Science - September 29, 2017

Parasite ruins African rice culture

Text:
Albert Sikkema

At a first glance, Rhamphicarpa fistulosa might look innocent, but it turns rice plants into slaves. PhD candidate Stella Kabiri investigated the effects of this parasitic weed that damages the African rice production at an ever-increasing rate.

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It is nothing new that Rhamphicarpa fistulosa has been parasitising rice in many African countries, but Kabiri studied how this vampiric plant exhausts the rice plants. She cultivated rice plants in pots, added various amounts of the weed’s seed and used periodic measurements to investigate how R. fistulosa affected the growth and production of the rice plants.

Standstill
Kabiri concluded that the weed impairs the rice plant’s photosynthesis and, depending on the density of the weed, also diminishes the growth of the rice plant by 22 to 71 percent. Furthermore, the amount of produced rice grains decreases by an alarming 78 to 100 percent. ‘Parasitism eventually caused a complete standstill of host plant growth, […]. This implies that ultimately the host plant was producing solely for the sake of the parasite’, Kabiri writes in this month’s Annals of Applied Biology. ‘Upon infection this Rice Vampireweed, however, turns into a genuine slave master, whereby it completely dominates its host.’

Vampire
The weed, which is colloquially called Rice Vampire Weed in Africa, is a parasitic plant that incidentally occurs in the rain-fed rice systems in Africa. Through pot experiments, Kabiri was the first to scrupulously follow the interaction between the rice and vampiric plants. R. fistulosa attaches to the roots of the rice plant. It is a facultative parasite, which means that the plant does not rely on a host and can propagate without one. The weed naturally occurs in low wetlands in Africa, areas in which rice cultivation has increased in recent years. The researchers at the Centre for Crop Systems Analysis in Wageningen and the Africa Rice Center in Côte d'Ivoire, who supervised Kabiri, therefore expect that this weed will become a plague of increasing importance for rice production.

False seedling bed
Kabiri’s research has also provided starting points to fight this parasitic weed through control measures. Firstly, the African rice farmer should create a ‘false seedling bed’: by creating a seedling bed prior to planting the rice, part of the weeds will germinate, which you can then pull before planting. What also helps is an improvement of the water management and soil fertility; the vampire occurs primarily on poor soil. R. fistulosa shares that last property with Striga, the best-known parasitic weed in Africa, which mainly parasitises maize and sorghum.


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