Genetic diversity of Striga hermonthica and Striga asiatica populations in Kenya J G GETHI*, M E SMITH , S E MITCHELL à & S KRESOVICHà *Kenya Agricultural Research Institute, KARI-Katumani, Machakos, Kenya,  Department of Plant Breeding, Cornell University, Ithaca, NY, USA, and àInstitute for Genomic Diversity, Cornell University, Ithaca NY, USA Received 5 September 2003 Revised version accepted 15 October 2004 Summary The parasitic angiosperms, Striga hermonthica and Striga asiatica, severely constrain cereal production in sub-Saharan Africa by causing huge losses in grain yield. Understanding the diversity of Striga populations is important because it allows identification of races or biotypes thus improving chances of breeding success. Amplified fragment length polymorphism (AFLP) ana- lysis was used to study genetic diversity among 17 populations of S. asiatica and 24 populations of S. hermonthica from Kenya. A total of 349 DNA fragments ranging from 51 to 500 bp were obtained from four EcoRI and MseI primer combinations. Genetic distances for S. asiatica populations ranged from 0.009 to 0.116 with a mean of 0.032. S. hermonthica populations had a genetic distance that ranged from 0.007 to 0.025 with a mean of 0.015. Only two clusters were found in S. asiatica populations whereas no apparent structure was evident in S. hermonthica pop- ulations. There was no evidence of isolation by distance for the two species. Although the low genetic diversity suggests Striga is relatively uniform across the popula- tions studied, it is possible that pathogenicity and virulence genes may be located in genomic regions that were not sampled. The data, however, does not provide evidence to support diversification of both Striga species in the region where the study was conducted. Keywords: amplified fragment length polymorphism, EcoRI, genetic diversity, Kenya, MseI, Striga asiatica, Striga hermonthica. Introduction The genus Striga, recently placed in the Orobanchaceae (Olmstead et al., 2001) includes the economically important witchweeds. Mohamed et al. (2001) described 28 species and six subspecies from Africa. Of these 22 species of Striga are endemic. The most important of 11 species that attack crops are Striga asiatica (L.) Kuntze and Striga hermonthica (Del.) Benth. parasitising cereals and Striga gesnerioides (Willd.) Vatke parasitizing cow- peas and other wild legumes (IITA, 1997). The origin of S. hermonthica is thought to be in the Nuba mountains of Sudan and Ethiopia and now it is widespread in many parts of Africa, as well as Yemen and Saudi Arabia (Musselman, 1987). S. asiatica has a wider distribution and is found throughout semi-arid areas of tropical and sub-tropical Africa, Asia, China and Australia (Musselman, 1987). Striga asiatica produces bright red flowers, but morphotypes with white, yellow and pink flowers occur in some regions. Flowers self-pollinate before opening when sticky pollen balls cling to the elongating style. S. hermonthica, also known as giant witchweed is an outcrossing species with purple flowers. Striga produces massive amounts of seed estimated at between 58 000 and 200 000 per plant (Parker & Riches, 1993), which can remain viable in the soil for a long periods of time (Bebawi et al., 1984). In sub-Saharan Africa, S. hermonthica has been estimated to affect cereal crops on over 21 million ha (Sauerborn, 1991) where farmers lose 20–80% of their yield, equivalent to 4.1 million tons of grain per year. This affects livelihoods of approximately 100 million people (Kanampiu et al., 2002). In western Kenya, it is estimated that 76% of land planted to maize (Zea mays L.) and sorghum (Sorghum Correspondence: J G Gethi, Kenya Agricultural Research Institute, KARI-Katumani, P.O. Box 340, 90100 Machakos, Kenya. Tel: (+254) 044 20330; Fax: (+254) 044 21122; E-mail: jgethi@yahoo.com Ó European Weed Research Society Weed Research 2005 45, 64–73