DMPP-enhanced control efficacy of chili pepper Phytophthora blight using ammonium bicarbonate: Action effects and mechanisms

Phytophthora blight of chili pepper, caused by oomycetes of Phytophthora capsici, has been reported to be a key limiting factor of chili pepper production worldwide. Increased public interest in protecting the environment and human health has prompted research in agronomic strategies that reduce the use of fungicides. Alternative control methods with high efficacy, low cost and limited environmental effect are high-priority research areas for sustainable agriculture. Under the same incubation conditions (soil moisture of 60% of field capacity, temperature of 25 ℃ and inoculation concentration of P. capsici of 500 CFU.g^-1), DMPP (1% applied pure N) and ammonium bicarbonate (AB) 100 mg(N).kg^-1 were added tofluvo-aquic soil and incubated for 15 d. Soil without any addition of DMPP and/or AB was set as the control. After incubation, DMPP or AB-treated soil was used to grow chili pepper in a pot experiment for 28 d. The effect of DMPP and AB application on disease incidence of Phytophthora blight of chili pepper was then compared. The soil physio-chemical and microbial responses (soil pH, electric conductivity, concentrations of different forms of nitrogen; numbers of total bacteria, fungi, P. capsici and ammonia-oxidizing bacteria) to the addition of DMPP and AB were determined. The relationship between Phytophthora blight disease incidence and soil phyiso-chemical and microbial characteristics was evaluated. The aim of the study was to investigate the control effects of DMPP-enhanced ammonium biocarbonate on Phytophthora blight of chili pepper and correlation with soil physio-chemical properties, and provided technological support for control of Phytophthora blight of chili pepper of greenhouse. The results suggested that compared with the control, the contents of soil ammonium nitrogen in DMPP and DMPP+AB treatments were higher, and the contents of nitrate and nitrite nitrogen significantly lower. The application of DMPP for 15 d decreased copied gene numbers of bacterial amoA and P. capsici ITS genes by 34.9% (P > 0.05) and 93.8% (P < 0.05), respectively. The copied 16S rRNA gene number increased by 54.7% (P < 0.05) compared with non-DMPP treatments. However, the copied numbers of fungal 18S rRNA gene and archaeal amoA gene were not significantly affected by DMPP. After incubation for 15 d, soil from each treatment was put into pots and ten chili pepper plants grown in each pot for 28 d. The P. capsici density was lowest in AB+DMPP (2.1×10⁵ copies·g^-1) treatment, followed by DMPP (15.4×10⁵ copies·g^-1). The control experiment had the highest number of pathogen (37.1×10⁵ copies·g^-1), which was 0.4-fold, 1.4-fold and 16.8-fold higher than those of AB, DMPP and AB+DMPP, respectively. The results from the pot experiment showed that the control treatment had the highest disease incidence (95.00%), followed by AB treatment (85.00%) and AB+DMPP treatment had the lowest disease incidence (32.20%). The efficacy of Phytophthora blight disease control by AB+DMPP treatment was 66.11%. Disease incidence was positively correlated with soil electrical conductivity, nitrate content and P. capsici population, but negatively correlated with soil pH, ammonium content and bacterial and fungal populations. The above results suggested that the control of chili pepper Phytophthora blight by the combined application of DMPP and ammonium bicarbonate decreased the number of ammonia- oxidizing bacteria, which, in turn, increased ammonium content, but also decreased soil nitrate content. Thus P. capsici population reduced under high concentration of ammonium, which effectively controlled chili pepper Phytophthora blight.