Walking along the streets, roadsides, and farms, dodder—a slender, twining, thread-like plant with bright stems ranging from pale green to yellow or bright orange—can readily be seen against the foliage of host plants. This parasitic weed has been identified by forestry researchers as a significant threat to forest health.
According to the Kenya Forestry Research Institute (KEFRI) Guidelines for Dodder Weed Management in Kenya 2020, the weed is rapidly spreading and has already been observed in about 12 counties, mostly in Eastern, Western, and the Rift Valley. Three years after the guideline was published, dodder continues to wreak havoc on trees and vegetation, spreading to more counties including Kiambu, Murang’a, and Nakuru.
James Karira, a farmer in Lower Kabete, Kiambu County, described the spread of dodder as alarming. “We used to see this plant a long time ago and never thought it would be a great threat to our crops, but now it is spreading like bushfire,” he said.
A few meters from Karira’s farm, Serah Wairimu shared that in her Gikuyu dialect, they nicknamed it ‘thiina,’ translating to poverty. “The plant does not have roots and depends on the host plant to survive,” she explained.
The local community, which practices subsistence farming, finds that fruit trees, exotic, and indigenous trees are not spared, with many being fully or partially covered by the weed. “We are worried that the spread of this weed will lead to food insecurity,” Wairimu said.
Apart from attacking crops, Karira explained that the weed has forced many people to uproot their live fences and replace them with concrete ones. “By attacking ornamental, native plants, and trees, it reduces the aesthetic value of landscapes,” he added.
KEFRI Acting Director Dr. Jane Njuguna said the parasitic plant obtains nearly all its nutrients and water from the host plant, earning it the nickname ‘spaghetti.’ “As dodder plants grow, they continually re-attach to the host, forming a dense mat of intertwined stems, and can grow and cover an entire tree or shrub with their thick, yellow-gold, orange, or green stems,” said Dr. Njuguna.
When dodder contacts a host, it produces structures called haustoria that penetrate the host’s vascular tissue and begin to extract nutrients and water, gradually leading to wilting and host death. The impact varies from moderate to severe reductions in host plant growth, and in some cases, complete loss of vigor and death. The severity of an infestation depends on the growth stage of the host plant at the time of initial dodder attachment, with the greatest reduction occurring when the parasite attaches to seedlings, often leading to death. Infected plants are also predisposed to secondary attacks by diseases, insect pests, and nematodes.
Kenya’s vision of increasing its tree cover from the current 12.13% to 30% by 2032 is threatened by the spread of dodder. Dr. Njuguna attributed its rapid spread to the weed’s prolific seed production, with each plant capable of producing several thousand seeds per season. Although only about 5% of the seeds germinate in the year following production, the remaining seeds can remain dormant and viable in the soil for over 20 years, depending on the species and environmental conditions.
“Dodder seeds are mainly spread from host plant to host plant by vectors such as birds and other animals, during pruning through the use of infected tools, through composting of infected material, by improper disposal of infested plant material, and by movement of infected soil and equipment,” Dr. Njuguna said. Other modes of dispersal include mud attached to shoes and car tires, and as contaminants in crop seed. Water also plays a role in seed dispersal, particularly for species near aquatic environments.
Dodder seeds germinate at or very near the soil surface, usually independent of host plant influence. The germinating seed sends up a slender, twining stem that coils around any object, including host plants. According to the guidelines, dodder species are found on a broad host range of both native and exotic plants, including trees, shrubs, hedges, and important cash crops such as tea and coffee. Dodder has a preference for certain species, with Thevetia peruviana (yellow oleander) being the most susceptible, commonly found along farm boundaries.
“The rapid spread and large host range indicate that dodder is likely to threaten the sustainability of many crops, shrubs, and trees if left unmanaged. It is projected that dodder invasion will lead to huge agricultural yield losses by 2029,” the guidelines revealed.
On managing the weed, Dr. Njuguna said the most immediate action is to eliminate or reduce the infestation before it gets out of control. “Farmers are advised to remove dodder stems by hand and prune to control the weed,” she said. Herbicides have been tried, but these chemicals also affect the host plants. A search in the registered Pesticide Control Product Board (PCPB) Product List 2019 indicates no herbicide or product has been registered for the management of dodder in Kenya. Attempts to control dodder using natural enemies (bio-control agents) have not been successful anywhere in the world. A search for biological control would require collaborative approaches with other relevant institutions and countries facing similar challenges.
