Science - June 3, 2004

Mycorrhiza may help to control ‘cereal killer weed’

Mycorrhizal fungi can suppress the germination, attachment and emergence of one of the most hated pests in semi-arid areas of Africa, the parasitic witchweed Striga. Venasius Lendzemo, a PhD-student from Cameroon, thinks that the fungi can contribute to the control of the cereal killer.

‘The bright pink flowers of Striga can completely cover sorghum fields in Africa. For the uninitiated in agronomy, the beautiful scenery is captivating, whereas smallholder farmers wonder why mother nature curses them with this scenery on a yearly basis,’ states Lendzemo in his dissertation. The attractive flowers of the witchweed Striga hermonthica, a close relative of broomrape and rattle, are misleading. Lendzemo describes its true nature as ‘a cereal killer weed’

Striga seedlings attach themselves and develop only on the roots of a compatible host, cereals in general and sorghum in particular. The parasite obtains nutrients and carbon at the expense of the host. According to Lendzemo, cereal yield losses are much larger than the biomass gain of the Striga plants. ‘Striga damage is also greater in soils with low fertility and soils that are more prone to drought.’ This means there is a direct link between agricultural intensification, as a result of increasing population pressure, increased damage caused by Striga and declining food security. In the worst scenario, whole villages need to be relocated. ‘This has already happened in north Cameroon’, states Lendzemo.

Alternatives

Heavy doses of nitrogenous fertilizers or injection of ethylene gas into the soil are effective ways of eliminating Striga, but prohibitively expensive for smallholder farmers. Lendzemo tried to develop an alternative way of tackling the witchweed at the below-ground stage by focusing on measures aimed at improving soil fertility. Certain arbuscular mychorrhizal (AM) fungi have a positive effect on the nutritional status of almost all tropical crops, and do not colonise Striga. His research therefore focused on whether AM fungi could play a beneficial role in Striga control, either by benefiting the cereal or by negatively affecting the cereal killer.

Lendzemo did experiments in pots and Petri-dishes and discovered that root exudates from AM colonised sorghum plants can suppress the germination of Striga seeds. Fungal inoculation also reduced the numbers of Striga attachments to sorghum plants. The negative impact on the witchweed was not related to the beneficial mycorrhizal effect on phosphorus uptake and biomass increment of sorghum. Field experiments, in which Striga seeds were added to planting holes of sorghum and maize with or without mycorrhizal inoculum, resulted in a 30 to 50 percent reduction in Striga emergence and a 40 to 63 percent reduction in Striga biomass. The inoculation also delayed the emergence of witchweed. This, however, did not result in cereal yield increases.

Confusing results

Lendzemo suggests several explanations for the confusing field results. ‘Large cereal plants may be a better substrate for the smaller number of Striga plants that emerge.’ He stresses therefore the importance of removing Striga plants from fields before the witchweed can set seed. ‘It would be illusory, and bad advice as well, to induce farmers to buy mycorrhizal inoculum. But farmers can easily manage their fields in ways that are favourable to AM fungi: by reducing soil disturbance and by preventing declines in soil organic matter.’

Lendzemo stresses the need for further research to understand the interactions in the field. Inventing ways of making use of these interactions in an integrated management system can hopefully stop ‘the downward spiralling yields of cereal crops as a result of Striga infection in Africa’.

Venasius Lendzemo MSc will defend his dissertation on June 8. His supervisors are Professor Martin Kropff (Crop and Weed Ecology) Dr Thomas Kuyper (Soil Quality).

Gert van Maanen

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