Selection of rhizobacterial strains for suppression of germination of Striga hermonthica (Del.) Benth. seeds

Striga hermonthica (Del.) Benth. is an obligate root-parasitic flowering plant that severely threatens cereal production in sub-Saharan Africa. A potential biological control option for reduction of crop yield–loss within the season of application is the use of soil-borne antagonists of S. hermonthica seed. This study was aimed at selecting soil-borne fluorescent pseudomonad strains capable of suppressing germination of S. hermonthica seeds and consequently reducing parasitism and damage to maize. An in vitro screening procedure was developed and was used to evaluate 460 fluorescent pseudomonad isolates from naturally suppressive soils. This resulted in the identification of 15 Pseudomonas fluorescens/P. putida isolates that significantly inhibited germination of S. hermonthica seeds. In a pot experiment using steam-sterilized soil, there was a significant reduction in the number of S. hermonthica plants on maize grown from seeds that were inoculated with any of the 15 bacterial isolates. Inoculation of maize seed with six of these isolates resulted not only in a reduced number of S. hermonthica plants, but also in an increased maize shoot biomass compared with the check. When soils inoculated with these bacterial isolates were left dried for 5 weeks after maize harvest and then planted with a second maize crop, no reduction in S. hermonthica parasitism was observed. This suggested that the bacteria did not persist in the soil after the first crop of maize. These results suggest that saprophytic fluorescent pseudomonads have potential for biological control of S. hermonthica in maize and that periodic application of bacteria, perhaps through seed treatment, may be necessary for sustained control.