Agriculture Reference
In-Depth Information
Fig. 14.1.
Knobby tubers result from secondary growth from lateral eyes. (Photo courtesy of M.J. Pavek,
Washington State University.)
Pointed-end, dumb-bell, and bottleneck
tubers result from irregular radial growth and
are characterized by a constriction along the
long axis of the tuber. The type of symptom is
determined by the time at which the growth
interruption occurred. Dumb-bell shaped tubers
have a constriction in the middle of the tuber
due to midseason growth interruption. Early-
season, growth-restricting stress leads to bottle-
neck or pointed stem-end tubers, and late-season
stress leads to pointed apical (bud) end tubers
that have a narrower bud end than basal
(stem) end.
Cultivars with long tubers tend to develop
malformations more readily than those with round
tubers; Russet Burbank is particularly susceptible.
No cultivar is completely free of tuber malforma-
tions. Tubers with pointed ends tend to have high
reducing sugar content and may develop a translu-
cent end (Iritani and Weller, 1973).
Conditions or stresses that modify plant
growth and temporarily slow or stop tuber
growth lead to tuber malformations. When
tuber growth resumes with favorable condi-
tions, growth and starch synthesis resume,
but only in certain areas of the tuber, com-
monly next to buds where cell division is most
active (Reeve
et al
., 1973). Abnormally shaped
tubers result. Tuber malformations are associ-
ated with wide fluctuations in soil and air tem-
peratures, moisture stress, defoliation due to
hail, frost or insects, an imbalance of fertilizer,
or a combination of these factors (Lugt, 1960;
Bodlaender
et al
., 1964; Hiller
et al
., 1985;
Krauss, 1985). Sudden or severe stress usually
results in a higher incidence of tuber malforma-
tions than continuous or gradually increasing
stress (Thornton, 2001a).
Factors that contribute to the incidence and
severity of tuber malformations include sin-
gle-stemmed plants that produce few tubers per
hill, poor stands, reduced tuber set caused by
Rhizoctonia or other reasons, and excessive vine
growth (Hiller and Thornton, 1993).
It is difficult to avoid this disorder com-
pletely because the unfavorable conditions that
cause tuber malformations often cannot be con-
trolled. Good cultural practices that promote
uniform stand, proper plant population, bal-
anced nutrition and even moisture will reduce
the incidence and severity of tuber malforma-
tions. When markets allow, growers should
plant cultivars that are less susceptible.
14.2 Tuber Cracking
There are two types of physiological tuber crack-
ing: growth cracks and thumbnail cracks (also
called air checks).
Growth cracks are shallow to deep fissures
in the surface of the tuber. They are caused
when internal pressure exceeds the tensile
strength of the surface tissues during tuber
enlargement and the outer periderm bursts
