Microbial Biopesticides: Controlling Xanthomonas citri in Cashew
The cashew tree (Anacardium occidentale L.) occupies an important place in the world because of its cashew nut. However, its cultivation is confronted with bacteriosis, a bacterial disease caused by Xanthomonas citri pv. Mangiferaeindicae. This disease is one of the main causes of the low yield per hectare of cashew nuts, which fluctuates between 350 and 500 kg/ha. In view of this, it is wise to find ways of controlling this disease. It is in this context the objective of this work was to produce bio-formulations based on bacteria isolated from the rhizosphere of cashew trees, in order to evaluate their effectiveness on the growth of the agent responsible for cashew bacteriosis (Xanthomonas citri pv. Mangiferaeindicae). Thus, two liquid formulations were made from Pseudomonas fluorescens and Bacillus subtilis isolated from the rhizosphere of cashew. Stability, in vitro antagonism and biocontrol tests against Xanthomonas citri pv. Mangiferaeindicae were performed. The results obtained showed an inhibition of the Xanthomonas citri pv. Mangiferaeindicae bacterium with inhibition zones of 8.13 ± 2.1 and 25.20 ± 3.9 mm in diameter respectively for the products formulated with Bacillus subtilis and Pseudomonas fluorescens. In biocontrol tests, both formulated products showed their ability to protect cashew plants against bacterial blight with reduction rates of 80.95 ± 2.3 % and 73.80 ± 5.2% for the Pseudomonas fluorescens and Bacillus subtilis formulations, respectively. These two formulations of bacterial, once tested in cashew plantations, could be used in the biological control of cashew bacterial blight in Côte d’Ivoire.
Food security is defined as access to safe and sufficient food for all. Meeting the food demand of a rapidly growing world population is becoming a major challenge for humanity. To meet the food needs of the population, agricultural productivity will have to be increased in a sustainable manner worldwide (Kumar et al., 2012). However, insect pests and plant pathogens (fungi, bacteria or viruses) contribute to the decline in agricultural productivity, which can be as high as 70%. Indeed, plants as well as harvested and stored products are subjected to attacks by many pathogens (Popp et al., 2013). This is the case for cashew (Anacardium occidentale L.) in Côte d'Ivoire.Cashew, a crop that plays an important role in the Ivorian economy because of its cashew nut, is a particular strategic and income-generating resource for farmers in the North, South, Centre and East of the country (Soro, 2012). However, despite the economic and nutritional importance of cashew, its cultivation is subjected to several phytopathological problems that compromise the quality and quantity of cashew yield (Silué et al., 2017). Bacterial blight is a bacterial disease of cashew caused by Xanthomonas citri pv. Mangiferaeindicae. This disease manifests itself by oily angular spots on the leaves surrounded or not by a halo-chlorotic. It attacks all the vital organs of the plant with high severity (Zombre et al., 2017). In Benin, a work of Afouda et al. (2013) revealed average severities of 32.96%. This high severity of bacterial blight could lead to a decrease in cashew nut yield. Also, Soro et al. (2017) found evidence of bacterial blight in cashew orchards in Côte d'Ivoire with relative incidences of 15%. To control this disease, producers resort to the use of chemical pesticides (Camara et al., 2015).
This strategy can be effective, but the repeated use of these chemicals generates harmful consequences for the environment and the health of the user. Indeed, these products favour the resistance mechanism in pathogens and the ecological imbalance due to the broad spectrum of action of most synthetic compounds. This would lead to the destruction of pests, but also of other populations in the ecosystem and can also cause serious health problems due to pesticide residues in foodstuffs (Kouassi, 2012). In order to mitigate the adverse effects of chemical pesticides, biological control agents are emerging as promising alternatives for the management of crop pathogens. Among these biological agents, microbial biopesticides (bacteria, fungi, viruses) are the most appropriate. Indeed, they offer advantages of higher selectivity and lower toxicity compared to conventional chemical pesticides (MacGregor et al., 2006). Recent studies have shown their importance in disease biocontrol (Pérez-Garcia et al., 2011). However, the formulation of microbial biopesticides is a key element in the design of control strategies for plant and crop diseases caused by plant pathogens (Nam et al., 2018).
During this decade, numerous works in greenhouse and field trials have shown the potential value of rhizosphere bacteria, including Pseudomonas fluorescens and Bacillus subtilis as biological control agents for plant pathogens (Akram, 2008). A work of Koua (2020) showed that B. subtilis strains isolated from the rhizosphere of cocoa trees in Côte d'Ivoire would be effective bioinoculants in the control of cocoa diseases in greenhouses such as swollen shoot. It would therefore be interesting to find a stable bacterial biopesticide formulation suitable for the control of bacterial diseases of cashew trees and thus find a sustainable solution to the problem posed by synthetic products in Côte d'Ivoire. The general objective of this work is to evaluate the efficacy of a formulation of bacterial biocontrol agents based on bacteria (P. fluorescens and Bacillus subtilis) isolated from the rhizosphere of cashew trees against Xanthomonas citri pv. Mangiferaeindicae.
Source : Microbial Biopesticides: Controlling Xanthomonas citri in Cashew | InformativeBD
