Experiments to determine the effect of the three viruses most common on Kenyan sweet potato varieties were carried out at the University of Nairobi Kabete farm. The sweet potato varieties were Bungoma, Kemb 10 and Ex-Shimba hills which were either virus-free or infected with Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic stunt virus (SPCSV) or a combination of SPFMV and SPCSV. Canopy photosynthetically active radiation (PAR) interception and the relative chlorophyll content of the plants were determined. At harvest, marketable tuber yield, harvest index (HI), leaf area index (LAI) and specific leaf weight (SLW) were measured.Infection with single viruses caused no or mild symptoms and had no significant effect on chlorophyll content of leaves of any variety. Dual infection with SPFMV and SPCSV resulted in severe symptoms characteristic of sweet potato virus disease complex (SPVD) and significantly reduced chlorophyll content. Only SPCSV and SPCSV + SPFMV reduced PAR interception, the latter combination especially. Generally, infection with single viruses increased the SLW but a decrease occurred in plants dually infected with SPFMV and SPCSV. Whereas SPVD significantly reduced the harvest index (78%) and tuber yield (98%) in the three varieties, infection with single viruses caused a reduction only in the varieties Bungoma and Kemb 10. SPVD reduced yield through reducing leaf chlorophyll content, PAR interception, LAI and HI. The single virus infections possibly reduced assimilate translocation from leaves to tubers as is inferred from high SLW. Effects of viruses on physiological processes and yield are discussed.
Light and water are among essential resources required for production of photosynthates in plants. A study on the effects of weeding regimes and maize planting density on light and water use was conducted during the 2001/2 short and 2002 long rain seasons at Muguga in -the central highlands of Kenya. Weeding regimes were: weed free (W1), weedy (W2), herbicide (W3) and hand weeding twice (W4). Maize planting densities were 9 (D1) and 18 plants m -2 (D2) intercropped with Phaseolus vulgaris (beans). The experiment was laid as randomized complete block design replicated four times and repeated twice. All plots were thinned to 4 plants m -2 at tasseling stage (96 DAE) and thinnings quantified as forage. Soil moisture content (SMC), photosynthetically active radiation (PAR) interception, evapo-transpiration (ET crop), water use efficiency (WUE), and harvest index (HI), were determined. Percent PAR was higher in D2 than in D1 before thinning but higher in D1 than in D2 after thinning in both seasons. PAR interception was highest in W2 but similar in W1, W3 and W4 in both seasons. SMC was significantly lower in W2 but similar in W1, W3 and W4. D2 had lower SMC than D1 in season two. Weeding regime significantly influenced ET crop, while planting density and weeding regime significantly influenced WUE and HI. D2 maximizes water and light use for forage production but results to increased intra-specific plant competition for water and light severely before thinning (96 DAE) that reduce grain yield in dual purpose maize, relative to D1.
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