Tuber Physiological Disorders - The Potato: Botany, Production and Uses - page 248

To reduce the occurrence of translucent
ends, use cultural practices that promote good
plant stands and uniform crop growth. Irriga-
tion should match water requirements of the
crop. Avoid moisture stress, especially during
early tuber growth. If irrigation is not possible,
use cultural practices that help to retain soil
moisture, such as proper spacing, hilling, and
cultivation. Grow cultivars that are less suscep-
tible to translucent end.

that the necrotic spots do not concentrate in
the pith or center of the tuber. There are nor-
mally no external plant or tuber symptoms
with IBS; the disorder is seen only after the
tubers are cut open. Several publications con-
tain color plates of tubers affected by IBS and
heat necrosis (Hiller et al ., 1985; Hiller and
Thornton, 1993; Stevenson et al ., 2001). Large
tubers (Iritani et al ., 1984; Sterrett et al .,
1991a) and tubers with high specific gravity
(Iritani et al ., 1984) have a higher incidence of
IBS. Cultivars vary greatly in IBS susceptibility
(Collier et al ., 1980; Sterrett and Henninger,
1991; Davies, 1998); the cultivars, Yukon Gold
and Atlantic, are highly susceptible, whereas
Desiree is very resistant.
IBS may develop soon after tuber initi-
ation (Olsen et al ., 1996), or during periods of
uneven or rapid tuber growth (Hiller et al .,
1985; Hiller and Thornton, 1993). Initially,
the necrotic spots occur toward the bud end of
the tuber, and over time, the color intensity
and area of the tuber affected increase (Ster-
rett et al ., 1991a). Symptoms may increase in
storage, particularly in tubers stored at higher
temperatures (Iritani et al ., 1984; Hiller and
Thornton, 1993).
The exact cause of IBS is poorly under-
stood. Localized calcium deficiencies have been
implicated as initiating cell death and tissue ne-
crosis associated with IBS (Collier et al ., 1980;
Davies, 1998). With restricted calcium (due to
environmental stress), the loss of membrane in-
tegrity is accompanied by oxidative damage,
which, when severe, leads to cell death (Davies,
1998). In resistant cultivars, activity of enzym-
atic antioxidants is significantly higher than in
susceptible cultivars (Davies, 1998). Tubers
with IBS have lower calcium concentrations
than unaffected tubers (Collier et al ., 1978,
1980; Sterrett and Henninger, 1991; Olsen
et al ., 1996). However, there is no clear threshold
for calcium below which the disorder is induced
(Davies, 1998). Ozgen et al . (2006) found IBS in-
cidence to decrease as tuber calcium content in-
creased from 100 to 250 µg g - 1 dry weight.
When calcium fertility or availability is low and
soil temperatures are high, the incidence of IBS
increases. Recently, Sterrett et al . (2006) found
that higher tuber manganese and sulfur levels
and lower phosphorus levels were associated
with lower IBS incidence. Tuber mineral status

14.13 Internal Brown Spot
and Heat Necrosis

Non-pathogenic necrosis in potato tubers has
been referred to by many different names: in-
ternal brown spot, internal heat necrosis, in-
ternal browning, internal rust spot, internal
brown fleck, physiological internal necrosis, and
chocolate spot (see Hiller et al ., 1985, and refer-
ences therein). These names more than likely
refer to the same disorder or similar disorders
that are difficult to distinguish from one another.
In the western USA, internal brown spot is the
accepted term for this disorder, in the eastern
USA, internal heat necrosis is the term used,
while in Europe, internal rust spot is customarily
used. Internal brown spot (IBS) will be used
here. For an in-depth review of this disorder, see
Yencho et al . (2008).
Symptoms include reddish-brown or
rust-colored groups of necrotic parenchyma
cells that appear spongy and suberized and are
lacking in starch (Baruzzini et al ., 1989). The
blotches or spots of necrotic cells may be small,
round, or irregularly shaped. In IBS, the
blotches are scattered throughout the flesh of
affected tubers. The spots can occur anywhere
in the tuber, but are more commonly found in-
side the vascular ring. The lesions seldom occur
outside of the vascular ring in the cortex. When
the spots are concentrated along or just inside the
vascular ring, some authors characterize the
disorder as internal heat necrosis; however, in-
ternal heat necrosis may occur anywhere in the
tuber (Yencho et al ., 2008). It has also been re-
ported that internal heat necrosis occurs in
tubers growing near the soil surface (Thorn-
ton, 2001b), while IBS can be found in tubers
throughout the profile of the potato hill (Hiller
et al ., 1985). IBS differs from brown center in