Antifungal Potential of Curcuma longa (Tumeric) and Zingiber officinale (Ginger) against Alternaria alternata Infecting Spinach in Kenya

Abstract

Spinach diseases caused by Alternaria spp. are one of the most significant devastating pathogens to spinach in Kenya and worldwide. Alternaria alternata has been associated with great losses in spinach both in total biomass yield and leaf quality. The pathogen has been traditionally controlled using synthetic fungicides which are expensive and harmful to both humans and environment. This study aimed at investigating the efficacy of the extracts of two plants; Curcuma longa (Tumeric) and Zingiber officinale (Ginger) against Alternaria alternata both in in vitro and in vivo conditions. Absolute ethanol, water, ethyl acetate and methanol were the solvents used in extraction of Curcuma longa and Zingiber officinale rhizome extracts. Decoctions were screened for antimycotic potential using the poisoned food technique. Results from this study revealed that Curcuma longa and Zingiber officinale extracts had varying degree of antifungal activity against the Alternaria alternata depending on the solvent used for the extraction and the concentration. Methanolic extracts of Curcuma longa and Zingiber officinale demonstrated the highest antifungal activity which was significant (p≤0.05) against the Alternaria alternata compared to ethanolic, ethyl acetate and aqueous extracts with percentage inhibition of 64% and 57%, respectively at the concentration of 50 mg/ml. At the concentration of 25 mg/ml and 50 mg/ml the percent inhibition on the fungal growth was not significant (p≤0.05) from the 8th day for all the solvents in both turmeric and ginger. Foliar spray with the extracts was found to be effective in lowering disease severity. Curcuma longa displayed the highest percent decrease index in comparison to Zingiber officinale with percent disease decrease index of 57.70% and 53.84%, respectively. The findings indicated methanol as the most suitable solvent for descending in the use of Curcuma longa and Zingiber officinale extracts in controlling Alternaria alternata associated with leaf spot of spinach in Kenya.