. The application in vivo of this phenazine molecule against S. cardinale reduced canker size indicating that the compound is directly involved in the control of the fungal pathogen by P. chlororaphis subsp. aureofaciens strain M71. Furthermore, the antagonist showed an interesting capacity for epiphytic fitness since it was able to establish itself on the crown of cypress plants and survive on it for more than three months.