Cowpea (Vigna unguiculata [L] Walp.) is primarily grown in drier regions of the world
where it is one of the most drought resistant food legumes. The southernmost region of
Africa is most probably the center of origin where many wild relatives of the species
are found in abundance. Botswana is one of the Southern African countries that has the
highest genetic diversity in respect of wild forms of cowpea. In Botswana, cowpea is
widely grown and plays an important role in the economy and diet of both the rural and
urban population. However, cowpea yield is negatively affected by different biotic and
abiotic factors. Witchweed (Alectra vogelii), an obligate root parasite, is one of the major
constraints to cowpea production in sub-Saharan Africa including Botswana. Up to 100%
yield losses of susceptible varieties have been observed in some farmers' fields. Control
of Alectra is difficult because of its enormous seed reserve in the soil that can germinate
and cause significant damage to susceptible cultivars even before the weed emerges
above the ground. Earlier studies indicate that B359, a landrace from Botswana, is a
source of resistance to the parasite and is being used in the breeding program. However,
this genotype produces only a few flowers per plant and there is an insufficient amount
of pollen for crossing. This study was conducted to identify additional or alternative
sources of resistance from both cultivated and wild cowpeas collected from Botswana
for developing Alectra resistant varieties which is perhaps the most feasible means of
reducing crop losses caused by Alectra attack. A total of 66 cultivated and 20 wild cowpeas
were tested under high Alectra pressure in a greenhouse at Sebele Agricultural Research
Station, Botswana, during 2008. Twelve of the cultivated genotypes, namely B426A,
B415, B413, B403, B400, B336, B399, B432, B416, B397, B404, and B359 showed
resistance to Alectra infection. Furthermore, 14 of the wild cowpeas did not show any
sign of infection. The majority of cultivated cultivars had thicker stems at above the soil
level and interestingly this morphological character showed negative correlations with
Alectra infection. Such cultivars showed puffy stems at early seedling stage. Generally
there was lower incidence of Alectra infection among cowpea genotypes with thicker
stems which could be used as a selection criterion if confirmed. Genotypes identified for
resistance to the parasite will be further evaluated under different growing conditions.