Epidemiological studies of downy mildew of oilseed poppy

Downy mildew is a major limiting factor of oilseed poppy production in Tasmania. However, little knowledge of the epidemiology of the disease currently exists. The objectives of this project were to taxonomically identify the downy mildew pathogen, characterise the spatiotemporal development of epidemics, analyse the effect of weather variables on epidemic development, and identify the means of overwintering by the pathogen. Phylogenetic analysis of the ribosomal DNA region, including the internal transcribed spacer regions and the 5.8S gene, indicated the downy mildew pathogen is Peronospora cristata, not P. arborescens as previously reported. Conidium dimensions were unable to distinguish between the two species. Under favourable disease conditions epidemics can develop rapidly, with disease incidence increasing in a field trial from 0.2 % to 100 % over a 40 day period during the 2001/2002 growing season. Epidemics were spatially aggregated after the onset of canopy closure, while the spatial pattern at an individual time was significantly associated with the spatial pattern that occurred 10 days prior. Under spatially aggregated plant densities the local area under disease progress curves (AUDPC) of both disease incidence and severity was positively correlated and spatially associated with high plant densities. These results indicate that downy mildew epidemics were dominated by secondary spread, from low levels of primary inoculum. Downy mildew infection was observed to decrease alkaloid content, but not capsule dry matter yield. Alkaloid content of capsules was significantly spatially dissociated with the local AUDPC of both disease incidence and severity in both the 2000/2001 and 2001/2002 growing seasons. Capsule dry matter yield in poppy crops was not consistently correlated or spatially associated with the local AUDPC of either disease incidence, or severity over both of these seasons. The forecaster model, DOWNCAST, developed for the prediction of epidemics of onion downy mildew (P. destructor), provided moderate prediction of sporulation and infection events during poppy downy mildew epidemics. Accuracy of prediction by the model was increased by increasing the critical limit for sporulation inhibition by nighttime rainfall to 3 mm, and decreasing the leaf wetness critical limit for infection when using Watchdog¬¨vÜ 450 dataloggers from 7.5 to 4.5. The principle means of overwintering by the downy mildew pathogen appears to be via the 'green bridge' provided by regrowth poppy plants, and other Papaver spp. Downy mildew oospores were found associated with the residues of poppy crops and survived at least 26 months burial in uncultivated soil. Peronospora cristata oospores were also detected in association with the seed of poppy, by a seed washing technique and molecular detection using the polymerase chain reaction. However, no evidence for primary infection resulting from oospores was recorded.