BRISTOL — Witchweed, broomrape, dodders and mistletoes: They sound sort of like pests. And, they are; these different families of weeds cause a great deal of destruction around the world, especially in Africa and Asia, and so far there’s no indication the weeds are being eradicated.

To the contrary – in at least one instance, they could be spreading. The good news is most of these weeds are not found in North America, but they still present a challenge to farmers here and elsewhere who are being tasked with feeding the world.

That is the upshot of an article in the most recent issue of the Weed Science Society of America (WSSA) journal. It’s called “Parasitic Weeds: A World Challenge.” The author is Chris Parker, a longtime weeds expert based in Bristol, England, United Kingdom (UK). Parker said witchweed, which are also called Striga, are not a threat to this region.

“I would not say Striga was a serious threat in the Midwest, but it does continue to be an immense challenge in the context of feeding the world as it is still quite inadequately controlled in Africa in spite of very considerable research efforts,” Parker said in an interview.

He added the United States became concerned when a species of witchweed was discovered in the Carolinas in 1955. The parasitic weed needs a host plant to complete its lifecycle. At that time 500,000 acres of corn were infested and tobacco crops were also considered vulnerable. People were concerned the pest could spread to the main Corn Belt, and so extra effort was made to eradicate it.

“It could be argued that this was something of an overreaction, as we now know that Striga only thrives on soils of low fertility as occur so widely in Africa,” Parker said.

Parker, who is retired from the UK Weed Research Organization, spent a sabbatical year at Purdue University in the 1960s. He said the “well-fed crops” in this area are unlikely to be infested with witchweed.

Unfortunately, the same can’t be said for broomrape, also known as Orobanche. Broomrape is also a family of parasitic weeds. While it doesn’t usually attack cereal crops or soybeans, it can attack tomato, carrot, potato, rapeseed and various legume crops.

“Control methods are very limited,” Parker stated.

He said there have been infestations of Orobanche ramosa in tomatoes in California that required “expensive fumigation treatments, so the risks are real. Another crop most definitely at risk is sunflower; Parker said another species of broomrape “devastated” sunflower crops across eastern Europe in the past and recently, in Spain.

He said the pest is avoided with the development of resistant varieties but the weed “keeps getting a step ahead,” so new varieties have to be bred in order to try to cope with it.

While these weeds have not seriously damaged any crops here except in a few rare instances, they are truly a menace worldwide. There are a number of species of witchweed, which are native to Africa. According to Parker’s paper, some species only attack wild plants, but others attack crops.

Striga asiatica, the species that infested the Carolinas’ crops, can also affect sorghum, bicolor, millets, rice and sugarcane. In Africa, infestation of sorghum can reduce photosynthesis by 62 percent. Some countries in southern Africa have an estimated 63-80 percent of their corn crops infested. The rice crop in Tanzania is affected.

Sometimes entire crops are destroyed, while average losses are 10-40 percent. He went on to write that the problem is made worse by either the lack of fertilizers or their expense, combined with intensive land use. These conditions reduce the soil’s fertility, which favors the growth of Striga.

“Meanwhile, the control measures are neither fully effective nor easy to apply,” he wrote.

In the United States, controls included the use of ethylene gas, methyl bromide, herbicides and expensive quarantine and monitoring procedures. But those options are too expensive for Africa. Parker said plant breeders have developed some resistant varieties, but many of those are not yet available or practical in Africa, either. Other species of witchweed do even more damage.

Witchweed and broomrape are of greatest concern, but dodders and mistletoes do damage to crops and forests in other parts of the world.

Parker concluded by writing it would be wrong to suggest no progress has been made in controlling these weeds, but in the case of Striga, the problem might just be getting worse despite all efforts. He said there “has to be continued effort” at controlling the witchweed and broomrape especially.

The best prospects for this seem to be in the area of resistant varieties, biological control and, eventually, the introduction of nitrogen fixation into cereal crops.