Agriculture Reference
In-Depth Information
loses its sink properties and stops growing (Lugt
et al
., 1964; Krauss, 1985). After vine killing,
carbohydrates are translocated to the secondary
tuber from the primary tuber, which will have a
reduced specific gravity and become glassy
(Lugt, 1960; van der Zaag, 1996). Secondary
tubers are physiologically younger than primary
or normal tubers, and when planted, will differ
in emergence and stem and tuber numbers pro-
duced ( Jefferies and MacKerron, 1987a).
Little tuber describes the phenomenon of
daughter tubers forming directly from an eye, or
a very short stolon from an eye of a seed tuber,
without sprout or foliage development. They
may form in storage prior to planting, or in the
field. Cultivars differ in susceptibility to little
tuber. Little tuber is associated with physiologic-
ally old seed tubers, with tubers that have been
stored at warm temperatures (20°C or higher),
then planted into cold (less than 10°C), dry soils,
or sprouted tubers that have been transferred
from warm storage to cold storage, then planted
at a later date (Hiller
et al
., 1985).
Cultural practices that encourage uniform
vine and tuber growth will help to minimize se-
cond growth. Ensure uniform growth with
proper plant spacing and population, correct
fertilizer rate and placement, and uniform soil
moisture levels.
To control little tuber, store seed tubers in
cool storage at
3-
4°C, avoid using seed tubers
stored for an excessively long time or aged at
high temperatures, and do not plant physiolo-
gically old seed into cold, dry soil. Storing seed
tubers in the light can slow physiological aging
(van der Zaag, 1996).
is suberized and protective; phellogen (cork cam-
bium) just beneath the phellem; and phelloderm,
the innermost layer just beneath the phellogen.
The phellem and phelloderm layers are derived
from the mitotic activity of the phellogen, which
is a single layer of meristematic cells. The phel-
lem is comprised of several layers of cells (ap-
proximately six to ten) and is the portion of the
periderm commonly referred to as the skin of the
potato. In immature tubers, the active meristem-
atic cells of the phellogen have thin radial walls
that fracture easily with mechanical pressure, al-
lowing the phellem or skin to separate or scuff off
(Lulai and Freeman, 2001), thus skinning the
tuber. As the phellogen becomes inactive as the
tuber matures, its cell walls thicken, become re-
sistant to fracture, and the skin becomes tightly
bound to the tuber (Lulai and Freeman, 2001).
Skinned tubers are highly susceptible to de-
hydration and invasion by pathogens (Lulai and
Orr, 1995). The skin of the bud end of the tuber
is more fragile and prone to skinning than the
stem end (Lulai and Orr, 1993). Under proper
wound-healing conditions, skinned areas of the
tuber will heal.
Scald (not to be confused with sunscald,
see “Greening” above) may develop when
skinned tubers are exposed to sun and warm
winds of high velocity and low relative humid-
ity after harvest (Whiteman and Lutz, 1954;
O'Brien and Rich, 1976). Relatively short ex-
posure (less than
4
h) is needed for damage
to appear. The skinned areas turn brown, due to
rapid superficial drying, and have a scalded
appearance. In storage, affected tubers often
decay and suffer weight loss.
To control skinning and scald, avoid har-
vesting immature tubers. Handle tubers care-
fully during harvest. Harvest tubers
14-
21 days
after vine kill to ensure proper tuber maturity
and skin set. Do not expose freshly harvested
tubers to sun, wind, or low relative humidity for
extended periods during or after harvesting.
14.7
Skinning and Scald
Skinning, or feathering, and scald are skin de-
fects of immature tubers. Skinning occurs when
immature tubers are harvested or handled. The
skin of the tuber is not “set” and is only loosely
held in place and easily scuffed or slipped off. The
skin may be completely or partially removed, ex-
posing the tissues beneath. Loose shreds of skin
adhering to the surface of the tuber will dry and
become thin and papery.
The potato periderm is composed of three
layers: phellem (cork), the outermost layer, which
14.8
Pink Eye
Initially believed to be caused by a pathogen,
current research suggests pink eye is a physio-
logical disorder (Lulai
et al
., 2006; Sabba
et al
.,
2008). It is characterized by puffy, pink-colored
