Non-host facilitators, a new category that unexpectedly favours parasitic weeds

In Europe, the current decrease of herbicide application increases and diversifies weed flora in crops. As a consequence, there is an increase in the number of pests that use weeds to spread. This change is a major threat to crop production. For instance broomrape-Phelipanche ramosa-is an obligate parasite of dicotyledonous species that reduces crop yield by up to 80 % of oilseed rape in France. In highly infected fields, broomrape can in turn infect various weed species and thus persist and even proliferate in the absence of host crops. Up to now, three categories of interactions between broomrape and plant species have been identified in monospecific stands: host, non-host and false host. In multispecific stands, broomrape germination and attachment have never been studied despite the use of multispecific crop associations to protect host crops with non-host species in tropical conditions. Therefore, we studied parasite germination and attachment of monospecific vs. multispecific stands, associating broomrape with host plants (oilseed rape), non-host plants (field bindweed) or a combination of both. Experiments were conducted in vitro for 6 weeks and in pots for 3 months. We measured the percentage of germinated broomrape seeds in the presence of field bindweed or oilseed rape. We analysed the attachment of the parasite on its host. Results unexpectedly show a nearly threefold increase in the infection of host oilseed rape in the presence of field bindweed. We thus proposed a new plant species category called 'non-host facilitator' and a new parasitic interaction process named 'facilitation'. The underlying mechanism is still unknown though we observed that field bindweed was able to support secondary parasite attachments originating from primary attachments on a nearby oilseed rape root system. Our discovery implies the rethinking of parasite management in arable crops, particularly the rotation or association of host with non-host crops, since the non-host crops could actually be non-host facilitators.