BIO-CONROL OF SELECTED BARLEY FUNGAL PATHOGENS USING Paenibacillus polymyxa KaI245 ISOLATED FROM SORGHUM RHIZOSPHERE IN WESTERN KENYA

Abstract

The use of agrochemicals in agriculture has several advantages but has been reported to cause phytotoxicity, hazardous to animals and the environment. In a quest to find alternatives, this study sought to test selected barley pathogens using a sorghum rhizobacterium identified earlier as Paenibacillus polymyxa KaI245. The selected barley pathogens were Drechslera teres f.sp teres (causal agent of barley net blotch) Rhynchosporium commune (causal agent of scald) and Puccinia graminis f. sp hordei (causal agent of barley stem rust). The objectives were; to isolate the fungal pathogens, screen P. polymyxa KaI245 for antifungal activity, by both dual culture technique and fungal inhibition by production of volatile organic compounds, enhancement of the bacterium’s antifungal activity using organic compounds, characterization of the antifungal compounds produced by the bacterium and greenhouse studies to establish efficacy of bacterial metabolites in controlling barley net-blotch. The fungal colonies were isolated from the symptomatic leaves and identified based on their cultural and morphological characteristics. There was marked variation in the morphology of D. teres. Some appeared grey on Potato Dextrose Agar while the majority were black. Conidia were cylindrical in shape having 3 to 6 septa. Some had a conspicuous scar. R. commune colonies were initially whitish pink and gradually turned white then grey. It is a slow grower with short and bulbous conidia. P. graminis f. sp hordei was identified from the brown pustules on the plant stems. Both the urediniospores and the teliospores were examined and identified under the microscope. The growth of mycelia in D. teres was inhibited by 47.3% in dual culture with P. polymyxa KaI245 while there was no significant inhibition by the bacterium on R. commune colonies. Paenibacillus polymyxa KaI245 cell-free supernatant obtained from Potato Dextrose Broth inhibited the mycelial growth of D. teres by 24.1%, but didn’t significantly affect the germination of teliospores, (P = 0.16). P. polymyxa KaI245 volatile organic compounds inhibited the growth of R. commune by 52.9%. These volatiles also had negative effects on the germination of P. graminis f. sp hordei teliospores. Germination dynamics of volatile exposed teliospores verses non-exposed differed significantly at P = 0.05 (chi sq = 27.53, df = 3, P = 1.4E-11). Efficacy of P. polymyxa KaI245 cell-free culture filtrate was performed in the greenhouse on barley plants infected with D. teres. Both the normal and double concentration of the crude bacterial suspension had a mean disease score of 1.8 which translated to 50% disease suppression compared to the positive control. Orius® 430 SC, a commercial synthetic fungicide recorded a mean disease score of 1.0 translating to 72.2% reduction in disease occurrence. Acetone and methanol were used in attempts to enhance the production of antifungal metabolites by the bacterium. Addition of 1% Acetone to the growth medium showed enhancement of the bacterium’s antifungal activity, with the inhibition of the fungal mycelium increasing to 27.9% down from 19.7%. Methanol stifled the bacterium’s ability to produce antifungal metabolites as was exhibited by reduced inhibition of mycelial growth of D. teres (10.3%). Characterization of antifungal metabolites produced by P. polymyxa KaI245 using GC-MS revealed 50 different metabolites. Addition of 1% acetone to P. polymyxa KaI245 growth medium led to the reduction of antifungal metabolites to 22 although 9 new metabolites were produced. Addition of 1% methanol on the other hand, also resulted in reduced number of antifungal metabolites (35) with 11 new metabolites produced. The bacterium metabolites have the potential to control the selected pathogens of barley as demonstrated in the case of D. teres.