Agriculture Reference
In-Depth Information
14
Tuber Physiological Disorders
Loretta Mikitzel*
New Brunswick Department of Agriculture, Aquaculture and
Fisheries, Potato Development Centre, Wicklow, Canada
Defects in potato tubers that are brought about by
unfavorable environmental factors that alter
normal plant and tuber growth are called
physiological disorders. These disorders are not
due to pathogens, insects, or nematodes, and are
often referred to as abiotic, non-infectious or
non-pathogenic disorders. As these disorders are
not caused by pathogenic microorganisms, they
should not be referred to as diseases. The major
causes of physiological disorders include im-
proper cultural, storage or handling practices,
high or low temperatures, and inadequate, exces-
sive or uneven soil moisture or nutrient levels.
Often, a disorder is caused by the combined effects
of environment and cultural practices.
Physiological disorders affect the external
and internal appearance of tubers. External dis-
orders, such as changes in tuber size, shape, or
skin color are readily apparent, but internal dis-
orders are often undetected until the tuber is cut.
These defects reduce the value of the crop be-
cause the tubers no longer meet market grade
standards. Losses due to physiological disorders
may be minor or very serious. Some cultivars are
more susceptible than others to particular dis-
orders. Physiological disorders occur erratically
from year to year and location to location. Since
they are often detected only after harvest, it is
difficult to identify the patterns in the field that
may aid in diagnosing the cause of the disorder.
Also, physiological disorders are slow to develop,
and manifestation of symptoms may not occur
for some time after the disorder is initiated.
Losses from many physiological disorders
can be minimized by using good cultural prac-
tices that promote uniform plant and tuber
growth over the course of the growing season,
and by maintaining proper storage conditions.
The cause of some physiological disorders is un-
avoidable or unknown, and they are therefore
uncontrollable.
14.1 Tuber Malformations
Tuber malformations include knobby (a type of
second growth), dumb-bell, bottleneck, and
pointed-end tubers. Tuber malformations are
also referred to as misshapen or rough tubers.
Knobby tubers are formed due to a loss of
apical dominance, resulting in secondary growth
in one or more lateral eyes (Krauss, 1985). The
growth results in protuberances that range in
size from a raised, swollen bud to very large
knobs, sometimes larger than the original tuber
(Fig. 14.1
). Knobs vary in size and shape, de-
pending on the stage of tuber development in
which the disorder began. The plant and the in-
terior of the tuber show no symptoms.
