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Potato tuber anatomy and susceptibility to common scab (Streptomyces scabiei)


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Khatri, BB 2008 , 'Potato tuber anatomy and susceptibility to common scab (Streptomyces scabiei)', PhD thesis, University of Tasmania.

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Common scab, caused by plant pathogenic Streptomyces sp., is a major soil and tuber
borne bacterial disease of potato. Whilst the disease seldom affects tuber yield,
infections produce unsightly blemishes on tubers that reduce their value. It causes
significant economic losses in the fresh, processing and seed market sectors of the
potato industry. Despite several decades of research activity, no single measure has
been developed that is able to provide effective control of this disease and comhlon
scab therefore remains a serious threat to the potato production throughout the potato
growing areas of the world. This project investigated links between tuber structural
attributes and susceptibility to infection, an area of study under-represented in the
literature and offering potential to deliver new knowledge on common scab disease
that may lead to improved approaches to disease management.
A series of experiments was conducted under field, glasshouse and hydroponic
conditions to investigate changes in morphology and anatomy of the potato tuber.
Inoculation treatments were developed to investigate relationships between structural
features at the time of inoculation and the subsequent levels of infection. A novel
hydroponic system was developed that allowed inoculation of individual tubers at
specific times or stages of development and permitted non-destructive observation of
the development of common scab symptoms. A second novel system involved the
use of netting in potting bags to separate tubers from the root zone and allowed
inoculation at different times during plant growth. This system facilitated study of
the pathogen infection window on the tubers, and pathogen-induced structural
changes in the tubers, under glasshouse conditions. Field studies were undertaken in
12 commercial crops across a range of production environments, and utilised seed
from a single source to allow assessment of site effects on tuber development and
disease incidence.
Many tuber anatomical and morphological features (stomata and lenticel number,
periderm thickness, lenticel structure, pattern of suberisation and internode expansion
rate on the tubers) were found to be highly variable under different field and
glasshouse conditions, and significant variability within crops/treatments was also observed. Lenticel diameter was determined by growmg environment, but other
structural features varied as much between tubers within sites or treatments as
between sites or treatments. Given the sporadic nature of occurrence of common scab
symptoms within and between crops, the observations of variation in structural
features was sufficient to indicate that anatomical features could be playing a role in
susceptibility to infection.
Detailed study of lenticel development on tubers revealed that two pathways of
lenticel formation exist in potato tubers. Formation of lenticels from stomata, the
commonly accepted pathway in the literature, occurred on all tubers, but when tubers
expanded rapidly, lenticels were found to have also formed directly from peridermal
rupture. In most tubers, the majority of lenticels were concluded to have formed from
this second pathway. The initiation of lenticel formation in both pathways was shown
to be via localised zones of increased cell division in the phellem layer, placing
pressure on the outer cell layers and leading to rupture of the periderm. Continued
cell division and expansion under the rupture site creates the characteristic raised
lenticular shaped structure of a mature lenticel. Suberization in lenticels was
observed to occur very late m Luber development in most glasshouse and
hydroponically grown tubers, and lenticels with no suberisation were found in
mature tubers under all growing conditions in the project.
No relationship was found between tuber anatomical features at the time of exposure
to the pathogen and either incidence or severity of common scab symptoms. While
trends were observed within trials, when data from all experiments were examined
there was no structural attribute that was consistently present or absent in treatments
that induced high levels of common scab symptoms. Under field conditions,
suberisation in the periderm was noted in young tubers in crops that subsequently
developed high incidence of common scab, but under glasshouse conditions nonsuberized
tubers at the point of inoculation were found to be highly susceptible to the
disease. No evidence was found of increased suberisation in lenticels during tuber
maturation being associated with reduced susceptibility to infection, with the timing
of suberisation varying markedly between trials and in many conditions not recorded
until very late in tuber development.
The window of tuber susceptibility to common scab disease infection was shown to
vary with the season or conditions under which the plants were grown. In addition,
the first direct evidence that different intemodes on tubers were susceptible to
infection at different times during tuber development was produced. Basal
intemodes, which are the first sections of the tuber to expand, are susceptible earliest
in tuber development with apical intemodes only becoming susceptible later in tuber
growth by which time the basal intemodes are no longer susceptible.
Basal intemodes in older tubers, which were less susceptible to common scab
infection, were shown to rapidly produce additional phellem cell layers when
exposed to the pathogen, whereas the same internodes in young, susceptible tubers
were slower to respond. A pathogen-induced suberin deposition in phellem and
filling cells was observed in less susceptible tubers in addition to the increased
thickness and number of cell layers in the phellem. This result suggested that the
phellem layer rather than the lenticels may be involved in common scab infection,
with infection occurring directly through the periderm, as suggested by Loria et al.
(2003), rather than through the lenticels as suggested by most other authors.
This project has contributed to a better understanding of the role and relationships
between pre-existing or induced structural features of the tuber and common scab
disease infection of potato. Further investigations on the role of phellem layers in
disease infection and pathogen-induced phellem layer responses are recommended.

Item Type: Thesis - PhD
Authors/Creators:Khatri, BB
Copyright Holders: The Author
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Copyright 2008 the author

Additional Information:

Available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references

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