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Epidemiological studies of downy mildew of oilseed poppy

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Scott, Jason Barry (2003) Epidemiological studies of downy mildew of oilseed poppy. PhD thesis, University of Tasmania.

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Abstract

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® 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.

Item Type: Thesis (PhD)
Keywords: Opium poppy, Downy mildew diseases
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:20
Last Modified: 11 Mar 2016 05:54
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