In-vitro evaluation of the anti-fungal activity of Tetradenia riparia and Ageratum conyzoides against Alternaria solani

Abstract

Early blight is a serious disease of tomato caused by Alternaria solani. Control of this fungus is by foliar application of fungicides, crop rotation and sanitation to prevent future outbreaks. However, use of fungicides is expensive and causes harmful effect to the environment and human health thus the need to search for alternative antifungal agents which could be effective and environmental friendly. The study investigated antifungal properties and MIC of two selected herbs using in-vitro techniques. Ethanolic extracts of Tetradenia riparia and Ageratum conyzoides were tested against A. solani isolated from infected tomato fruits. To isolate the pathogen, tomato fruits with typical symptoms of early blight were collected from Kyaliwajala market in Wakiso district. They were surface sterilized using 70% ethanol, rinsed with sterile distilled water and dried with sterile filter papers. The fungi was picked and placed in petri dishes containing Potato Dextrose Agar and kept at 25o for 7 days. Pure culture was obtained by sub-culturing the isolates on PDA media. Pathogen was identified following the cultural and morphological characteristics criteria. Cultural characteristics were observed directly by pigmentation on medium and mycelial growth pattern on PDA plates. The morphological characteristics of A. solani that were observed under the microscope include large, long-beaked spores. In order to confirm the identity of the isolates obtained, a pathogenicity test was conducted under in-vitro conditions using healthy tomato fruits. The crude ethanolic extracts were then tested against A. solani using the Agar well diffusion method and the minimum inhibition concentration was determined. Mancozeb was used as the positive control. Both extracts showed a significant effect on reducing fungal growth as showed by zones of inhibition, however the higher the concentration, the less the fungal growth (Fig. 7). The MIC for T. riparia was 15% and 25% for A. conyzoides (Fig. 8). Therefore T. riparia is a better antifungal agent at lower concentration as compared to A. conyzoides against A. solani. More studies need to be done to validate these plants under field conditions and there after phytochemical screening and isolation of active compounds.