Purple Witchweed

Striga hermonthica

Purple Witchweed (Striga hermonthica), an annual parasitic broadleaf weed of the Orobanchaceae family.

Family: Orobanchaceae
Growth habit: Parasitic broadleaf
Life cycle: Annual
Invasive status: Native parasitic weed, but a devastating agricultural pest across its range
Native region: Sub-Saharan Africa.
Reproduction: Seed - produces enormous numbers of dust-like seeds that can remain viable in soil for over a decade, germinating only in response to a host root's chemical signal.

Environment

Habitat & Growing Conditions

Habitat: Cereal fields, particularly in nitrogen-poor soils across the savanna zones of sub-Saharan Africa.
Climate requirements: Semi-arid to sub-humid tropics.
Soil types: Thrives especially in poor, nitrogen-depleted soils, where host crops are already stressed.

Impact

Affected crops: Sorghum, maize, pearl millet, rice, sugarcane - attaches to host roots underground before emerging
Competition impact: Most of the damage happens underground before the parasite even emerges - it can destroy 30-100% of a cereal crop, and is considered one of the biggest biological constraints on African food security.
Economic impact: Estimated to cause billions of dollars in cereal losses annually across Africa and affects the livelihoods of over 100 million people.

Management

Control methods: Resistant/tolerant crop varieties, delayed planting, intercropping with trap or catch crops (e.g. the 'push-pull' system using Desmodium), and improving soil fertility (Striga is worse in poor soils).
Organic control: Hand-pulling before the parasite flowers and sets seed (it must be pulled before flowering, since damage is already done by the time it emerges), soil fertility improvement, and push-pull intercropping.
Chemical control: Herbicide seed-coating on resistant maize varieties (imazapyr-resistant maize) is used in parts of East Africa; conventional post-emergent herbicides act too late since most damage is already done underground.