Biological control of Phytophthora blight by Pseudomonas protegens strain 14D5

Abstract

Foliage blight caused by Phytophthora nicotianae and Ph. tropicalis is an important disease of annual vinca (Catharanthus roseus). In this study, 121 strains of Pseudomonas species originally recovered from recycled irrigation water were evaluated in vitro and in planta for their biological control potentials against this disease. We found 12 strains belonging to Ps. granadensis and Ps. protegens that reduced the radial growth of colonies of Ph. nicotianae isolate 3A12 by >91% in 6-well plates. They also reduced the radial growth of 3A12 and Ph. tropicalis isolate 7G9 by at least 82 and 54%, respectively, in 10-cm plates. In planta, Ps. protegens strain 14D5 reduced the infection by isolate 3A12 on four cultivars of annual vinca by 36 to 59%. Disease control efficacy of 14D5 against isolate 7G9 ranged from 23 to 45% depending on the plant cultivar. Secondary metabolites produced by strain 14D5 significantly reduced the radial growth of both Phytophthora isolates 3A12 and 7G9 in liquid media. Treatments containing these metabolites also reduced encystment, germination, and survival rates of zoospores of isolate 3A12, and foliage blight severity on C. roseus ‘Pacifica Punch Xp’ caused by 3A12. Treatments containing live cells and metabolites of 14D5 resulted in higher zoospore mortality and disease reduction than those only containing metabolites. These findings indicated that producing anti-Phytophthora secondary metabolites is likely the major mode of action of stain 14D5, while other mechanisms such as parasitism might be involved in its biological control against Phytophthora foliage blight.