Phylloplane bacteria antagonistic to the sunflower pathogen Alternaria helianthi

Abstract

Thirty-eight of the 61 isolates of bacteria obtained from diseased and healthy sunflower leaves inhibited the germination of conidia and growth of germ-tubes of Alternaria helianthi in vitro. Inhibition included reduced conidial germination, germ-tube swelling causing vesicle formation, excessive germ-tube branching, lysis of germ-tubes, absence of sporulation and a reduced rate of hyphal growth. Some bacteria appeared to be endoparasitic, persisting inside the lumina of conidia and causing erosion of the conidium wall which resulted in the destruction of conidial cells. Bacteria attached themselves to conidia, hyphae and conidiophores and, in the field, are probably dispersed with the fungus. Five bacterial isolates that showed a high level of inhibition were identified as members of the genus Bacillus and comprised three species, B. subtilis, B. cereus and B. mycoides. The inhibitory substance produced by an isolate of B. subtilis was heat stable and therefore probably not an extracellular enzyme. Inhibition caused by the 38 isolates was not due to acid production by the bacteria. The size of lesions formed by A. helianthi was reduced when an isolate of B. mycoides was applied to sunflower plants with the conidial suspension. B. mycoides had no effect on infection by A. helianthi when applied to sunflower leaves 24 h prior to inoculation with conidia of A. helianthi.