Soil-borne strain IC14 of Serratia plymuthica with multiple mechanisms of antifungal activity provides biocontrol of Botrytis cinerea and Sclerotinia sclerotiorum diseases

Plant-associated strain IC14 of the Gram-negative bacterium Serratia plymuthica isolated from soil around melon roots was shown to suppress a wide range of phytopathogenic fungi in vitro. Foliar application of strain IC14 protected cucumber against Botrytis cinerea gray mold and Sclerotinia sclerotiorum white mold diseases of leaves under greenhouse conditions, reducing disease incidence by 76 and 84%, respectively. The strain possessed chitinolytic and proteolytic activities, produced the antibiotic pyrrolnitrin [3-chloro-4-(2′-nitro-3′-chlorophenyl)pyrrole] and siderophores, and secreted the plant growth hormone indole-3-acetic acid. An endochitinase with an apparent molecular mass of 58 kDa, was estimated to be the main secreted chitinolytic enzyme. Two mutants, one with increased chitinolytic activity and the second deficient in chitinolytic activity, were obtained by miniTn5-insertion mutagenesis. Neither mutant differed appreciably from the parental strain in the production of other antifungal compounds or in suppression of B. cinerea and S. sclerotiorum on plates or in the greenhouse, suggesting that chitinolytic activity is less essential for biocontrol of these pathogens by strain IC14. The obtained results present novel information concerning the potential of the soil-borne S. plymuthica strains as biocontrol agents of foliar diseases caused by plant pathogenic fungi.