Characterisation of phaeoisariopsis griseola, phytochemical screening and fungicidal activity of selected plant extracts against phaeoisariopsis griseola of common bean var. GLP 1127 mwezi moja

Abstract

Common bean is an important legume and a cheap source of proteins. Phaeoisariopsis griseola causing Angular Leaf Spot disease (ALS) is a major constraint for common bean production in Kenya contributing to yield losses estimated at 80%. ALS disease has been difficult to control using cultural practices and resistant varieties which necessitates the need to search for an alternative management option. Synthetic fungicides too have numerous side effects, so the plant-based products are getting more popularity as they are safe to use, easily available and cheap. Most plant extracts possess antifungal activity. Azadirachta indica, Allium sativum and Tithonia diversifolia exhibit antimicrobial activity though little is known about the success of using these crude extracts to control Phaeoisariopsis griseola of common bean. The study was to characterize Phaeoisariopsis griseola, phytochemically screen and determine the fungicidal activity of Azadirachta indica, Allium sativum and Tithonia diversifolia extracts against Phaeoisariopsis griseola of common bean. The specific objectives were to: morphologically profile Phaeoisariopsis griseola from infected common bean, phytochemically screen the plant samples of Azadirachta indica, Allium sativum and Tithonia diversifolia and determine their effects on spore germination using solvent extracts of water, ethanol and methanol, effects on growth, yield and disease severity and incidence. The study was conducted at Maseno University, Department of Botany and greenhouse. Specimens of Azadirachta indica, Allium sativum and Tithonia diversifolia were collected from Maseno university and its environs. The plants were identified by taxonomists in the Department of Botany. Specimens were then air-dried and ground into fine powder for extraction. Methanol, ethanol and distilled water were used for extraction. Phaeoisariopsis griseola were isolated from five diseased common bean plants collected from Ugenya, Bondo and Sabatia sub-counties for morphological profiling and pathogenicity tests done. The pathogen was plated on PDA media and identified based on their cultural and microscopic characters by using taxonomic guides. Fungicidal activity of the extracts was determined using concentrations of 50, 75 and 100% in vitro. Synthetic fungicide and water were used as positive and negative controls respectively. In vivo evaluations were conducted in the greenhouse using common bean plants Var. GLP 1127 Mwezi moja. Six kilograms of solar sterilized sandy-loam soil was used and two common bean plants Var. GLP 1127 Mwezi moja grown per plastic pot and inoculated with 60ml suspension 2.5 x 106 spore/ml of Phaeoisariopsis griseola after 21 days. Seven days after disease inoculation, the plants were sprayed by 100% concentration of the extracts with four replications. The plastic pots were arranged in a greenhouse in a completely randomized design. Data was collected on morphological profiling, inhibition of spore germination for the test pathogen, growth, yield and disease index after 7 days until physiological maturity. The data were subjected to Analysis of variance and treatment means separated and compared using Duncan test (P=0.05). Morphological profiling of isolates of Phaeiosariopsis griseola from Bondo, Sabatia and Ugenya sub-counties revealed variations in terms of hyphae and spores’ characters. Alkaloids, tannins, flavonoids, terpenoids, saponins, cardiac glycosides and sterols were present in nearly all the plants with flavonoids being absent in Azadirachta indica and sterols, saponins and alkaloids being absent in Tithonia diversifolia. All the plant extracts significantly decreased spore germination of Phaeiosariopsis griseola in vitro (P≤0.05) to a mean of 7.6 for Allium sativum, a mean of 7.5 for Azadirachta indica and a mean of 7.3 for Tithonia diversifolia. The most effective was Allium sativum extract. Highest fungicidal effects were observed on plants inoculated with Ugenya isolate followed by Bondo and Sabatia isolates respectively. Moreover, Allium sativum and Azadirachta indica at 100% concentrations had the highest fungicidal effect while Tithonia diversifolia and the other concentrations had moderate to low fungicidal effect proportionate to the concentration percentage. Methanol had the highest extraction potential while water solvent had the least effect. The extracts had higher significant effect on growth index and disease index (P<0. 001). The extracts had no significant effect (P=0.955) on pod weight. From this study plant extracts of Azadirachta indica, Tithonia diversifolia and Allium sativum had higher potential for pathogen control at higher concentrations (100%) hence are recommended as potential botanicals for the control of ALS disease of common bean Var. GLP 1127 Mwezi moja.