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Tittel: Dispersal and survival of downy mildew in rocket and other cruciferous crops
Forfattere:Nordskog, B., Le, V., Hladilova, J., Gauslå, E. & Hermansen, A.
Navn på utgivelse: NJF Report
Publikasjonstype: Proceeding fra int. konf. uten ISBN-nr
Volum (nr): 7
Nr: 9
Side(r): 111
Utgivelsesår: 2011
Avdeling: Bioforsk Plantehelse
Fagområde: Plantesjukdommer

Downy mildew, caused by Hyaloperonospora parasitica s.l., represents an increasing threat to Norwegian production of cruciferous vegetable crops. Although the pathogen has been present in Norway for over a century, the intensified production regimes of field vegetables has contributed to an increase in disease incidence and severity. The project “Effective and sustainable control strategies for downy mildew in rocket, broccoli and cauliflower” were initiated by vegetable growers and financed by the Norwegian Research Council, “Jordbruksavtalen”, “Fondet” and vegetable growers for the period 2009-2011. As part of the project we have surveyed the presence of downy mildew in cruciferous crops and weeds, tested possible cross inoculation between different cruciferous crops and weeds, and tested how long detached spores can survive in air.  

The survey of downy mildew incidence in cruciferous crops (mainly rocket, broccoli and cauliflower) and weeds were performed by local extension officers in the Counties Buskerud, Rogaland, Vestfold, Østfold, and Hedmark. In addition, 65 growers were asked to survey and report findings in their fields. During the project period downy mildew incidence was mainly observed in wild rocket (Diplotaxis tenuifolia) and broccoli, while a few incidences of downy mildew were reported in swede, oil seed rape and the weed Sheperd’s purse (Capsella bursa-pastoris).

The host range of isolates collected in the survey was assessed on a selection of cruciferous species.  Seedlings of wild rocket (‘DI902’), cultivated rocket (Eruca sativa), cauliflower (‘Freedom’), broccoli (‘Ironman’), oil seed rape (‘Valo’) and swede (‘Vige’) were inoculated by spraying the cotyledons with a spore suspension. One week after inoculation, the seedlings were incubated over-night at high RH and scored for positive infections by observation of visible sporulation on the cotyledons. Preliminary results show that downy mildew from oil seed rape can infect swede, cauliflower and broccoli. Isolates from broccoli were partly virulent on cauliflower. An isolate from swede were virulent on broccoli, while an isolate from rape were virulent on cauliflower, broccoli and swede. An isolate from Sheperd’s purse was virulent on broccoli. Isolates from wild rocket were not virulent on any of the other plants in the trial. The results indicate a risk of cross infection between different cruciferous species, although also supporting the ongoing discussions on dividing H. parasitica into separate species.

The possibility of long distance dispersal of viable spores depends on how long the spores can survive in free air. Spores from freshly sporulating rocket seedlings were transferred to filter paper and exposed to solar radiation or in shade. Preliminary results show that spores were not able to germinate after 6 hours exposure to the sun, while shaded spores were able to survive for 24 hours or more. These results will be included in the development of a forecasting model for downy mildew on cruciferous crops.  

ISSN: 1653-2015