| Tuber
External Growth Defects: GROWTH CRACKS
Large
growth cracks are due to desynchronized growth between inner and
outer tuber tissue. Sometimes this is referred to as bursting. Cracked
tubers are culled unless the cracks are shallow enough that the
tubers could be used for processing.
Development
and Appearance
Longitudinal
fissures develop when the core tissue inside the tuber grows faster
than the outer tissues, the periderm. In other words, the internal
pressure of the tuber is greater than the tensile strength of the
skin or surface tissue. The resulting fissure or crack can extend
the entire length of the tuber and may be shallow or a half inch
deep. Growth cracks undergo wound-healing and show a characteristic
suberized appearance. When healed, they seldom become infected with
a pathogen. Growth cracking can also be associated with plant infection
by a few relatively uncommon viruses such as yellow dwarf virus
and spindle tuber viroid. More importantly, growth cracking can
be caused by exposure to members of two new herbicide families,
imidazolinones such as Pursuit and sulfonylureas such as Ally and
Accent.
Causation
The
sudden and rapid growth of internal tuber tissue or growth cracking
is primarily due to an uneven availability of soil moisture and
rapid, uneven uptake of water. This occurs when heavy rain or excessive
short-term irrigation is followed by a period of dryness. Growth
cracking is exacerbated when plants are spaced widely apart or when
tuber set is unusually low. Uneven fertilization placement worsens
the cracking. This is also true of excessive nitrogen or poor timing
of nitrogen fertilization especially during the mid-bulking period.
Low soil boron may also worsen the amount of cracking.
Susceptibility
Varietal
susceptibility to growth cracking is not well documented or understood.
It may be related to the speed of tuber growth during mid-bulking
and to root growth.
Cultural
Practices
Three
cultural practices to minimize growth cracking are uniform plant
spacing, adequate soil moisture with consistent irrigation scheduling
and uniform fertilization especially avoiding excessive and late
applications of nitrogen.
Harvest
Cracking
Thumbnail
cracking is shallow and random. Appearance is that of semi-circular
breaks, half-moons, and are associated with exposure of very turgid
(hydrated) tubers to very dry conditions. Surface splitting or air
cracking is a slight separation of the skin. This is associated
with exposure of very turgid tubers to sudden cold temperatures.
Both these types of cracking are associated with harvest but should
not be confused with shatter bruise which has a distinct appearance
and is caused by impacts. Thumbnail and air cracks heal slowly and
are subject to infection and dehydration in storage.
Elephant
Hide
This
disorder is called elephant or alligator hide on russet varieties
and fish skin or turtle back on red and white varieties. It is characterized
by a thick russet appearance to the skin due to furrowing or cracking
of the skin. Causes and practices to control it are not known. Russet
Burbank seems to be especially susceptible.
Summary:
External Disorders |
