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Skin Spot Management - Polyscytalum pustulans qPCR assay

Publication Date: 
22 December 2010
Author/Contact :
J Peters, J Woodhall (Fera) & G Harper, G Stroud (SBCSR)


Polyscytalum pustulans causes a serious but intermittent blemish disease of potatoes in store. Symptoms of the blemish disease on tubers, known as skin spot, normally become visible many weeks after harvest. Store managers, seed suppliers and purchasers have difficulty, at present, in making an early assessment of the risk of stocks being infected at harvest. Conventional and real-time assays for the detection of P. pustulans DNA were developed as part of a previous PCL-funded project (R285). Preliminary data from a follow-on PCL-funded project, R294 (2007-08, see page 20 onwards), demonstrated that the real-time assay had potential to enable the early prediction of skin spot levels during storage. The work presented in this report (pages 4-19) for project (R413, 2008-10) confirms that there was a good relationship between
levels of P. pustulans DNA in peel at harvest and skin spot development in tubers
following a 20-week storage duration.
The conclusions are:
• Over two years of trials involving the testing of 106 sets of tuber samples, there was
a good relationship between P. pustulans DNA levels in tuber peel at harvest and
skin spot incidence on tubers after storage. This relationship promises to enable
skin spot risk assessment and management strategies to be made prior to storage.
• During the 2008-09 season, at the end of a 20-week storage period, 51% of 53
tuber samples developed visible symptoms resembling skin spot and 87% had
detectable levels of P. pustulans DNA. During the 2009-10 season, 100% of a
further 53 samples collected developed visible skin spot after storage and all
samples had detectable levels of P. pustulans DNA.
• It is suggested that for samples tested at or around harvest, P. pustulans DNA
levels of <103 pg DNA/g peel represents low skin spot risk; 103 to 104 pg DNA/g
peel represents moderate skin spot risk; and > 104 pg DNA/g peel represents high
skin spot risk.
• There was broad agreement between results presented in this report and those
reported in a previous PCL-funded project (R294) for samples collected at or
around harvest
• The discrimination between low and high risk samples, in terms of skin spot
development, was good in progeny tubers collected at harvest. However, this
disagrees with the findings of R294 where better discrimination was achieved when
testing tubers at the end of a 20-week storage period. A likely reason for this is that
tubers investigated in R294 developed higher skin spot severity than was found on
tubers in this report.
• This work indicates that crop duration is a significant component of the disease
epidemiology. This, as a factor, has been under-investigated.
• Future sampling should involve the removal and testing of peel from the entire
surface of the tuber, or as a minimum include cores taken from around the stolon
and eyes.
• It is recommended that further work is done to refine the relationship between
pathogen DNA levels in peel and skin spot levels by considering factors such as
cultivar, crop duration and geographical location of crop.

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