Effect of Phosphorus and Desmodium on Striga Hermonthica (Del) Benth . Incidence in Maize (Zea Mays L.) Based Cropping System in Western Kenya.

Abstract

Continuous cultivation of land in w. Kenya has resulted in depletion of soil nutrients and increase in S. hermonthica incidence. Maize yields are less than', L t ha' although the potential is 4_5t ha". However, incorporation of some desmodium species in cereal based systems is known to reduce Striga seeds through allopathic mechanism. We tested the hypothesis that phosphorus catalyses production of desmodium root exudates and its effectiveness in controlling Striga weed. Three pot experiments were conducted at KARI- CIMMYT Striga Research Station - Kibos. A field experiment was also sited at Ngiya and Ndori in Siaya County. The objectives of the experiments were:- to determine the effects of P fertilized desmodium species on Striga germination, seed viability, density, Striga biomass, seed bank, soil fertility, maize yield components as well as desmodium nodulation and biomass production. The 2 by 8 factorial pot experiments were laid on a split plot design with desmodium species as main plots while fertilizer rates (0, 10.3,20.6,30.9,41.2,51.5,61.8 and 72.1mg P kg -I soil) as sub plots. All pot experiments were replicated 3 times. Treatments for field experiment included the two desmodium species as main plots while Prates (0, 23, 46 and 69 kg P20S ha" ) formed the sub plots. The treatments were replicated 4 times. Data was subjected to ANOVA. Significantly different means were separated by LSD at 5% level of probability. Results revealed that mature desmodim plants (21 to 33weeks) were more effective in suppressing Striga incidence compared to the young (8 to 21 weeks) plants. Exposing Striga seeds to root exudates for 42 days increased seed germination by 9 and 13% under D.intortum and D.uncinatum treatments respectively. Continuous exposure of seeds to the exudates for 42 days significantly (p ~ 0.05) reverted viable seeds to dormant status, desmodium species not withstanding. Application of phosphorus at ~ mg 51.5 mg P kg" soil significantly (p::;0.05) increased nodulation in D.intortum while D.uncinatum had nodule number significantly increased at 72.1 mg P kg" soil. Soil pH was reduced by 0.26 and 0.11 units at Ndori and Ngiya sites respectively. Application of phosphorus at 46 and 69 kg P20S ha" increased soil organic carbon by 0.2% and 0.25% respectively at Ngiya. There is need to research on the mechanism involved in reverting viable seeds to dormant status and quantify biologically fixed nitrogen associated with the respective desmodium species.