Agriculture Reference
In-Depth Information
( Jefferies and MacKerron, 1987b; Thornton,
2001a). High internal cellular tension develops
from tissue expansion during rapid tuber
growth. Growth cracks will suberize, or wound
heal, and eventually become covered with normal-
looking skin. They occur more often at the bud
end and tend to run longitudinally on the tuber.
There may be one or more cracks per tuber.
Cracks formed early in the season will be flatter
at harvest than those formed later (van der Zaag,
1996). Cultivars vary greatly in susceptibility to
growth cracking.
Growth cracks occur when there are rapid
changes in tuber growth rate, as when a long,
dry period, or period of high temperature stress,
is followed by rain (Jefferies and MacKerron,
1987b). Rapid water uptake results in large
changes in tuber turgidity and growth rate
(Thornton, 2001a). Susceptibility to growth
cracks is increased by wide plant spacing and
low tuber set. If fertilizer placement or timing
cause uneven tuber growth, growth cracks may
develop (Strand, 2006).
Growth cracks may also develop when plants
are infected by
Potato yellow dwarf virus
,
Potato
mop-top virus
,
Potato spindle tuber viroid
, or when
plants are exposed to certain herbicides (Thornton,
2001a). Carnegie and McCreath (2010) indicated
that
Potato virus A
(PVA) and
Potato virus Y
(PVY)
might also cause growth cracking in some potato
cultivars. Growth cracks caused by virus are diffi-
cult to distinguish from those caused by physio-
logical conditions. Identifying the cause of such
growth cracks requires observing foliar symptoms
and/or laboratory analysis.
To reduce the incidence of growth cracks,
when possible, one should plant cultivars that are
less susceptible, maintain uniform growing condi-
tions by maintaining adequate soil moisture and
fertility throughout the growing season, and ad-
just seed piece spacing to ensure uniform plant
spacing and adequate plant population.
Thumbnail cracks or air checks are ran-
dom, shallow, arch-shaped breaks in the skin
caused by handling cold, very turgid tubers. The
cracks become visible after the tuber is exposed
to drying conditions. These cracks are due to
mild physical impacts and are generally superfi-
cial. Freshly harvested tubers exposed to the air
may also develop thumbnail cracks without an
impact. Similar cracks may develop in storage.
Breaks in the skin of tubers are often related to
high tuber hydration and temperatures below
7.2°C (Strand, 2006). Thumbnail cracks heal
slowly, especially at low relative humidity, lead-
ing to tuber dehydration and increased potential
for tuber rot.
To minimize thumbnail cracks, do not har-
vest tubers when pulp temperatures are below
10°C, adjust machinery so drop heights are min-
imized to reduce impacts, and wound heal at
high relative humidity. In addition, tuber pulp
temperature should be maintained above 10°C
when handling tubers that have been removed
from storage.
14.3
Enlarged Lenticels
Lenticels are small, inconspicuous pores scat-
tered (one per square centimeter of surface) on
the tuber surface. They are essential for gas ex-
change and oxygen diffusion into the tuber for
respiration (Wigginton, 1973). When tubers
lack oxygen, as in excessively wet, waterlogged
soils or in very compacted dry soils, the lenticels
swell and become prominent. The lenticels be-
come enlarged (hypertrophied or proliferated)
and protrude beyond the periderm, forming
raised masses of corky tissue approximately
0.5 mm in diameter on the tuber surface (Lulai,
2001). Enlarged lenticels, which resemble mini-
ature kernels of popped maize, can look like scab
lesions, but are smaller and lighter in color. They
detract from the tuber appearance and are sites
of entry for decay organisms. It is important to
note that some potato cultivars appear to be more
susceptible to enlarged lenticels than others.
Enlarged lenticels, often referred to as water
spot or water scab (Thornton, 2001a), may also
form after harvest or in storage when free mois-
ture is allowed to remain on tuber surfaces.
To prevent enlarged lenticels, avoid over-
irrigation during the growing season. Provide
good field drainage, especially in low-lying areas
that may stay wet for long periods of time. Do
not allow free moisture to form on tubers after
harvest.
14.4 Greening
Tuber greening is the light-induced formation of
green color just below the skin of the tuber due
