European Journal of Plant Pathology 107: 849–860, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Vegetative growth of sorghum and Striga hermonthica in response to nitrogen and the degree of host root infection Woldeyesus Sinebo 1 and D.S.H. Drennan 2 1 Holetta Research Centre, P.O. Box 2003, Addis Abeba, Ethiopia (Fax: +2511370377; E-mail: iar@telecom.net.et or wsinebo@hotmail.com); 2 School of Plant Sciences, Whiteknights, P.O. Box 221, Reading, RG6 6AS, UK Accepted 23 July 2001 Key words: complete infection, nitrogen, partial infection, Sorghum bicolor Abstract The effects of nitrogen and the extent of sorghum root infection by Striga hermonthica on host-parasite association during vegetative growth were studied using a split root system in a 3 × 3 factorial combination of N (37 mg on one, 18.5 or 37 mg on both root-halves) and Striga (no, one or both root-half infection). High N increased sorghum shoot weight by 22% more than low N, but did not significantly affect Striga growth 64 days after transplanting sorghum (DAP). Striga reduced sorghum stem height and weight by 22% and 25% at 38 DAP, and by 34% and 36% at 64 DAP, respectively. Leaf weight was not affected. Striga stimulated root growth 38 DAP, but not 64 DAP. In partially infected sorghum, 64 DAP, the parasite shoot number, shoot height and shoot dry weight were 36%, 46% and 35%, respectively and host shoot dry matter was 142% of those in fully infected plants, indicating an inverse relationship between the degree of host root infection and the level of resistance. The results suggest that sorghum released resistance-confering substances to the infection points after sensing infection. When infection points are widely distributed as in fully infected sorghum, less of such substances appear to render the host more vulnerable. Abbreviations: DAP – days after transplanting sorghum; vs. – versus; df – degrees of freedom. Introduction Sorghum (Sorghum bicolor (L.) Moench), in sub- Saharan Africa, is grown by subsistence farmers on more than 48 million hectares. Striga hermonthica (Del.) Benth. is the witchweed that parasitises this crop in low input well-drained infertile soils where drought is also a problem (Nour et al., 1986). The estimated grain-area in Africa infested by Striga is 21 million hectares, and the loss in grain production is 4.1 million tons. The potential loss in grain produc- tion in Africa alone might amount to 44 million tons (Sauerborn, 1991). The parasite affects the host directly through compe- tition and indirectly by inducing pathological effects. The latter starts before the emergence of the para- site and accounts for 80% of the loss in production (Press et al., 1990). Striga infection results in stunted shoots. Grain and stem weights are reduced but often the roots remain relatively unaffected (Egly, 1971; Graves et al., 1989; Press and Stewart, 1987; Raju et al., 1990). As a result, plant allometry is altered. Studies have shown that the parasite tends to do poorly in the presence of high levels of added N with a concomitant improvement on host growth (Last, 1960; Bebawi and Farah, 1981; Bebawi and Abdelaziz, 1983; Gworgwor and Weber, 1991; Cechin and Press, 1993a,b; Mumera and Below, 1993; Raju et al., 1990). Nitrogen delays emergence, and reduces infestation and growth (height and weight) of the parasite. The mechanism by which N delays or reduces the effect of Striga in sorghum is not clearly understood. Nitrogen may inhibit seed germination and radicle extension (Pesch and Pieterse, 1982) by weakening