Efficacy of Plant Extracts and Antagonistic Fungi as Alternatives to Synthetic Pesticides in Management of Tomato Pests and Diseases

Abstract

In the last few decades, farmers have relied on synthetic pesticides to manage crop pests and diseases. This is because synthetic pesticides are easily available, have quick knock down effect, have varied modes of action and are reliable. However, synthetic pesticides are not easily degraded, they leave residues in crop products, are expensive, are harmful to the user and are an environmental hazard. In addition, some pests and disease pathogens have developed resistance to synthetic pesticides and farmers have had to increase frequency of pesticide application since target markets demand aesthetically presentable produce. Presence of residues in fresh vegetables has led to increased interceptions and the produce has been denied access to lucrative markets. This has led to reduced export volumes, loss of market reputation, loss of employment and loss of income. The objective of this study was to evaluate the efficacy of plant extracts and antagonistic fungi compared to the synthetic pesticides in the management of pests and diseases of tomato under in vitro and field conditions. Extracts from different plants were screened for activity against economically important fungal pathogens of tomato. Plant samples were extracted in ethanol and concentrated using a rotary evaporator. The extracts were tested for activity by poisoned food technique, which involved incorporating the extract into the culture media and sensitivity of the fungal pathogen was determined by measuring the pathogen colony radial growth. The most active extracts were further evaluated together with antagonistic fungi for efficacy in managing tomato pests and diseases under field conditions. Their efficacy was compared to synthetic pesticides, a commercial botanical and a commercial antagonist applied weekly. Data was collected on incidence and severity of early and late blight, population of white fly, damage by Tuta absoluta leaf miner, fruit yield and quality. xiv Out of the ten plant extracts evaluated Turmeric (Curcuma longa), lemon (Citrus limon), garlic (Allium sativum) and ginger (Zingiber officinale) showed significant activity against test pathogens. However, the test fungal pathogens varied in sensitivity to the different plant extracts with Alternaria solani being the most sensitive while Fusarium oxysporum f.sp. lycopersici was the least inhibited. Turmeric (Curcuma longa) extract was the most active and reduced mycelia growth of all the tested plant pathogens by up to 45% in vitro. It reduced mycelia growth of Alternaria solani by up to 70% while mint (Mentha piperita) was the least active. Under field conditions, plant extracts were effective in reducing populations of whiteflies and Tuta absoluta by up to 50% while the antagonists reduced the same pests by up to 30% compared to the negative control. Plant extracts and antagonistic fungi significantly reduced early and late blight diseases of tomato. Turmeric extract reduced early blight by up to 30% and late blight by up to 50% compared to the treated control. Majority of yield under grade 1 and 2 was from plants treated with a commercial botanical with neem as the source of extract while garlic (Allium sativum) extract had the highest yield under grade 3. Plant extracts and antagonistic fungi reduced pest and disease damage of fruit yield by up to 40% and 60%, respectively compared to the untreated control. The comparative effectiveness of plant extracts and isolated antagonistic fungi with the synthetic pesticides and the commercialized antagonists and botanicals is proof that the crude products have significant potential. Therefore, there is need for comprehensive explorations into the local environment and more plants and organisms be identified and screened for antimicrobial properties and thereafter tapped and made available to farmers. This will help the average farmer reduce the production costs, have higher income and at the same time have clean, safe and quality produce for high value markets. Key words: Plant extracts, antagonistic fungi, tomato, plant pathogenic fungi