| Tuber
External Growth Defects: DEFORMATIONS
There
are several types of tuber deformations or malformations such as
dumbbell, kidney, pointy end, and hot dog (elongated). Tubers with
these defects are also referred to as “being rough.” They are due
to problems associated with apical buds and longitudinal growth.
Harvest quality or grade is affected and tubers are discarded as
culls.
Development
and Appearance
During
a period of stress, longitudinal growth slows or may even stop.
When favorable conditions return, tuber growth resumes, “stop and
go” growth. Therefore, appearance is due to irregular longitudinal
growth, often because of a constriction. Pointy stem-end or Bottleneck:
Growth disruption occurred during early bulking and the constriction
is at the stem end. Dumbbell and Elongated (hot dogs): Growth disruption
occurred at mid-bulking. Dumbbells have a constriction in the middle.
Kidney-shaped tubers tend to have a slighter mid-section constriction
and tend to curve, giving the kidney-shape appearance. Elongated
tubers show little lateral growth; they also tend to be gnarled
and curved. Pointy bud-end: Late-season growth disruption resulting
in a slight constriction at the bud end. Often dumbbells and pointy
bud-ends are associated with jelly
end rot or glassy end.
Knobbiness
also sometimes referred to a secondary growth is due to stimulated
growth of lateral buds in one or more eyes. Protruding eyes is a
similar defect. The size and shape of knobs depends on the growth
stage of the tuber when stress occurs. As with the deformations
reviewed above, high-temperature stress is the cause for stimulating
this abnormal, lateral-bud growth. Susceptibility to knobs varies
with varieties. Practices to lessen knobbiness are the same as for
deformations. Unlike pointy end and dumbbells, there are no internal
defects or rots associated with the formation of knobs. Knobby potatoes
are considered culls, lowering marketable yield.
Causation
These
deformations are primarily due to high temperature stress in the
field often but not necessarily exacerbated by water stress. Note,
water stress or drought alone does not cause these deformation.
The severity of the deformation increases with higher temperatures
and longer high-temperature periods. Basically, high temperatures,
above 80oF, decrease cell division and lower the supply of carbohydrates
available to the tuber. Other factors that exacerbate temperature-induced
deformation are excessive nitrogen application before a high temperature
period, uneven nutrient or moisture supply, hail and frost.
Susceptibility
Potato
varieties vary considerably in their sensitivity to high-temperature
field stress. The rule of thumb is that longer-tuber varieties are
more susceptible than rounder-tuber varieties. For example, long
whites such as Kennebec are more susceptible than round whites as
Atlantic. Russet Burbank is one of the most sensitive of all varieties.
Cultural
Practices
Since
weather cannot be controlled, when planting susceptible varieties
choose areas with cooler climates. Irrigate adequately during early
bulking. Field capacity should be maintained higher than 80%, closer
to 90%, during tuber growth (Potato Production Stages: Scheduling
Key Practices, Univ. Nebr. Coop. Ext. Circ. # 95-1249). Avoid exacerbating
stresses such as excessive nitrogen.
Summary:
External Disorders |
