The influence of time and severity of Striga infection on the Sorghum bicolor - Striga hermonthica association Promotor: Prof. dr. M.J. Kropff Hoogleraar in de Gewas- en Onkruidecologie Co-promotor: Dr. ir. L. Bastiaans Universitair docent, leerstoelgroep Gewas- en Onkruidecologie Promotiecommissie: Prof. dr. W.H.O. Ernst (Vrije Universiteit Amsterdam) Prof. dr. J.M. van Groenendael (Radboud Universiteit Nijmegen) Prof. dr. ir. H. van Keulen (Wageningen Universiteit) Prof. dr. Th.W. Kuyper (Wageningen Universiteit) Dit onderzoek is uitgevoerd binnen de onderzoekschool: Production Ecology and Resource Conservation The influence of time and severity of Striga infection on the Sorghum bicolor - Striga hermonthica association Aad van Ast Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit prof. dr. M.J. Kropff in het openbaar te verdedigen op vrijdag 2 juni 2006 des namiddags te vier uur in de Aula. Aad van Ast (2006) The influence of time and severity of Striga infection on the Sorghum bicolor - Striga hermonthica association. van Ast, A. - [S.l.: s.n.]. Ill. PhD thesis Wageningen University. - With ref. - With summaries in English, French and Dutch ISBN: 90-8504-399-9 “Gelukkig hij, die de oorzaken van de dingen heeft leren inzien”. “Felix qui potuit rerum cognoscere causas” Vergilius Aan Ans Abstract Ast, A. van, 2006. The influence of time and severity of Striga infection on the Sorghum bicolor - Striga hermonthica association. PhD thesis, Wageningen University, Wageningen, The Netherlands, with summaries in English, French and Dutch, 154 pp. This thesis presents the results of a study on the interaction between the parasitic weed Striga hermonthica (Del.) Benth. and sorghum (Sorghum bicolor [L.] Moench). The main objective of the study was to investigate the effects of time and level of Striga infection on the interaction between host plant and parasite. Consequences for sorghum performance and the growth and development of the parasite were examined. A comparison between two sorghum cultivars differing in level of Striga tolerance, revealed that the absence of a negative effect of Striga infection on photosynthetic rate and a delayed time of first Striga infection both contributed to the lower extent of yield reduction of the tolerant cultivar. Likewise, in an experiment with a wide range of Striga seed infestation levels, it was observed that higher soil infestations levels did not only result in a higher Striga infection level, but also in an advanced time of first Striga infection. The importance of time of infection was further investigated in a pot experiment in which the time of infection was artificially delayed. Striga parasitism and reproduction, and the detrimental effects of Striga on crop performance could be strongly reduced by delaying the time of first infection. Prospects of reducing Striga parasitism by means of cultural control methods that are based on the principle of a delayed onset of Striga attachment were assessed. In a pot experiment, the combination of shallow soil tillage, deep planting and the use of transplants resulted in a four-week delay in first emergence of the parasite, a strongly reduced infection level of the sorghum host and highly improved sorghum yields. Evaluation of these methods under field conditions resulted in a 85% reduction in Striga-infection level, but as no delay in time of parasite infection was established, no beneficial effect on crop yield was obtained. Potential causes of the absence of a delay in Striga infection time under field conditions were discussed and alternative options for establishing a delayed infection in the field were proposed. Keywords: Striga hermonthica, Sorghum bicolor, infection time, infection level, tolerance. Preface When writing this preface, I realize only too well how many people have contributed, directly or indirectly, in the foreground or in the background, to the completion of this thesis. Here, I like to take the opportunity to thank all of them for their contributions. I will thank you all personally, but here I can only thank a few people by name. First of all I thank my promotor, Prof. dr. Martin Kropff, for the confidence he showed in me when I asked him, shortly after his appointment as professor in Crop and Weed Science, if he was willing to supervise the PhD project I envisaged. Martin, I also like to thank you for your stimulating enthusiasm for the project, for your scientific input, and for the opportunity you gave me to carry out my research besides my regular work for the department. I am deeply indebted to my daily supervisor and co-promotor Dr. Lammert Bastiaans. His stimulating attitude, his clear ideas when discussing the experimental designs, his thorough and constructive comments on the manuscripts, and the lively discussions that resulted from them, strongly contributed to the realization and quality of this thesis. Lammert, I am very grateful for all the time you invested in me, and for your continuous confidence in a good final result. Besides, I recall with great pleasure our travels to Mali and Burkina Faso, where we combined business with pleasure. Also our common interest in sports, both active and passive, contributed much to our friendship. Lammert, I look forward to our continued collaboration in education and research for many years to come. My first steps in research on parasitic plants and weeds I took under the inspirational supervision of two persons who had since long won their spurs in this field : Dr. Siny ter Borg of the former department of Vegetation Science, Plant Ecology and Weed Science of Wageningen Agricultural University, and Dr. Arnold Pieterse of the Royal Tropical Institute in Amsterdam. I gained my first knowledge and experience in the field of parasitic plants as a research assistant in the projects on Orobanche (broomrape) conducted by Siny and Arnold. I am very grateful to the both of you for transferring your knowledge, and exciting my interest in this group of extraordinary plant species. Arnold, I also like to thank you for being my guide in Mali during my first visits to this fascinating country and its friendly population. I thank Prof. dr. Thom Kuyper, Dr. Tjeerd Jan Stomph and Dr. Harro Bouwmeester for their interest in my research and for the discussions that put my work in the broader perspective of food production and security. The research was enabled by many people around me. First, I like to thank Jet Drenth and Ans Hofman, not only for their assistance in the greenhouse experiments, but also for taking over several of my duties in the department, which gave me time to carry out this PhD-research. I am grateful to Peter van de Putten and Peter van Leeuwen for their assistance in the photosynthesis measurements, and to Jacques Withagen for his advice on the lay-out of the experiments and the statistical analysis of the results. Gon van Laar edited the manuscript of the thesis in her well-known, accurate way, for which I am very thankful. I sincerely thank Gijsbertje Berkhout for her expertise in the financial settlement of my research activities. Une partie de la recherche a été effectuée à l’Institut d’Economie Rurale (IER) du Mali à la Station de Recherche Agronomique de Cinzana. Je voudrais remercier la direction de l’IER, et particulièrement dr. Bourema Dembélé, Directeur Scientifique, qui a bien voulu mettre à ma disposition les services et structures de son organisme. Monsieur Seriba Katilé, mon cher ami, merci beaucoup pour ton assistance dans l’organisation et l’installation des expérimentations au champ. En suite je voudrais remercier Madame Katilé et Madame Hawa Coulibaly pour leur hospitalité et les bons repas durant mes séjours à Cinzana. Les assistants de recherche, Siaka Dembélé, Amadou Tembely, Mamadou Ely Coulibaly, Mahamadou Ousmane Diarra et Badjan Bouaré sont sincèrement remerciés pour leurs contributions dans les expérimentations au champ. Je vous remercie tous pour m’avoir accueilli et m’avoir mis à l’aise à la station de recherche agricole à Cinzana. The research presented in this thesis has partly been conducted within the framework of the EU INCO-DEV program “Improved Striga Control in Maize and Sorghum” (ISCIMAS). The collaboration between the various research groups was very inspirational to me. For this I thank all the participants of the programme: Prof. dr. Mike Beale and Mrs. Vicky Child (IACR – Long Ashton Research Station, UK), Dr. Doulaye Traoré, Dr. Oumar Ouédraogo and Mrs Rokia Naomo Traoré (Institut de l’Environnement et des Recherches Agricoles; INERA, Burkina Faso), Mr. Sériba Katilé (Institut d’Économie Rurale; IER, Mali), Dr. Harro Bouwmeester (Plant Research International; PRI, The Netherlands) and Dr. Paula Westerman (Crop and Weed Ecology group, Wageningen University; CWE, The Netherlands). I also thank my fellow PhD students in Striga research. I am grateful to Jonne Rodenburg, Tom van Mourik, Venasius Wirnkar Lendzemo and Gideon Ayongwa for the interesting discussions on our mutual research projects, and for the friendly atmosphere in which the discussions took place. Venasius, I also like to thank you for your friendship and for being such an excellent host during my stay in Cameroon. I am looking forward to future collaboration. In the course of the years, many students contributed to the Striga research of the department. I am grateful, therefore, to the following students for their dedication and stimulating collaboration: Yvette Bakker, Paul Belder, Harm-Jan Bisschop, Anaïs Bodé, Matthijn de Boer, Roy Gorkink, Petra Hogenboom, Slavica Ivanovic, Erik Jansen, Hans Keller, Diaz Kroeze, Marjolein Kruidhof, Marcius Kuster, Venasius Lendzemo, Cyril Maissan, Bianca van Mourik, Lester Narvaez, William Quaedvlieg, Barbara Remerij, Emiel Schreur, Emiel Snel, Esther Tempel, Tineke Troost, Ramon Welsing, Gerbrand Wilbrink, Erik Wijnker, Tieme Zeilmaker and Chiel Zwarts. When performing the experiments in the tropical greenhouses of Wageningen University, I could always call on the assistance of the staff of the Wageningen Plant Sciences Experimental Centre.