Abstract

In Malawi the parasitic weeds (witchweeds) for cereals and legumes exist simultaneously. The predominant species for cereals is Striga asiatica, while for legumes it is Alectra vogelii, which causes damage in groundnuts (Arachis hypogea L.), cowpeas (Vigna unguiculata) and soybeans (Glycine max.). Growing legumes is a control measure for S. asiatica in cereal crops. Studies were initiated in 2000/2001 at Chitedze Research Station to screen amongst several entries of groundnuts, pigeon peas (Cajanus cajan), sunhemp (Crotalaria spp.), soybeans, velvet beans (Mucuna spp) for resistance to A. vogelii. In 2002/2003 a similar experiment was repeated, utilizing the promising entries. In 2003/2004 a screen house pot study was conducted at Bunda College to evaluate soybean lines with and without A. vogelii infection. Results from the 2000/2001 screening study indicated that amongst the crops soybean are the most susceptible to Alectra. Within soybeans, Bossier, Ocepara-4 and 501/6/12 were the least susceptible, with < 1.0 plant m^-2, compared to 5.0 plants m^-2 with 427/5/7. All pigeon peas and green manure entries had no Alectra emerging on them. Similar results were obtained when the study was repeated with a few entries in 2002/2003. In a screen house study of 2003/2004, soybean varieties Bossier and Ocepara-4 had the least Alectra on them (<0.33 plants pot^-1), while other entries such as Kudu had 7.0 plants pot^-1. There was significant (P < 0.05) reduction in plant dry mass, grain weight, and pod number due to Alectra infection). The following conclusions were drawn from these studies:- 1). A. vogelii significantly affects crop growth, 2). Pigeon peas and green manure crops of mucuna and crotalaria are immune toAlectra, 3). Farmers with S. asiatica and A. vogelii infestation in their fields should therefore use resistant soybean types (Bossier, Ocepara-4), pigeon peas or green manures as trap crops for S. asiatica, while avoiding damage or build of Alectraon their farms, 4). As is always the case, an integrated management approach should be advocated in management of the parasitic weeds.

Key words: Integrated witchweed management, legumes rotations, Alectraresistance.