Cytological investigations on colonization of sorghum roots by the mycoherbicide Fusarium oxysporum f. sp. strigae and its implications for Striga control using a seed treatment delivery system

The application of the potential Striga-mycoherbicide Fusarium oxysporum f. sp. strigae (Foxy 2) by seed coating is an appropriate option for delivering and establishing the biocontrol agent in the rhizosphere, the infection zone of the root parasitic weed Striga hermonthica. Cytological investigations using light and transmission electron microscopy were performed to assess the pattern and extent of colonization of sorghum roots and shoots by Foxy 2, applied as film-coat on seeds. Germination of Foxy 2-treated plants was similar to un-treated plants. During sorghum root development, mycelia of Foxy 2 started from seed coat colonizing root surfaces. The intensity of root colonization increased with time. However, hyphae were not found in the apical zone of the roots. Hyphae growing on the surface of the roots were observed penetrating rhizodermal cells including root hairs, and colonizing the intercellular space and the cells of the cortical parenchyma. Even after four weeks, hyphae were not penetrating the endodermal layer to invade the tissues of the central cylinder. In addition, hyphae of Foxy 2 were also completely absent in the xylem vessels in sorghum shoots even after 11 weeks of sowing, which further proves the non-pathogenicity of the fungus to sorghum. Foxy 2 showed high efficacy in controlling Striga in combination with both tolerant and susceptible sorghum varieties in a root chamber experiment, as reflected by the high percentage of diseased Striga seedlings (95% and 86%, respectively), recorded after 26 days of sowing. The intensity of fungal root colonization, which coincides with the peak of Striga attachments to its host root, will ensure high fungal efficacy and will facilitate the practical use of Foxy 2 against Striga using seed treatment delivery. Indeed, this proof of sorghum non-pathogenicity of the fungus will encourage the acceptance of Foxy 2 by authorities and farmers in Africa.