Kari 3rd Scientific Conference Programme and Abstracts October 1992

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Kenya Agricultural Research Institute
The parasitic angiosperm Striga (Striga hermonthica Del. Benth) infects tropical cereals causing heavy losses. Investigations were under taken in Western Kenya in 1989 and 1990 to determine if the occurrence of Striga in maize (Zea mays L.) could be mediated by amount, source and timing of N availability; and if the efficacy of N is contingent upon its regulation of assimilate partitioning. An additional objective was to determine how level is influenced by genotype. One experiment evaluated N rates of 0, 30, 60 and 90kg. ha-l supplied as either urea, Ca(N03)2' NH4N03 or (NH4)2S04 plus the nitrification inhibitor dicyandiamide (DCD), while a second experiment evaluated similar N rates applied at 14, 21, and 35 days after planting. Regarding genotypes, one experiment evaluated the maize cultivars KCB, H635, H512 and H622, while another examined 16 diverse genotypes. Although Striga infection generally decreased with increasing N availability, the impact was partially dependent on the severity of infestation. All N rates decreased infection in 1990, while only 90kg. N reduced infection in 1989. Under high parasite intensities in 1989, only urea reduced Striga (26%), while in 1990, infection was significantly reduced (30%) by all sources of N. In both years, N applied at 28 DAP resulted in the least infection. Although assimilate partitioning during vegetative growth was unresponsive to N treatments, N availability during reproductive growth altered partitioning in favor of the ear over vegetation. This alteration resulted in increases in grain yield (64%) and harvest index (27 o/r), and a decrease in the concentration of total non-structural carbohydrate (TNC) (16%). An up to 3-fold range was observed among maize genotypes in their resistance to Striga. In general, Striga infestation decreased with shorter maturities such that the earliest genotypes to mature, were also the most resistant. This resistance response appears related to ability of early maturity genotypes to efficiently partition assimilates to the ear before incipient competition from Striga. Full season genotypes have initially weaker reproductive sinks which allows for the build-up of TNC in their stalks and roots. Excessive TNC could then serve as substrate to support growth of Striga. This effect would explain the more intensive infestation in full season genotypes and N deficient plants as stalk TNC was significantly correlated with seasonal Striga density in both experiments. Based on these findings, judicious use of N fertilizer and selection of resistant genotypes can be important components of an integrated Striga control strategy, particularly in subsistence agricultural systems in Africa and Asia where this parasite poses a significant threat to food production.