Isolation, Characterization and Potential Use of Alternaria Brassicicola in Biocontrol of Striga Hermonthica on Zea Mays in Siaya County, Kenya
Ogada, Abogno Reuben
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The witchweed [Striga hermonthica (Del.) Benth] is a highly successful parasite of cereal in Sub- Saharan Africa, leading to enormous economic losses. In Kenya, the weed reduces maize yield by between 35%-100%. Previous studies done on the efficacy of “Foxy-2” in Kenya were not effective in biocontrol of the weed. Nevertheless, Striga has continued to increase both its host range and area under infestation. This study aimed at isolation, characterization and potential use of Alternaria brassicicola as a new bio-agent for Striga hermonthica control on maize farms. A. brassicicola were isolated from cabbage leaves infected with Alternaria dark leaf spot disease in Limuru and Nyeri farms, Kenya. The colony, conidiophores and conidia of each isolate, were viewed under X40 Leica DM 500 microscope (Leica, Germany). DNA of the isolates was successfully extracted by the CTAB method. The amplified ITS regions of 5.8S rDNA were molecularly analysed and sequenced using ITS 1 and ITS 4 primers. Striga seeds (0.25mg) from Kibos were sterilized and inoculated with 5 ml spores suspension of the isolates and preconditioned for 11 days. To determine germination inhibition, 3 ml of 0.1 ppm GR-24 was added on the 11th day and incubated overnight at 30ºC. The radicle formed were viewed by Leica MZ7F Microscope (Leica, Germany) and analysed using ImageJ (version 2019) software. In the greenhouse trial, sterilized S. hermonthica seeds were inoculated with A. brassicicola isolates that showed ≥ 62% germination inhibition. The inoculated seeds were mixed separately in each pot with soils (heated at 100ºC) sampled from 51 Striga infested farms each in Bondo, Ugenya and Gem. Seeds of local white maize landrace known as ‘Rachar’ were planted 7 days later at a depth of 5-10cm in each pot. Morphological and molecular analysis confirmed that the isolates were either strains of A. brassicicola, A. alternata or A. brassicae. The isolates significantly (P ≤ 0.0001) inhibited germination by between 50.9±25.19 (NY019) to 73.3±11.23 (LM013) over the control. Soil characteristics did not influence (P ≥ 0.05) the emergence of inoculated Striga seeds. A. brassicicola isolates significantly (P ≤ 0.05) reduced Striga emergence by between 29.1% (LM017) to 78.9% (LM019a). However, isolate NY021 enhanced Striga seeds emergence by 30.4% over the control. This study established that some strains of A. brassicicola have the potential to inhibit the germination and emergence of Striga seeds. Therefore, the fungus could be used as a possible bio-agent in the production of mycoherbicide against Striga weed to enhance cereal’s productivity.