The efficacy of Moringa oleifera plant extracts against selected fungal and bacterial plant pathogens infecting selected vegetable crops in Zimbabwe.

Abstract

Diseases and pests are among the major constraints to horticultural production worldwide, and this has been further exacerbated by mono-cropping production systems in response to increased food demands of an ever-expanding population. As a result, farmers have resorted to excessive chemical use in order to manage diseases and pests, and maintain high yields. However, excessive chemical use has been associated with negative environmental and health effects. Numerous studies have been carried out to determine antifungal and antibacterial properties in traditional medicinal plants aimed at developing bio-pesticides which can be utilized together with synthetic pesticides in integrated disease management strategies. One such alternative is the use of Moringa oleifera extracts. Currently, in-vitro studies carried out on Moringa antimicrobial action have mainly focused on controlling human enteric pathogens. It is against this background that this study was conducted during the 2014 – 2016 cropping seasons. The main objectives of the study were to: 1) determine farmers’ perceptions on vegetable disease incidence, prevalence and disease control methods in relation to seasonality and prevailing climactic conditions. 2) Evaluate effectiveness of Moringa leaf and seed plant extracts in suppressing growth and development of bacterial (Pectobacterium atrosepticum, Hall, Pectobacterium carotovorum subspp. brasilienses, Bur., Dickeya dadantii, Dick.) and fungal (Pythium ultimum, Trow., Rhizoctonia solani, Kuhn., Fusarium solani, Mart., Phytophthora infestans, Mont.) plant pathogens in-vitro. 3) Determine whether Moringa aqueous extract concentration influences the efficacy of its antimicrobial activity. 4) Determine whether Moringa bark aqueous extracts could exhibit antibacterial activity against four different pathovars of the Xanthomonas campestris, Pammel., pathogen. 5) Determine the antifungal and antibacterial efficacy of Moringa leaf, seed and bark aqueous extracts against selected fungal and bacterial pathogens infecting two crops Lettuce (Lactuca sativa, L.) and Cabbage (Brassica olearacea, L.) grown under greenhouse and open field conditions respectively. Initially, a survey was carried out to determine the perceptions of horticulture farmers on crop disease incidence and control methods in the sub-humid areas of Zimbabwe. Survey results revealed that farmers face more outbreaks of fungal diseases compared to bacterial diseases. In addition, farmers have noted an increase in disease incidence over the past 5 – 10 years. The survey further revealed that the majority of the respondents depend on chemicals to control diseases in their crops. Only a small percentage of the farmers use cultural or mechanical alternative disease control methods. However, none of the respondents utilize botanical or bio-pesticide disease control strategies to manage these diseases. There is need to raise awareness among farmers regarding the negative health and environmental effects of increased chemical use and the potential of using bio-pesticide strategies in plant disease management. Field and greenhouse trials were conducted over three growing seasons using the completely randomized block and split plot experimental designs set up as factorial trials. The laboratory results exhibited the efficacy of Moringa leaf, bark and seed aqueous extracts in significantly controlling the growth of fungal (Pythium ultimum, Rhizoctonia solani, Fusarium solani and Phytophthora infestans) and bacterial (Pectobacterium carotovorium subspp. brasiliensis, Pectobacterium atrosepticum and Xanthomonas campetris pv campestris) pathogens. There were significant interactions between Moringa aqueous extract source and concentration which influenced the antimicrobial action of the extracts (P = 0.001). The results from the greenhouse and field studies revealed that Moringa leaf, seed and bark aqueous extracts significantly controlled bottom rot (Rhizoctonia solani) and stem/root rot (Fusarium solani) diseases in lettuce and black rot disease (Xanthomonas campestris pv campestris) in cabbages (P < 0.05). Disease suppression was more effective at higher concentrations of the Moringa aqueous extracts, whilst the highest disease severity occurred at the lowest Moringa aqueous concentration levels. However, Moringa seed aqueous extract demonstrated higher antibacterial activity against black rot disease and antifungal activity against test pathogens (P < 0.05). Moringa seed and leaf aqueous extracts also significantly enhanced head weight and diameter in lettuce. Moringa aqueous extracts can therefore be considered as bio-pesticides in Integrated Crop Disease Management strategies, and these can be a viable, and environmentally friendly alternative to chemical use. Based on our findings, it is recommended that further in-vivo studies to improve extraction protocols, and determine ideal application methods be carried out to improve Moringa aqueous extract bio-pesticide efficacy. These should be carried out in selected crop pathogens of economic importance. Currently, there is very little literature regarding in-vivo crop, pathogen and bio-pesticide interaction studies with Moringa. Training workshops and demonstration plots to impart knowledge and skills to farmers on preparation and utilization is key to enhance uptake.