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The rise of parasitic weeds: a growing problem for European farmers?
Weeds are a major challenge in arable farming and fruit and vegetable production. With increasing restrictions on herbicide use, effective weed management is becoming more and more difficult. Furthermore, the transition to more sustainable farming and the consequences of climate change can create opportunities for new species such as parasitic weeds.
What are parasitic weeds?
Parasitic weeds are plant species that depend on crop plants to fulfil all or part of their life cycle. They attach themselves to the roots or stems of the crop plants to steal water, nutrients and sugars and to alter the crop plants’ hormone balance. In arable farming and open field vegetable production they can cause severe weed problems. Effective control options are sparse and harvest losses can be substantial.
“In Southern Europe and the Balkans, parasitic weeds are already a major problem in a wide range of crops, including sunflower, carrot and tomato,” says crop and weed ecologist Jonne Rodenburg of Wageningen University & Research. “In Europe there are more than 30 species of parasitic plants known to be weeds or with the potential to become weeds. The main species are the stem parasitic dodders, such as field dodder and clover dodder, and the root parasitic broomrapes, such as clover broomrape and hemp broomrape.”
Spread in Northwest-Europe
There is evidence that certain parasitic weeds are starting to spread in Northwest Europe. Factors such as less intensive soil tillage, the use of cover crops and
flower-rich field margins, combined with a warmer, drier climate, may be
contributing to this trend.
Clover broomrape on Dutch arable land
On De Biesterhof, an organic farm near Nijmegen (the Netherlands), a recent new infestation of clover broomrape (Orobanche minor) was discovered in white clover sown as green manure. “We saw clover broomrape for the first time on our farm this summer,” farm manager Howard Koster explains. “We assume the species was
introduced along with the clover seeds we used. At first, we were excited to
see a new plant species on our land, but other organic farmers advised us to
thoroughly scout the fields and remove all individuals as this can quickly
develop into a serious weed problem.”
Similar observations in the UK
This observation is not restricted to the Netherlands: farmers in the UK are seeing similar developments. Ruth Naudé, an independent agronomist in Suffolk, recently discovered broomrape in a legume cover crop mix sown under the UK government’s recently launched Sustainable Farming Incentive (SFI) scheme.
Naudé: “I have since been in touch with other members of the Association of Independent Crop Consultants, and a few of them have now also come across broomrape on farms. We are very concerned that this and other novel weeds are being introduced into the UK via imported, unregulated seed mixtures, which are in much greater demand since the SFI scheme was introduced.” If crop diversification is indeed contributing to the spread of parasitic weeds, this poses a serious challenge in terms of promoting and implementing sustainable agricultural practices.
“Weeds like broomrape could have a massive impact on our ability to grow legume crops successfully. We need to understand much more about them and the impact they have on cropping rotations. Losing another cover crop option because of contaminated seed mixtures would have a significant detrimental effect on both farming and environmental measures,” Naudé says.
Europe-wide study
To gain a better understanding of these developments, researchers from Wageningen University & Research (the Netherlands), the University of Greenwich and ADAS (both in the UK) are conducting a Europe-wide study. Through a recently launched online survey in different European languages, the research team is looking for input from arable farmers, advisors and researchers – both from those already experiencing these weed issues and from those who have not yet encountered them. As Lucie Büchi of the University of Greenwich explains: “We have so far only anecdotal evidence that parasitic weeds are on the rise in Northern and Western European agriculture, and we are seeking confirmation of this. We would also like to know if farmers are familiar with these types of weeds and if they are prepared for any new infestations.”
John Cussans of ADAS adds: “The farmers in Kent I am working with are truly concerned about these weeds as they know just how difficult they are to control”. “And,” says Jonne Rodenburg, “if we know which species are spreading, in which crops or cropping systems and what the main drivers are of this spread, we will have entry points for both research and extension to ensure future problems are either prevented or contained early on.”