Efficacy of Mondia whitei (Hook.f.) on Management of Early Blight (Alternaria solani E.) and Late Blight (Phytophthora infestans Mont.) and on Growth and Yield of Tomato (Solanum lycopersicum L.)

Abstract

In Kenya, tomatoes (Solanum lycopersicum L.) are a significant vegetable in horticulture production and consumption. Most tomato farmers use synthetic fungicides to control fungal diseases on tomato crops. However, this method has been proven to be environmentally and health hazardous, and sometimes ineffective as the pathogens develop resistance. To this effect, there is a possibility of exploiting bio-extracts from plants, as fungicides. This study that was carried out at Masinde Muliro University of Science and Technology sought to assess the impact of Mondia whitei Hook.f. extracts on tomato plant growth and antifungal activity both in-vitro and in-vivo, and subsequently screen for phytochemicals. Mondia whitei roots were harvested washed, dried, milled and extracted in methanol (MeOH), dichloromethane (DCM) and ethyl acetate (EtOA) through maceration technique. The extracts' in-vitro antifungal properties against Alternaria solani E. and Phytophthora infestans Mont. at concentrations of 10% and 20% were assessed using the pour plate method. Four treatments were used in a completely randomized design (CRD) for the experiment: Two levels of the extract concentrate (10% and 20%), positive (0.25% ridomil fungicide) and negative (Blank) control which were replicated three times. In-vivo experiment was laid in a greenhouse using CRD with five treatments: Three levels of the extract concentration (2.5%, 10% and 20%), positive and negative control which were replicated four times. Phytochemical screening was done using standard laboratory qualitative techniques. Mondia whitei extracts completely inhibited the in-vitro growth of A. solani and P. infestans at both concentrations of 10% and 20% of all the solvent extracts. Compared to the negative control, which recorded an average disease incidence of 57.86% and 80% for A. solani and P. infestans, respectively, the in-vivo M. whitei treated plants showed a significant (p≤0.05) reduction in disease incidences, with 20% extract concentration recording a disease incidence of 18.5% and 35% for A. solani and P. infestans, respectively. Furthermore, compared to the negative control, which recorded disease severity index values of 8.29 and 10.42 for A. solani and P. infestans, respectively, the plants treated with 20% extract concentration had significantly lower (p≤0.05) disease severity index values of 4.43 and 7.14 for A. solani and P. infestans, respectively. The experiment also showed a significant (p≤0.05) increased growth rate of all growth parameters, with increase in extract concentration. 20% M. whitei extract concentration recorded 133.96cm and 117.71cm for height, 9.81cm and 8.44cm for leaflet size and 17 and 15 compound leaf numbers for A. solani and P. infestans respectively. Whereas, negative control recorded 93.49cm and 99.30cm for height, 7.83cm and 7.13cm for leaflet size and 10 and 8 compound leaf number for A. solani and P. infestans respectively. Secondary metabolites detected were cardiac glycosides, flavonoids, glycosides, tannins, carbohydrates, terpenoids, volatile oils, saponins, alkaloids and steroids. The results of this investigation show that M. whitei exhibits potent fungicidal effects against P. infestans and A. solani. The plant accumulates a diversity of phytochemicals that possibly confer the fungicidal activities. Findings from this study present M. whitei as a potentially safe, affordable and environmentally sound alternative to synthetic fungicides against A. solani and P. infestans in tomato. It also presents the potential use of M. whitei to treat early and late blights, and has provided important leads for the development of new plant-based antimicrobial fungicide.