Genetic Diversity of Alternaria solani and Phytophthora infestans isolates and their Management using crude Extracts and Essential Oils of Selected Plants in Kenya

Abstract

Kenya is rated 6 in Africa with an annual overall production of 397,007 tons of tomato fruit. Tomato production in Kenya is constrained by many biotic and abiotic factors and among them, diseases such as early and late blight caused by Alternaria solani and Phytophthora infestans, respectively. Farmers use synthetic chemicals to manage the diseases. However, there has been major concern because they are costly, toxic to human beings and the environment. Some of the products have been rejected in the international markets because of high chemical residue levels. Some of the pesticides have not been effective because of the pathogens undergoing genetic mutations that result to resistant strains. This study sought to assess the genetic diversity of pathogens causing early and late blight in tomato and their management using plant extracts and essential oils from selected plants. Isolation of A. solani and P. infestans was done from infected tomato plant samples collected from Mwea, for morphological and molecular characterization. Crude extracts and essential oils of garlic, ginger, Mexican marigold and tick berry were tested in-vitro to determine their efficacy against the two pathogens. Alternaria solani and P. infestans were cultured in Potato Dextrose Agar and V8 agar respectively after amending the media with different plant extracts and essential oils. Pathogen radial growth was monitored per treatment over time. Tomato plants were grown in the greenhouse and inoculated with spores of cultured isolates of A. solani and P. infestans. The inoculated plants were sprayed with essential oils and ridomil th synthetic fungicide as a control and disease development was monitored. Data collected was subjected to analysis of variance to test for statistical significance among treatments. Means were separated using Students Newman Keuls test at 95% level of confidence. Molecular characterization of the pathogens showed that A. solani had high genetic variation, while P. infestans showed low genetic diversity. Characterization of the essential oil compounds revealed that the test plants had many compounds and terpenes were the majority. The in-vitro experiment revealed that both the crude extracts and the essential oils were fruitful in suppressing the growth of the test pathogens. However essential oils were more effective than the crude extracts (p<0.05). Among the crude extracts, garlic was the most effective biocontrol. Ginger and garlic essential oils had similar effect to Ridomil  synthetic fungicide (Metalaxyl-M and S-isomer, Mancozeb) which showed 100% pathogen growth inhibition. In the greenhouse experiment, essential oils and Ridomil  synthetic fungicide did not differ significantly (p>0.05) in lowering disease severity. The tomato plants treated with Mexican marigold had the highest plant height (78.208±1.28) and also the highest number of leaves (20.5±0.93) in plants inoculated with P. infestans. The treatment also exhibited the highest fruit yields. The effects of other treatments did not vary significantly (p>0.05). Essential oils used in the greenhouse experiment had similar effects to Ridomil fungicide and therefore can be tapped for their antimicrobial efficacy. This study concluded that there exists genetic variation within A. solani than in P. infestans in Mwea, Kenya. The source of variation within them needs to be studied. This study recommends that plant extracts and essential oils from ginger and garlic be used as bio-pesticides in management of early and late blight diseases in tomato as they portrayed similar efficacy to ridomil synthetic fungicide. Simpler and cheaper methods of essential oil extraction can be explored to make the oils available for use as pesticides.