Activity of extracts from selected Tanzanian spices on major fungal pathogens and blight diseases of Tomato

Abstract

Tomato (Solanum lycopersicum L.) is affected by many fungal diseases and farmers rely on synthetic pesticides for management. Detrimental effects associated with injudicious use of chemical pesticides have caused a demand for alternative crop protection products. The objective of this study was to evaluate the antifungal activity of selected spices against major fungal pathogens and blight diseases of tomato. An ethnobotanical survey for ginger and turmeric and evaluation of awareness of botanical pesticides use and collection of commonly consumed spices was conducted in Tanzania. Ethanolic extracts from spices were prepared by maceration and tested for antifungal activity in posisoned food bioassay against Phytophthora infestans, Alternaria solani, Fusarium oxysporum f.sp. lycopersici and Pythium spp in vitro, their effect on seed germination and efficacy on severity of early and late blight diseases of tomato in. The most active extracts were fractionated in solvents with varied polarity, analyzed for biochemical composition and tested for fungicidal activity. Results indicated that among the tested spices, clove extract was the most active, inhibiting all the fungal pathogens (100%). Combined effect of the most active extracts showed that clove combined with either ginger, black pepper or turmeric inhibited growth of P. infestans (100%). The activity of solvent fractions was lower compared to the crude ethanolic extracts, except for clove fractioned in n hexane which completely inhibited the growth of P. infestans. Therefore, the most active compounds were better extracted in ethanol. Gas chromatography- mass spectrometry analysis of the clove’s n- hexane fraction showed high abundance of eugenol (74%) which is likely to be responsible for the high antifungal activity. High concentrations of the spice extracts deterred and slowed germination but low concentrations stimulated seed germination of up to 98%. Under field conditions black pepper extract reduced severity of late blight by 40% while clove extract reduced severity of early blight by 35% compared to the untreated control. The findings herein are proof of activity of spice extracts under in vitro and field conditions. This study recommends that the most active extracts be considered for development of a broad botanical fungicide for management of fungal diseases of tomato.