Occurrence of fusarium headblight of wheat in Nakuru district and its management using biocontrol agents
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Fusarium head blight (FHB) has recently re-emerged as a devastating disease of small grains throughout the world. Different management strategies have been tried but control of the disease has not yet been achieved. Survey for the occurrence of FHB of wheat was carried out during the 2006 cropping season in five agro-ecological zones of Nakuru. The disease incidence was determined as the number of blighted heads over the total number of heads in a 10m2 while severity was determined as the proportion of the spikelets bleached. Wheat and maize grain samples were collected from the same farms and causal fungi isolated and identified to species level. Pathogen isolation was by plating on agar culture media. The DON contamination was determined using competitive ELISA method. Laboratory and greenhouse studies were also conducted to evaluate the efficacy of selected antagonists namely, Bacillus sp, Alternaria sp, Trichoderma sp and Epicoccum sp in management of PI-rn. The greenhouse experiments were arranged in a completely randomised design. Data on severity was determined as the proportion of spikelets bleached and yield was determined as ten-ear weight and yield per pot. The laboratory experiment involved growing the pathogen and the antagonists in dual cultures. Antagonism in culture against Fusarium graminearum was determined as reduction in colony diameter. Most farmers used seeds from their neighbours (58%) or their own seed 25% for planting. Most also (92%) rotated wheat with wheat or maize. Head blight was found to be highly prevalent (88-100%) and mean incidence ranged 4 to 9%. Severity ranged from 15-37 %. The main fungal genera isolated from wheat heads and grains were Fusarium sp, Alternaria sp and Epicoccum sp. Fusarium sp was most prevalent (42%) in .infected wheat heads while Epicoccum sp was most prevalent (35%) in harvested wheat grain. Only Fusarium sp and Penicillium species were found in maize grains. Fusarium poae had the highest isolation frequency followed by F. graminearum and F. chlamydosorium in both diseased heads and wheat kernels. Fusarium verticilloides was the most frequently isolated (58%) from maize. Fusarium graminearum was found in both wheat and maize. There was no significant difference on the Fusarium species over the AEZS. There were no significant differences on the DON levels over the AEZS. The mean DON levels were 249 and 221 ug/kg for wheat and maize respectively. The DON in wheat ranged from 99-3691lglkg whereas that in maize ranged from 13.6-634 ug/kg, The four antagonists significantly reduced colony diameter with the highest reduction being in paired cultures with Trichoderma (64%) and least effective was Epicoccum (44%) colony growth reduction. The antagonists had little or nonsignificant reduction of disease severity. Two antagonists Alternaria and Epicoccum significantly reduced the DON levels. The results indicate that head blight in Nakuru district is due to a complex of Fusarium species and that it leads to varying levels of DON over the AEZs. It showed that Fusarium graminearum could be major causal agent of head blight in wheat and ear rot in maize. There was indication of a possible cross contamination between maize and wheat. It is therefore important to sensitize farmers on methods of reducing disease and DON levels and the importance of better rotational programmes. It was also clear that the antagonists effectively reduced colony growth in culture but little success in greenhouse tests. The reduction of DON by Alternaria and Epicoccum species was also pronounced. More research on efficacy 'of other antagonists should be done and determination of wheather the reduction in DON was accompanied by an increase in other toxins.