Potential use of anaerobically digested manure slurry to suppress Phytophthora root rot of chilli pepper

• The mycelial growth and spore germination of Phytophthora capsici were significantly inhibited by anaerobically digested pig (ADP) and dairy slurry (ADD).
• Application of ADP and ADD significantly reduced the incidence of Phytophthora root rot of chilli pepper and filter-sterilization reduced disease suppression only to a certain degree.
• The better control of Phytophthora root rot by ADP was attributed to the higher concentration of ammonium and the particular structure of humic substances.

Anaerobic digestion is a promising way to treat the large amounts of animal manures produced from animal husbandry in terms of efficient recovery of energy by the anaerobic digestion and of more sustained use of the slurries. Side effects such as suppression of soil borne diseases are not yet well investigated and understood. The objective of the study was to evaluate the potential use of anaerobically digested slurry (ADS) to suppress Phytophthora capsici, the causative agent of Phytophthora root rot of chilli pepper. Mycelial growth and zoospore germination of P. capsici were inhibited by ADS and application of ADS significantly reduced the disease incidence under bioassay conditions. The percentage of reduction in zoospore germination of P. capsici and disease incidence by anaerobically digested pig slurry (ADP) were significantly greater than anaerobically digested dairy slurry (ADD). Filter-sterilization reduced disease suppression only to a certain degree. Exogenous applied ammonium and humic substances (HS) isolated from ADS suppressed P. capsici and the percentages of inhibition of hyphal growth and zoospore germination by HS extracted from ADP were greater than those derived from ADD. It is hypothesized that that the better control of Phytophthora root rot by ADP was attributed to the higher concentration of ammonium and the particular structure of HS.