Genetic Diversity And Virulence Study Of Seven Striga Hermonthica Ecotypes From Kenya And Uganda On Selected Sorghum Varieties

ABSTRACT

Parasitic weeds are a serious problem in agricultural production, causing large crop losses in many parts of the world and particularly Africa. One of the most economically important parasitic weed is Striga. Striga weed is considered to be the greatest biological constraint to food production in sub-Saharan Africa (SSA). This genus includes races; Striga hermonthica, S. asiatica, S. gesnerioides, S. aspera and S. forbesii that are considered to be most harmful to crops. Striga hermonthica is the most widespread among the species in the semi-arid tropical African zones. The wide geographical distribution set conditions for genetically structured populations. The genetic variations among the weed populations allow for quick breakdown of resistance in crops hence making control of the weed difficult. Efficient and effective control of S. hermonthica requires knowledge on inherent genetic variability within local and regional races of the weed. However, the genetic diversity and virulence of S. hermonthica ecotypes in Kenya and Uganda on selected sorghum varieties remain unknown. This study evaluated genetic diversity among seven S. hermonthica populations from locations in Kenya and Uganda using 5 primer sets of Expressed Sequence Tags – Simple Sequence Repeats (EST-SSR). The size of the amplified fragments ranged from 87 to 202 base-pairs. The total number of bands detected across all the primer combinations after correcting for repeatedness was 38. The genetic diversity among the seven populations was moderate as revealed by the Nei‟s genetic distance values which ranged from 0.122 to 0.710 with an average of 0.33. AMOVA revealed low genetic differentiation among the populations (Fst = 0.100). This study also evaluated twelve sorghum varieties for their response to S. hermonthica infection. It was established that the varieties resistance responses to S. hermonthica varied widely. The phenotype of resistant interaction was characterized by inability of the weeds haustoria to penetrate the sorghums root endodermis due to intense necrosis and in rare cases the parasites radicle growing away from the host root. The resistant sorghum varieties were the Asareca W2, Asareca AG3, N13 and the Wild type which had low mean number of S. hermonthica plantlets growing on their roots (˂ 1.0) while the most susceptible varieties were Sap 027, Epurpur which had the highest mean number of S. hermonthica plantlets growing on their roots (˃8.0). According to Tukey‟s Honest Significant Difference test, there was high significant difference in the means of number of Striga growing on the roots of sorghum varieties, Striga dry biomass and S. hermonthica length between the susceptible and resistant ones (P˂ 0.05). This knowledge holds great potentiality since resistant sorghum germplasm tested will be sourced and targeted to the seven specific geographical areas where virulence of the specific S. hermonthica populations was characterized