Unexpected fast development of branched broomrape on slow-growing Brassicaceae

In France, oilseed rape is getting highly infected since 1990 by the branched broomrape Phelipanche ramosa (L.) Pomel. Branched broomrape infection causes serious yield losses ranging from 5 to 100 %, notably in the Mediterranean area. P. ramosa is parasiting the plant roots. The growth of P. ramosa on Brassicaceae weeds has not been studied quantitatively so far, except for the model species Arabidopsis thaliana. Since P. ramosa has a fast development rate on the fast-growing A. thaliana, P. ramosa development should be slower on other slower-growing Brassicaceae species. Here, we cultivated in the laboratory seven Brassicaceae weed species including Capsella bursa-pastoris, Capsella rubella, Cardamine hirsuta, Lepidium campestre, Lepidium draba, Sinapis arvensis and A. thaliana as control, during 3 weeks. We counted the number of P. ramosa individuals that have reached the following growth stages: germination, attachment, tubercle, bud and underground stem. We then assessed the development rate of P. ramosa by calculating the odds ratio of attachment or higher development stages of P. ramosa on Brassicaceae, with A. thaliana as the reference. We found that five Brassicaceae species had an odds ratio ranging from 0.9 to 2.4. These ratios are thus similar or higher than that of the A. thaliana reference. This finding shows for the first time that P. ramosa develops faster on the five Brassicaceae species. This finding is also unexpected because A. thaliana is a fast-growing plant, whereas the five Brassicaceae species have a longer life cycle. Therefore, this observation demonstrates for the first time that P. ramosa development depends on others factors than the speed of plant host development.