Agriculture Reference
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Fig. 11.5. The different lesion types of common potato scab caused by Streptomyces scabies . (a) Superficial
discrete lesions formed on the surface of the tuber. (b) Raised discrete lesions. (c) Pitted discrete lesions
varying in depth from 3 to 5 mm may develop from raised discrete lesions on the tuber surface.
(d) Coalescing superficial lesions, often referred to as russeting, as when they cover the entire surface
of the tuber, the appearance resembles that of the skin of a russet tuber. (e) Raised coalescing lesions.
(f) Pitted coalescing lesions.
chains with cross walls that eventually constrict
and break off into individual spores. When the
spores mature, they develop gray or melanin pig-
mentation. When the spore comes into contact
with a suitable host, it will germinate and the in-
fection process begins (Fig. 11.6 ) . The pathogen
can infect the tuber directly, but may also invade
lenticels or any open wound on the surface of
the potato tuber. On recognizing infection, the
potato produces several layers of cork cells as a
defense. The pathogen may then re-infect the
tuber through its protective cork layer. Lesion
size will also vary, depending on when infection
occurs. The earlier a tuber becomes infected,
generally the larger the lesion. The phytotoxin,
thaxtomin, is strongly believed to be linked to the
infection process; however, its mode of action is
not known (Fry and Loria, 2002; Loria et al .,
2008). Preliminary studies have indicated a nec-
rotic response on tuber tissue when thaxtomin is
applied directly.
Common scab infection is characterized by the
formation of raised suberized lesions containing
cross-linked hydroxycinnamic acid amides and
feruloyltyramine trimers in the tuber periderm
(King and Calhoun, 2005, 2010). These
compounds contribute to the formation of a
polyphenolic barrier to further attack. Not all
polyphenolics were effective as anthocyanins, a
class of pigmented polyphenolics, and were not
found enriched in lesions, and tubers high in an-
thocyanins were also not resistant to common
scab (Tai et al ., 2013). Polyphenolic biosynthetic
enzyme gene expression is regulated by myelo-
blastosis (MYB) and basic helix-loop-helix (b-HLH)
transcription factors and expression of two MYB
and three b-HLH genes was correlated highly
with common scab resistance (Tai et al ., 2013).
The use of irrigation as a way to manage
this disease has been observed since the early
1920s, although it is difficult to maintain ad-
equate soil moisture (Lapwood et al ., 1973). Soil
moisture may also enhance infection risk by other
pathogens. In semi-arid growing conditions, potato
plants must be watered for 6-8 weeks after the
onset of tuberization to achieve a lasting degree
 
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