In vitro and in vivo assessment of the potential of compost and its humic acid fraction to protect ornamental plants from soil-borne pathogenic fungi

Five humic acid fractions (HAs) were isolated from various organic substrates used for plant growth in containers, including both conventional ones, i.e. peat (P) and coconut fiber (CF), and newly introduced ones, i.e. compost (Co) and mixtures at 20% and 60% (v/v) of Co with P (Co20 + P and Co60 + P) and Co with CF (Co20 + CF and Co60 + CF). The effects of these HAs at 50 and 300 mg L−1 were evaluated on the mycelial growth of two soil-borne phytopathogens, Pythium ultimum (PYT) and Fusarium oxysporum f. sp. callistephi (FOC), in a series of in vitro experiments. Furthermore, in experiments performed in a climatic chamber, the bulk substrates were tested on the growth and health of two ornamental plant species: impatiens (Impatiens walleriana Hook) and China aster (Callistephus chinensis L. Nees), repeatedly infected respectively with PYT and FOC. In general, any HA at either dose reduced significantly the radial growth of PYT mycelium after 42 h, whereas Co-HA and P-HA at the higher concentration showed a high inhibition on PYT during the whole experiment. A significant inhibition of FOC growth was exerted by all HAs, especially at the higher dose, during the 192-h experimental time. No significant correlation exists between the extent of the inhibitory action on the two fungi and the chemical and functional properties of HA. The only exception is the existence of a negative correlation between the inhibition of PYT and the HA oxygen content. The various substrates used in trials conducted in vivo caused different plant growth response. The treatments with Co alone or its mixtures at 60% with P or CF were lethal for both plants, even before the first fungal inoculation. The substrates Co20 + P and Co20 + CF were very effective in controlling the fungal attack and increasing the growth of impatiens and China aster.