The cabbage root fly Delia radicum (Diptera: Anthomyiidae) and downy mildew Peronospora parasitica (Oomycete: Peronosporales) in the vegetable brassica-oilseed rape agroecosystem

Abstract: Cruciferous plants (Brassicaceae) represent economically important crops for Swiss agriculture, comprising i.e. vegetable brassica and oilseed rape (Brassica napus L. emend. Metzg.). However, cultivation techniques of the crops differ, for example in terms of plant protection measures. To prevent economic losses in the production of vegetable brassica, measures to protect plants from infestation with the cabbage root fly Delia radicum (Diptera: Anthomyiidae) have to be applied and infection with downy mildew Hyaloperonospora (= Peronospora) parasitica (Oomycete: Peronosporales) must be prevented. In contrast to vegetable brassica production, D. radicum and P. parasitica are not known to cause significant economic losses in oilseed rape production in Switzerland.To determine the importance of oilseed rape as source of pest and pathogen inoculum within the vegetable brassica-oilseed rape agroecosystem, we monitored the abundance of D. radicum and analysed plants for H. parasitica infection.The abundance of D. radicum was monitored in 2012 in a field with vegetable brassica, i.e. cauliflower (Brassica oleracea var. botrytis L.), and in three fields with oilseed rape in the region of Lucerne, Switzerland. For monitoring, we used one yellow water trap per field. The distance between the traps ranged from 330 m to 1400 m. Captured flies were identified and sexed in the laboratory. Additionally, oviposition of D. radicum was checked weekly at ten randomly selected plants per field. Monitoring revealed that the time of emergence of D. radicum is not influenced by the presence of host plants. However, our results showed that overwintering site and host plant availability influenced the initial population size of D. radicum in spring.Seedlings of vegetable brassica with visible H. parasitica symptoms, and seeds and seedlings of oilseed rape with or without visible symptoms of H. parasitica were analysed with molecular techniques. The results of the investigations revealed that vegetable brassica seedlings, seeds and seedlings of oilseed rape were infested with H. parasitica. Furthermore, the results of the molecular analyses showed that all collected samples of H. parasitica from vegetable brassica and from oilseed rape belong to the same population.In conclusion, with an increase in the oilseed rape production area in Switzerland, undisturbed overwintering sites and infestation levels of D. radicum are increasing in vegetable brassica and in oilseed rape cultures. Additionally, the distribution and epidemiology of H. parasitica in vegetable brassica and in oilseed rape is enhanced by the increasing oilseed rape production area in Switzerland as well.