Influence of compost tea, powdered kelp, and Manzate 75 on bacterial-community composition, and antibiosis against Phytophthora infestans in the potato phylloplane

The search for replacements for synthetic fungicides has prompted the examination of a range of plant-based composts and extracts as alternative crop-protection agents. A study was established to compare the relative efficacy of the commercially available Jolly Farmer (JF) compost tea, of an Acadian SeaPlants Inc. (ASL) powdered-kelp product, and of Manzate 75 DF (dry flowable) as foliar protectants against the potato late blight pathogen Phytophthora infestans. Population density, community structure, and antibiosis ability of phylloplane bacteria present on potato foliage ('Russet Burbank') were compared before and after foliar treatment. Bacteria recovered using culture-dependent means were characterized by fatty acid methyl ester (FAME) analysis and partial sequencing of their 16S ribosomal RNA genes. Autochthonous bacterial-population densities on the phylloplane varied with year, and tended to be lowest following Manzate 75 DF treatments. Phyllobacteria communities recovered from posttreatment phylloplanes often bore little resemblance to their tank-mixture counterparts, for all three treatments, and were significantly (P = 0.05) less effective at inhibiting P. infestans growth, in vitro, compared with foliar communities before the treatments. Manzate 75 DF was the most effective treatment (P = 0.05) for controlling P. infestans infection and disease development in vivo. Both JF compost tea and ASL powdered kelp were similar in their inability to protect potato leaves from P. infestans severity. Phyllobacterial populations established following Manzate 75 DF treatment proved to have significantly (P = 0.01) fewer antibiotic-producing strains (in vitro) than naturally occurring leaf populations prior to foliar treatment. Accordingly, it appears that Manzate 75 DF derives little if any additive or synergistic plant-protectant properties from the naturally established phylloflora.