Controlling witchweed infestations in Africa

By Lominda Afedraru

August 5, 2019

Farmers who grow cereal crops in most African countries are all too familiar with the challenges presented by striga, a parasitic plant also known as witchweed that infests farmers’ fields and causes lower yields, or even no harvest at all.

Now African scientists are breeding maize that can resist this pest plant as extension agents are offering farmers various solutions for improving yields in areas where the invasive weed is especially prevalent.

Striga infests the roots of the host plant, especially cereals belonging to the grass family. It survives by siphoning water and nutrients from host plants such as maize, rice, millet and sorghum, thus stunting and wilting the hosts. Striga has affected production in numerous crops, but the problem is particularly serious for those who grow maize, which is considered a staple food crop by a majority of Africa’s population.

While 80 percent of striga species are found in Africa, the weed is a truly global challenge. It began showing up in maize fields in both the US and Australia in the 1950s before the massive application of chemical inputs was used to eliminate it.

That experience has African scientists, particularly those in Uganda and Kenya, devising news means for farmers to overcome this challenge in a bid to improve production of the affected crops.

Crop and economic losses

A January 2018 report in the journal PLOS Pathogens stated that striga has expanded from its native range in the Semien hills of Ethiopia and the Nubian Hills of Sudan to over 40 countries in Africa. In infested fields, striga causes 20 to 100 percent crop losses, which in turn lead to significant economic losses. For example, striga causes an estimated loss of $111 million to $200 million annually in African rice fields alone.

In a 2011 Swedish University of Agricultural Science publication, researchers Jenny Anderson and Marcus Halvarsson found that in Nigeria’s Northern Guinea Savanah, sorghum has lost 50 percent of its vigor due to striga. In Kenya, sorghum production where there is striga infestation is at 550kg per hectare, compared to 1,200kg per hectare where the weed is not present.

Dr Micheal Otim, the head of cereal and legumes research programs at Uganda’s National Agricultural Crops Resources Research (NARO), has identified four major striga zones in East Africa: the Lake Victoria zone, the inland dry zone found in Tanzania, the inland moist zone in Uganda and a conterminous coastal zone along the Indian ocean in both Kenya and Tanzania.

The most affected of these is the Lake Victoria zone, where striga is said to cause 50 to 80 percent crop losses across the entire region. Tanzania has the largest area of striga infestation totaling over one million hectares of land and over one-third of its three million acres under maize production are infected. Uganda has 262,000 hectares of striga infestation and 32 percent of its maize is under infestation, while Kenya has 216,000 hectares of striga-hit cropland, with most of it found near Lake Victoria.
Across East Africa, the economic impacts are substantial, totaling over $568 million a year.

Finding solutions

Dr. Charles Lwanga Kasozi, a senior research officer in the cereals section at NARO has been working with a team of scientists both in Uganda and Kenya to evaluate striga infestation in farmer fields since 2013. He says that the weed poses a serious challenge to farmers growing cereal crops in East Africa.

“The weed attaches its roots to the host plant and withdraws water and nutrients from the host plant causing stunting and wilting. This causes 20 to 100 percent damage to the host plant,” he said. “ When it grows up, it produces seed which will germinate depending on the soil fertility and growth like chemicals produced by host plants, but its seeds can be kept dormant in the soil for up to 20 years.”

Kasozi said scientists in Uganda and Kenya have been breeding maize varieties which confer resistance to the weed. The scientists are advising farmers to use herbicide-resistant maize varieties, namely the Longe7H IR (Imazapyr resistant) variety bred by the Uganda company Nasesco Seeds in collaboration with the scientific team at the National Crops Resources Research Institute (NaCRRI). Many farmers are already growing this variety both in Kenya and Uganda.

Other maize varieties are being developed that will be able to resist striga and grow well in the presence of an infestation. These varieties, which have a built-in genetic makeup that confers resistance, are being bred in partnership with the International Institute of Tropical Agriculture (IITA) and CIMMYT, and are expected to be released within two years.

In addition to the new varieties, another measure farmers are embracing is what is known as “push-pull technology,” where the host crop is intercropped with desmodium, a plant also known as tick clover, to stress striga germination and increase soil fertility.

Farmers are also encouraged to conduct early weeding in order to limit striga plants growing in cereal crop fields. When too many of the weeds are already in the field, farmers are encouraged to grow Crotalaria to act as a trap.

Isaac Wamatsembe, an agricultural inspector at Uganda’s Ministry of Agriculture, Animal Industry and Fisheries, said that when his teams conduct their routine field work, they advise farmers to guard against striga by leaving the land to fallow for two to three years before planting a cereal crop for the second time on the same land.

He notes that across Africa the weed is a menace to farmers, particularly affecting women who are engaged in the weeding and thereby prevented from carrying out other routine day-to-day work.

Wamatsembe added that children are also sometimes affected because their parents tend to use them as source of labor, making the children weed maize, rice, sorghum and millet fields before going to school.