Weeds are a major challenge in arable farming, vegetable, and fruit production. With increasing restrictions on herbicide use, effective weed management is becoming more difficult. The transition to more sustainable farming and climate change may also introduce new weed problems. An example of such new problems concerns parasitic weeds. It entails plant species that depend on crop plants to fulfill (part of) their life cycle. They parasitize on the roots or the stems of the crop plants to steal water, nutrients and sugars and to alter the crop plants’ hormone balance. When such plants invade arable fields they can become weed problems with high associated crop losses and few effective control options. Jonne Rodenburg from Wageningen University: “In southern Europe and the Balkans parasitic weeds are already a major problem in a wide range of crops, including sunflower, carrot, tomato, faba bean, hemp, clover and many more. There are more than 30 species of current and potential parasitic weeds in Europe but the main species are the stem parasitic dodders – such as field dodder and clover dodder – and the root parasitic broomrapes – such as sunflower broomrape, bean broomrape, hemp broomrape and clover broomrape.”
There is anecdotal evidence that certain parasitic weeds are spreading in Northwest Europe. Factors such as reduced soil tillage, the use of cover crops and flower-rich field margins, as well as a warmer, drier climate, may be contributing to this trend. On “De Biesterhof”, an organic farm near Nijmegen (the Netherlands), for instance, a recent new infestation of clover broomrape (Orobanche minor) was discovered in white clover sown as green manure (see picture). Farm manager Howard Koster explains: “We observed broomrape for the first time on our farm this summer. We assume that the species has grown from seed introduced through the clover seeds we used. At first we were excited seeing this new species on our farm, but other organic farmers advised us to carefully scout our farm and remove all individuals as this can quickly develop into a really serious weed problem!”.
This observation from the Netherlands does not stand on its own; farmers in the UK have made similar observations. Ruth Naudé, an independent agronomist working with farmers across Suffolk, recently observed broomrape in a legume cover crop mix that had been planted under the recently introduced Sustainable Farming Incentive (SFI) scheme implemented by the UK government. She said “Having since been in contact with other members of the Association of Independent Crop Consultants, a few of us have now also come across broomrape on farms. We are very concerned that this and other novel weeds are being introduced to the UK via imported, unregulated seed mixtures which are in much greater demand since SFI was introduced”.
Obviously, farm plant diversification efforts as a potential driver for parasitic weed introductions would not be good news for the promotion and implementation of sustainable farming practices. Ruth Naudé: “Weeds like broomrape could have a massive impact on our ability to grow successful legume crops. We need to understand much more about them and the impact they may have on cropping rotations. Losing another break crop because of contaminated seed mixtures would have significant detrimental effect on both farming and environmental schemes.”
To gain a better understanding of these developments, researchers from Wageningen University & Research (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 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 anecdotic evidence that parasitic weeds are on the rise in North and West European agriculture, and we are seeking confirmation of this. We would also like to know if farmers know this type of weeds and if they would be prepared for any new infestations”. John Cussans of ADAS adds: “The farmers in Kent that I am working with are truly concerned about these weeds as they know just how difficult they are to manage”. “And …” says Jonne Rodenburg “… if we know which species are spreading, in which crops or cropping systems and what are the main drivers for this spread, we will have entry points for both research and extension to ensure future problems are either prevented or contained early on.”
The researchers still welcome new respondents to their survey. So, are you a farmer or working in the sector, you can take part in this study, also if you’ve never heard about this problem before! The survey (already available in English, French, German, Italian and Dutch) takes max. 10 minutes and can be accessed by scanning the QR code below this article or by visiting: tinyurl.com/parasiticweeds.