THE SIGNIFICANCE OF ANTIBIOTIC PRODUCTION BY PSEUDOMONAS-AUREOFACIENSPA147-2 FOR BIOLOGICAL-CONTROL OF PHYTOPHTHORA-MEGASPERMA ROOT-ROT OFASPARAGUS

Pseudomonas aureofaciens PA147-2 produces an antibiotic (Af(+)) which inhibits the growth of fungal phytopathogens on phosphate buffered pot ato dextrose agar (PBPDA). To determine the role of the antibiotic in disease suppression in vivo, PA147-2 and an antibiotic-deficient Tn5 m utant (Af(-)) PA109, were tested for their ability to suppress root ro t of Asparagus officinalis seedlings caused by Phytophthora megasperma var sojae, in the glasshouse. Seedlings coinoculated with the pathoge n and the wildtype strain PA147-2, showed a significantly reduced leve l of infection and disease severity compared to seedlings inoculated w ith 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 homologo us recombination, gave similar levels of disease suppression as the wi ldtype. This suggests the antibiotic produced by PA147-2 is important for the control of P. megasperma in vitro as well as in planta. Treatm ent of seedlings with PA147-2 and derived strains including the Tn5 mu tant (Af(-)) strain improved plant weight by 40-100% in the presence a nd absence of the pathogen suggesting PA147-2 also has a direct growth stimulatory effect independent of antibiotic production.