Modelling cropping system effects on branched broomrape dynamics in interaction with weeds

Prod 2018-202e SPE EA GESTAD INRA AGROSUPInternational audienceBranched broomrape, Phelipanche ramosa (L.) Pomel, is a parasitic plant which infects many crop and weed species. As a holoparasite, it entirely relies on host resources to survive and reproduce. It is a major pest in oilseed rape in France. Its management has to be thought as a part of the overall weed management because infected weeds either must be killed to limit crop infection or can be used to stimulate suicidal parasite germinations. In order to design efficient pest management strategies, we modelled the effects of cropping systems on P. ramosa dynamics in interaction with weeds in a model called PHERASYS. PHERASYS functions and parameters were based on the literature and on our own experiments. The model simulates processes that determine the multiannual parasite dynamics, i.e. in situ seed mortality, dormancy and germination, and subsequent parasite attachment and survival on host plant until seed production and release. Seed mortality and dormancy were assessed during a 2-year field experiment. Seeds display a very low mortality (7% per year). Seasonal dormancy lasts from January to mid-April following seed release and from mid-March to the end of June the next year. We characterized interactions between P. ramosa on one hand, and crops and weeds on the other hand by listing host, non-host and false-host (non-hosts that stimulate parasite germination) species reported in the literature, and their ability to stimulate P. ramosa germination in in vitro experiments. Our greenhouse experiment showed that parasite survival until fructification depends on host biomass. We predicted parasite seed biomass from total parasite biomass with a «harvest index» measured on parasites harvested in fields. PHERASYS was connected to the FLORSYS model which simulates multi-annual weed dynamics and crop production at a daily time step in cropping systems. FLORSYS includes a submodel (RSCone) which simulates root systems where parasitic seeds attach following germination stimulated by root exudates. PHERASYS is an innovative approach as it models the complete lifecycle of P. ramosa within cropping systems for the first time. PHERASYS will help to design efficient rotations and weed management strategies to manage P. ramosa as it allows to test the effect of crops and weeds used as trap (false-hosts) and catch (hosts destroyed before parasite reproduction) plants. Different tillage strategies and sowing densities, which affect the probability that parasitic seeds encounter host roots, will be explored by simulation. The first results already point to a few potential parasite management improvements. For instance, burying seeds by tillage is not efficient to deplete the parasite seed bank, since the in situ seed mortality is low, but delaying crop sowing until parasite seeds are dormant should reduce crop infestation. Weeds with a short life cycle may be used as catch plants during fallow as they die before parasite reproduction