Control mechanism of phytophthora blight of chilli pepper by anaerobically digested pig slurry - The roles of ammonium and humic acid in biogas slurry

Anaerobic digestion is promising in treatments of large amounts of animal manure from animal husbandry. It has been apparent that new methods of utilizing anaerobically digested slurry were needed to achieve sustainable management of husbandry wastes. Anaerobically digested slurry has shown suppressive effects on different soil-borne pathogens and its application to soils has been proposed as a new way to control several plant diseases. The objective of this study was to evaluate the potential use of anaerobically digested pig slurry (ADP) to suppress Phytophthora capsici, the causative agent of phytophthora blight in chilli pepper. The study also aimed to clarify the roles of ammonium and humic acid in ADP inhibition of pathogen growth. A pot experiment was conducted to study the effect of untreated anaerobically digested pig slurry (ADP1), ammonium-intensified pig slurry (ADP2), ammonium-removed pig slurry (ADP3) and humic acid-removed pig slurry (ADP4) on controlling phytophthora blight in chilli pepper. The results showed that ADP1 effectively reduced the incidence of phytophthora blight with a relative control efficiency of 40.8%. Disease incidence of plants treated with ADP3 and ADP4 was not significantly different from that of pathogen control (PC). This suggested that removing ammonium or humic acid from ADP significantly reduced ADP control efficiency. Real-time PCR analysis showed that P. capsici number in rhizhospheric soils of ADP2 was the least (2.51×10³ copies·g^-1), with significantly larger numbers for ADP3 (8.19×10³ copies·g^-1) and ADP4 (1.38×10⁴ copies·g^-1) treatments. When P. capsici density in the soil dropped below 9.54×10³ copies·g ^-1, disease incidence became strongly correlated with P. capsici count. Different ADPs had different effects on the growth of soil microorganisms. The numbers of soil total bacteria, fungi and actinomycete were largest in ADP1 treatment. The least number of fungi was observed in ADP2 treatment. In-vitro experiments showed that NH4+ concentrations of 500 mg·L^-1, 1 000 mg·L^-1 and 1 500 mg·L^-1 reduced zoospore germination by 77.6%, 81.8% and 95.4%, respectively. Zoospore germination rates in treatments with 25 mg(C)·L^-1, 50 mg(C)·L^-1, 75 mg(C)·L^-1 and 150 mg(C)·L^-1 of humic acid extracted from ADP1 decreased by 27.8%, 54.5%, 70.0% and 87.5%, respectively. The study suggested that ammonium and humic acid were major ADP inhibitors of P. capsici growth. ADP was promising in terms of inhibiting and controlling soil-borne diseases.