Evaluation of non-aerated compost teas for suppression of potato diseases

The use of water extracts from composted organic waste to suppress crop diseases has received attention in some parts of the world over recent decades. These extracts, also known as compost tea, are produced from compost that is fermented in water (either with or without aeration) for a specific period, filtered, and applied as a foliar spray or soil drench to control plant diseases. Compost tea incorporates soluble organic matter, beneficial microorganisms and macro and micronutrients, thus it is claimed that application to soil-plant systems improves soil biology and fertility for subsequent plant growth. Non-aerated compost tea (NCTs) production is a low-cost method to prepare an anaerobic tea from compost and previous studies have shown that NCTs are effective in limiting a wide range of foliar and soil borne disease in most of the pathosystems evaluated. However, how production factors (e.g. compost type, timing of application, concentration, and addition of adjuvants) influence effectiveness of NCTs has not been fully explored. The growing awareness of negative consequences on human and environmental health associated with reliance on synthetic pesticides has led to increased interest in alternative crop management solutions. In developing countries, such as Ethiopia, alternatives that are low-cost are particularly worthy of further development. Therefore, there is a compelling reason for researching NCTs as disease suppressants by designing studies to better understand the involved factors and mechanisms and inform practical use. The research presented in this thesis was designed to evaluate suppressive efficacy of different types of NCTs against economically important pathogens of potato (Solanum tuberosum L.), an important food source in Ethiopia, in different experimental settings. Initially, in vitro bioassays of NCTs from two Tasmanian compost sources (a commercial product and a vineyard compost) were tested against fungal pathogens of potato: Alternaria solani and Rhizoctonia solani. Firstly, it was demonstrated that the biotic component of the two NCTs was required to inhibit growth of the pathogen mycelium, while NCTs sterilized by filtration led to no suppression of pathogen growth. Secondly, it was found that maximum inhibition differed with NCT type and pathogen, for example, application of the commercial compost tea resulted in up to 74% inhibition of A. solani mycelium, 85% of R. solani (isolate 422) but only 36% for R. solani (isolate 299). An assay with detached potato leaves tested the efficacy of the NCT derived from the commercial compost, with or without an additive carbohydrate gum, against brown leaf spot symptom caused by Alternaria alternata and found that while application of the compost tea significantly reduced disease severity, there was no measured difference when gum was added. Important physico-chemical characters of the NCTs used in the above experiments, and their respective parent compost were described. In addition, the bacterial and fungal communities were analysed by high throughput next-generation sequencing of the 16S rRNA and ITS genes and found to contain higher numbers of bacterial operational taxonomic units than fungal. The microbial community structure of the NCTs and chemical characters extracted from the parent composts are important factors affecting the suppressive abilities of the NCTs. Studies with pot-grown potato plants were carried out in Ethiopia with NCTs made from different compost sources and using the economically important bacterial wilt pathogen Ralstonia solanacearum (biovar II). Three different kinds of mature and cured compost sources typically present in Ethiopia (agricultural waste compost, solid municipal waste compost and vermicompost) were used to prepare NCTs. Compost source and timing of application were the two most important factors that influenced reduction of bacterial wilt symptoms. In this experiment, NCTs made from agricultural waste compost were more effective to reduce disease severity than those made from solid municipal waste or vermicompost tea. Moreover, application of NCT at the same time that potato tubers were planted and inoculated with pathogenic R. solanacearum led to greater disease reduction than when applied 7 days before or after introducing the pathogen inoculum to the soil and planting the tubers. The most effective treatment resulted in a 2.5-fold reduction in disease compared to the non-treated controls, based on the “area under the disease progress curve” parameter. The effectiveness in suppression of bacterial wilt development of these same NCTs when combined with tree derived gum sources as adjuvants was further investigated in pot-trials in Ethiopia where compost tea was applied as a drench to planted tubers and then a spray as plants emerged. In this case, both the NCT type and type of the added gum (gum myrrh or opoponax) were found to be important factors for enhancing bacterial wilt suppression. As for the previous study, the NCT made from agricultural waste led to greater disease suppression than the other NCT types, and addition of myrrh gum led to the greatest reduction (over 2-fold) of disease severity. Therefore, the addition of gum was shown to be beneficial. Given the agricultural waste NCTs were generally most effective in controlling several key pathogens of potato, either in-vitro or in-vivo, an on-farm composting trial was carried out using the readily available agricultural waste materials pertinent to small scale farming practices in the central high land area of Ethiopia. Factors examined included different plant substrates (maize, wheat and cowpea straw and grasses, or coffee husk) and compost ages. The pit-composting method used led to long mesophilic phases (which did not exceed 45°C) as opposed to aerobic and open windrow compost methods that can reach up to 70°C in the thermophilic stage. Physico-chemical parameters of both the parent compost and the NCTs were measured for samples collected at three different stages of composting and values of pH, EC, exchangeable cations and nutrients, and the C: N ratio were found to be in the optimum range when compared with literature reports of agricultural waste compost materials. The shifts in microbial communities of NCTs over the stages of composting were thoroughly studied based on next-generation sequencing of 16S and ITS genes for bacterial and fungal communities, respectively. Bacterial phyla such as Proteobacteria, Bacteroidetes, Firmicutes, Verrucomicrobia, and Chloroflexi were identified with marked shifts in relative abundances over stages of composting. Similarly, fungal taxa, mainly Ascomycota, Basidiomycota, Cryptomycota, Entomophthoromycota and Glomeromycota, were abundant phyla accompanied with variation in relative abundances across the different stages of composting considered in the experiment. In conclusion, this thesis demonstrates that non-aerated compost teas that are prepared from a mixture of on-farm waste materials can reduce both soil borne and foliar disease of potato, and that efficacy can be further enhanced when carbohydrate rich gum is added as an adjuvant. The potential of this low-cost resource to be made and used on-farm in field situations should be trialled to develop practical recommendations to enable integration in to crop protection strategies, particularly in developing countries.