Evaluation of tick-trefoil (desmodium species) for drought tolerance and control of witchweed \{striga hermonthica (del.) benth.\} in grain sorghum and branched broomrape (orobanche ramosa l.) in tomato

Parasitic weeds of the genera Striga and Orobanche pose a severe problem for agriculture because they are difficult to control and are highly destructive to several crops. The present study was undertaken to evaluate local and exotic Desmodium species for drought tolerance and for ability to suppress S. hermontica on sorghum and O. ramosa on tomato. A series of laboratory, green house and field experiments was conducted during the period 2006 to 2009 at the Gezira Research Station Farm, Wad Medani, Central Sudan and the Gedarif Research Station, eastern Sudan. Laboratory experiments studied the i) effects of temperature and drought on germination of Desmodium species and ii) influence of Desmodium species root exudates on germination of S. hermonthica and O. ramosa. Five Desmodium species; the exotic, D. uncinatum, D. intortum, D. distortum, D. tortuosum, and two collections of the local species D. dichotomum, obtained from Damazin and Kadugli were used. Laboratory experiments showed that gennination of Desmodium spp. increased with increasing temperature and time. Gennination was delayed at 15 deg C while temperatures of 25 to 30 deg C resulted in optimal germination. PEG 8000 at low concentration (50 g/L) had no adverse effect on germination. However, PEG 8000 concentration at 100 g/L it delayed gemunation. At 200g/L PEG 8000 resulted in complete or near complete suppression of germination of D. uncinatum. In all Desmodium species radical length significantly decreased with increasing PEG 8000 concentration. Undiluted root exudates of D. distrotum induced higher germination (35%) of S. hermothica than the other species. None of the Desmodium species root exudate induced germination of O. ramosa. D. uncinatum and D. dichotomum curtailed Striga attachment to sorghum roots. Potted D. tortuosum, was slightly more drought tolerant than the other species. At its lowest level of infestation (5mg/ pot), Striga emergence significantly declined at the highest D. uncinatum population density. D. dichotomum when planted 60 days prior to sorghum was more effective in suppressing the parasite than when planted 30, 90 days prior to sorghum or planted on the same day as sorghum. In field trials, D. uncinatum and D. intortum gave the lowest stand in comparison with other species. None of the Desmodium species showed regeneration in the second season. Field trials at GRSF revealed that sorghum planted in plots previously sown to D. dichotomum and D. tortuosum displayed 39 and 77% increase in grain yield, respectively. In season 2008/09 trials at GRSF, employing two sorghum genotypes, intercropping with D. dichotomum reduced Striga emergence, Striga biomass and increased yield by 161 %. At Gedarif (2007/08), intercropping of sorghum, cv. Korokollo, with D. dichotomum increased grain yield significantly. Intercropping with other Desmodium species increased grain yield over the sole crop, albeit not significantly. Field trials on tomato undertaken in 2006/07 showed that Desmodium species, planted between holes or in the same hole as tomatoes, reduced O ramosa emergence and increased tomato yield significantly. However, the increase in yield was not significant in season (2007/08)