OCCURRENCE AND MANAGEMENT OF ALTERNARIA LEAF SPOT IN SPINACH USING EXTRACTS FROM GINGER AND TUMERIC PLANTS

Abstract

Spinach (Spinacia oleracea) is an economically important vegetable crop in Kenya cultivated by small-scale farmers for domestic consumption. Alternaria leaf spot caused by Alternaria alternata is one of the diseases of spinach which cause low yields and poor quality crop in Kenya. The pathogen has been traditionally controlled using synthetic fungicides which are expensive and harmful to both humans and environment. The present study aimed at establishing the occurrence of Alternaria alternata in the farmer’s fields, characterize the causal pathogen as well as test the efficacy of the extracts of two plants; Curcuma longa and Zingiber officinale against Alternaria alternata both in vitro and in vivo conditions. Extensive surveys were conducted in September 2018 in seven selected sub-counties namely; Mosop, Chesumei, Aldai, Nandi hills and Emgwen of Nandi county, and Kapseret and Moiben in Uasin Gishu county. In each farm the disease incidence was established and diseased samples collected for isolation, identification and characterization in the laboratory. Absolute methanol, ethanol water and ethyl acetate were the solvents used in extraction of Curcuma longa and Zingiber officinale rhizome. The extraction technique used was the modification of the homogenization in solvent. The solvent-to-sample ratio of 10:1 (v/w) solvent to dry weight ratio was used. The decoctions were screened for antimycotic activity using the poisoned food technique. The study revealed that the disease occurred in all the sub-counties surveyed. Out of the forty farms surveyed 57.50% had leaf spot disease. The highest disease incidence was observed in Mosop sub-county (41.31%). Overall disease incidence ranged between 25.79% and 50.51% among the farmer’s fields. When isolates were studied in the laboratory, their morphological characteristics varied from fairly compact to luxuriant mycelial growth, texture varied from feathery to cottony. The growth rate was between 2.98 and 4.05 mm/day, and the spore yield after 12 days was between 4.68×104and 7.4×104 conidia/ml. Curcuma longa and Zingiber officinale extracts displayed varying degree of antifungal activity against the Alternaria alternata depending on the solvent used for extraction and the concentration. Methanolic extracts of Curcuma longa (64.56%) and Zingiber officinale (57.37%) demonstrated the highest antifungal activity against Alternaria alternata, which was significantly different (p≤0.05) as compared to ethanolic, ethyl acetate and aqueous extracts 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 both tumeric and ginger. The results further showed that percentage inhibition increased with increasing concentration of the extracts. The findings indicated that methanol is the best solvent for extraction. The extracts from Curcuma longa displayed the highest percent decrease reduction in comparison to Zingiber officinale of 57.70% and 53.84%, respectively. These results have demonstrated that the use of Curcuma longa and Zingiber officinale extracts in control of Alternaria alternata associated with leaf spot of spinach is possible. The extracts can be incorporated into integrated disease management to reduce/eliminate spread of Alternaria as an alternative to synthetic fungicide application.