THE SIGNIFICANCE OF ANTIBIOTIC PRODUCTION BY PSEUDOMONAS-AUREOFACIENS PA147-2 FOR BIOLOGICAL-CONTROL OF PHYTOPHTHORA-MEGASPERMA ROOT-ROT OF ASPARAGUS

Pseudomonas aureofaciens PA147-2 produces an antibiotic (Af(+)) which inhibits the growth of fungal phytopathogens on phosphate buffered potato dextrose agar (PBPDA). To determine the role of the antibiotic in disease suppression in vivo, PA147-2 and an antibiotic-deficient Tn5 mutant (Af(-)) PA109, were tested for their ability to suppress root rot of Asparagus officinalis seedlings caused by Phytophthora megasperma var sojae, in the glasshouse. Seedlings coinoculated with the pathogen and the wildtype strain PA147-2, showed a significantly reduced level of infection and disease severity compared to seedlings inoculated with the pathogen alone. However, 100% of seedlings treated with Af(-) mutant PA109 were diseased. Furthermore, a strain derived from mutant PA109, restored to Af(+) through allele replacement of Tn5 by homologous recombination, gave similar levels of disease suppression as the wildtype. This suggests the antibiotic produced by PA147-2 is important for the control of P. megasperma in vitro as well as in planta. Treatment of seedlings with PA147-2 and derived strains including the Tn5 mutant (Af(-)) strain improved plant weight by 40-100% in the presence and absence of the pathogen suggesting PA147-2 also has a direct growth stimulatory effect independent of antibiotic production.