Effects of Glyphosate on Yield and Shelf Life of Yam: A Case Study in Wulensi in the Nanumba Traditional Area, Ghana
Abstract
This study was undertaken to elucidate the effect of glyphosate on the soil mycoflora of yam farms and yam rots as a followup on a previous study which suggested that glyphosate use had no effect on yam rots as held by farmers. This study was set up at the field level and in the laboratory to investigate these concerns. Two yam varieties; “laribako” and “olodo” were grown in a replicated control trial design under the conditions of chemical weed control (use of glyphosate) and manual weed control in three replicate sites in the Wulensi traditional area of the Nanumba south district. The transient effect of glyphosate on fungal population and diversity were determined by serial dilution method on Cooke’smedia. Combined methods of soil plate fungi isolation and food poison was used to determine the effect of normal and double the normal rate of glyphosate application on fungal population and diversity. Fungal species belonging to the genera Aspergillus, Fusarium, Mucor, Penicillium, Rhizopus, Rhodotorula and Trichoderma were isolated. The positive control (fungicide) totally inhibited fungal growth. Generally, there were no significant differences (P≤0.05) among the fungal population recorded for the glyphosate treated soils and the negative control (water). The glyphosate treated soil initially recorded a lower fungal population, which subsequently increased with passing days. The recorded fungal population ranged from 3.6×105 to 7.7×105 CFU/gfor 10 days old glyphosate treated soil and non-treated soil respectively. The normal and double the normal rate of glyphosate application did not significantly (P≤0.05) affect the fungal population among the various treatments, but influenced fungal diversity over a short period of time. The study revealed after the storage period, that there was no difference (P > 0.05) in rots between yam treated with glyphosate herbicide and yam manually weeded. The study further revealed that glyphosate application at the recommended rate had no adverse effect on the mycoflora of the soil under consideration; hence did not distract the natural mycoflora of the soil which could have resulted in the proliferation of fungal pathogens causing yam rot.
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Introduction
The commercialization of farming in Ghana coupled with the dwindling numbers of farm level workers, has led to the adoption of herbicides in crop production. Yam farmers in particular, have over the years, relied on herbicides for weed control on their farms. Despite the huge relief brought to the yam subsector through the use of herbicides, there are concerns regarding the quality of yam under the use of herbicides. A section of the public believes that the use of these herbicides makes the yam tuber susceptible to rots.
Yam tuber rota major post-harvest challenge in yam production reduces the quality and quantity of stored yam tubers: resulting in huge economic losses. Among the various varieties of yam, tubers of Laribako have been documented as the most susceptible to rot (Wumbei et al.,2019; Demuyakor et al., 2013). Yam tuber rot pathogenic fungi including Aspergillus flavus, A. niger, A. oryzae, Botryodiplodia theobromae, L. theobromae, Fusarium culmorum, F. oxysporum, F. moniliforme, Penicilium spp, Rhizoctonia solani, Rhizopus stolonifera and Sclerotium rolfsii (Wumbei et al., 2022; Dania et al., 2020). These fungi are normal mycoflora of soil but under certain conditions have the potential of becoming opportunist plant pathogens. Since yam tubers develop in the soil, several soil microbes including fungi exist on yam tuber skin as normal mycoflora, some of which may become pathogenic causing yam tuber rot during storage.
Herbicide application has the potential of influencing plant disease development by either enhancing the resistance or susceptibility of plants to diseases (Hammerschmidt, 2018). Also, the application of a herbicide such as glyphosate may reduce the population of beneficial soil microbes and increase that of soil borne pathogens which can facilitating the risk of disease development in crops where glyphosate was previously applied; especially, when the plants are planted too soon after glyphosate application (Hammerschmidt, 2018). The rate at which glyphosate is applied can influence its effect on plant cultivated on glyphosate applied lands (Van Bruggen et al., 2021). Researchers have evaluated the normal and double the normal rate of glyphosate application on soil microbial community and concluded that, although the composition of soil microbes may be disrupted, the microbial community seem to recover within a short period of time (Vázquez et al., 2021; Lupwayi et al., 2020). The long-term application of glyphosate in no-till systems only caused minor changes in soil microbial communities which seem to recover with time (Lupwayi, et. al., 2020; Arango et al., 2014). According to Kepler et al. (2020) and Spinelli et al. (2021) glyphosate application does not make crops susceptible to diseases, especially when the crops are planted a few days after application of glyphosate. This is because glyphosate does not persist in soil as residue since it is readily degraded by some soil microbes (Spinelli et al., 2021, Haney et al., 2000).
An initial study was conducted to assess the impact of glyphosate application on yam tuber storability. This was conducted using a replicated control trial design with two yam varieties in three locations in the Wulensi traditional area, the result of which was suggestive that the herbicides are not responsible for yam rots. The current study is therefore a repetition of the earlier study with an aspect of soil health analysis using fungi as bioindicator to determine if the application of glyphosate distorted the normal ecology of mycoflora in BBN, NKG and SDB soils in favour of pathogenic once aiding in yam tuber rot. 1.1 Objectives: The main objective of the study was to determine if the use of herbicides contributes to yam tuber rots. The other objectives were to determine the influence of herbicides use on the yield of yam and whether the use of herbicides can lead to residue problems in yam.
Conclusion
The application of glyphosate had no adverse effect on the mycoflora of the soil under consideration; hence did not distract the natural mycoflora of the soil which could have resulted in the proliferation of fungal pathogens causing yam tuber rot. The greater number of rotten tubers recorded for Laabako than olodo shows that the rot was not facilitated by the applied glyphosate, but the susceptibility of Laabako tubers to rot.
The lack of difference in yield between glyphosate treated plots and manually weeded plots shows that weeds are an important factor affecting yield, but the method of weed control does not affect yield.
