Tuber Physiological Disorders - The Potato: Botany, Production and Uses - page 250

caused by Potato leaf roll virus . Unlike the diseased
tubers, tissues damaged by low temperature will
fluoresce on exposure to ultraviolet light (Hrus-
chka et al ., 1969).
Potato tubers are able to be supercooled to
-7°C, and remain unharmed if allowed to
warm up (Thornton, 2001b; Boydston et al .,
2006). However, if these tubers are jarred or
handled, ice crystals will form and cause cell
death.
To minimize low-temperature injury, en-
sure adequate soil depth over tubers, with proper
hilling, and harvest before the danger of severe
frost. In storage and in transit, maintain tem-
perature above 3°C. Intermittent warming dur-
ing long-term storage at low temperatures can
prevent low-temperature injury (Hruschka
et al ., 1969). If tubers with low-temperature in-
jury must be stored, use high airflow rates to
minimize losses.

particularly when the plants are still green and
the weather is hot (Sieczka and Thornton,
1993). The disorder is most noticeable when
quick-acting chemical desiccants are used, but
may also occur when the haulm is destroyed
mechanically or by heavy frost. Growing loca-
tion appears to contribute to the appearance of
this disorder (Halderson et al ., 1985). The ten-
dency for vascular discoloration is increased
when vines are less mature at the time of top
killing (Halderson et al ., 1985). Vascular discol-
oration may be present at harvest or may in-
crease in intensity during the first few months of
storage.
To control vascular discoloration, use
chemicals that kill the haulm gradually. Avoid
rapid vine-killing methods during hot weather,
especially if the soil is dry.

14.16 Internal Anthocyanin

Discoloration

14.15 Vascular Discoloration

On occasion, atypical anthocyanin synthesis
occurs in the flesh of cultivars where normally
there is none. Some cultivars are prone to in-
ternal anthocyanin discoloration, while others
are rarely, if ever, affected. Abnormal pigmen-
tation has been seen in many potato cultivars;
Record and Ulster Sceptre in Great Britain, Ro-
seval in France (Storey and Davies, 1992), and
Nooksack, Hilite Russet, Frontier Russet, and
Russet Norkotah in the USA and Canada (Olsen
et al ., 2003).
When affected tubers are cut, purplish red
stripes and flame-shaped discolorations can be
seen in the perimedullary region. The purple or
red color occurs when the natural balance be-
tween anthocyanin precursors and inhibitors
is disrupted (Storey and Davies, 1992). This
disorder appears to be associated with light, as
it is observed mainly in green tubers (van der
Zaag, 1996). It is more prevalent in tubers
found near the soil surface. Flesh pigmentation
has also been associated with low temperat-
ures (below 10°C) in the field or storage (Olsen
et al ., 2003). When cooked, the discoloration
becomes slightly less apparent, or may dis-
appear if pale.
To reduce the development of anthocyanin
discoloration, ensure adequate hilling in the field.

Vascular discoloration is known by several
names - vascular necrosis, stem-end discolor-
ation, stem-end browning, stem-end necrosis,
vascular browning, phloem necrosis, and
xylem ring discoloration. It is characterized by
discoloration of varying intensity in the vascu-
lar ring at the stem end of the tuber. Symptoms,
visible after the tuber is cut, include necrotic,
light tan, reddish-brown or dark brown streaks
at the point of stolon attachment. The discolor-
ation is usually confined to within 1 cm of the
stem end, but in severe cases may extend the
length of the tuber. Vascular discoloration af-
fects the marketability of table-stock and pro-
cessing potatoes. This vascular discoloration
often causes concern, because it resembles vas-
cular discoloration caused by Verticillium or
Fusarium wilt.
Sometimes, the cells at the point of stolon
attachment will collapse to form a necrotic
brown lesion (van der Zaag, 1996; Wale et al .,
2008). This lesion may be invaded by fungi or
bacteria. Vascular discoloration has no effect on
seed quality, and affected tubers are viable as
long as no pathogen invades the lesion.
Vascular discoloration occurs when plant
haulms are killed rapidly under dry conditions,