MANAGEMENT OF SORGHUM ANTHRACNOSE USING BIOCONTROL AGENTS PRODUCED BY SORGHUM RHIZOBACTERIA IN WESTERN KENYA

Abstract

The use of sorghum rhizobacteria antifungal agents in the management of sorghum anthracnose was made possible by isolating a total of 294 rhizobacterial isolates from sorghum rhizosphere soil of different agro-ecological zones of western Kenya and screening them in vitro against isolated sorghum foliar fungal pathogens, viz: Alternaria alternata (leaf spot), Aspergillus candidus (seed rot), Alternaria longissima (Grain storage mould), Botrytis cinerea (grey mould), Colletotrichum gloeosporioides (anthracnose), Colletotrichum sublineolum (anthracnose, red stalk rot), Exserohilum turcicum (leaf blight), Fusarium equiseti (associated with Fusarium moniliforme; pathogenicity still questionable), Fusarium moniliforme (stalk rot) and Nigrospora oryzae (ear rot). Screening yielded 101 sorghum rhizobacterial isolates that showed antagonism against the sorghum test fungal pathogens. Ninety five of them inhibited the pathogens by ≥ 30% whilst those that inhibited at least one of the test pathogens by ≥ 70% were 35. The acrisol soil type consistently produced antagonistic rhizobacterial isolates regardless of prevailing soil conditions. Of the 35, two rhizobacterial isolates labeled KaI245 and MaI254 inhibited fungal growth of all the pathogens tested, an indication that the two isolates produce potent broad-spectrum antifungal agents. Fungal growth inhibition produced by isolate KaI245 were greater and honestly significantly different from those of isolate MaI254 at p < 0.05 hence the former was considered to be the best antimicrobial agent producer. Biochemical tests and the API system identified isolate KaI245 as Aeromonas hydrophila while isolate MaI254 was identified as Bacillus megaterium. Aeromonas hydrophila KaI245 produced appreciable amounts of antifungal agents in liquid media containing potato infusion and glucose. Its culture-filtrate exhibited antibiotic activity against all the test sorghum pathogenic microorganisms. Purification of the antifungal agent-culture-filtrate by column chromatography using both polar and non-polar solvents enhanced the activity of the filtrate. This was reflected by a 65.47% increase in size of the clear zones of inhibition produced against C. sublineolum when the purified culture-filtrate was used. Paper chromatography combined with bioautography of the isolate’s culture-filtrate produced one continuous zone of inhibition against C. sublineolum, an indication that the culture filtrate contained one active compound. Optimal antifungal agents production in synthetic media include: fructose as the carbon source, an incubation period of 6 days with shaker flasks agitated at a speed of 180 rpm, an initial synthetic medium pH of 7, and an incubation temperature of 28oC. In vivo tests in the greenhouse showed that Aeromonas hydrophila KaI245 antifungal agent-culture-filtrate concentrated twice significantly delayed and suppressed (p < 0.05) the development of sorghum anthracnose caused by C. sublineolum on sorghum leaves. This compared favourably with Folicur®430SC, a systemic fungicide used in the control of a range of fungal diseases including sorghum anthracnose. The results of this research provide a novel, sustainable and environmentally friendly method of controlling cereals diseases using sorghum rhizobacteria.