Commercial Potato Production and Cultural Management - The Potato: Botany, Production and Uses - page 98

Plant maturity harvest. Vine desiccation and removal is effect-

ive in stopping late blight spore production on
foliage, which may reduce contamination of
healthy tubers during harvesting operations and
storage. In addition, seed growers may remove
foliage from healthy plants prematurely to pre-
vent virus spread by late-season aphid infest-
ations.

Cultural management decisions are typically
based on a predetermined harvest date or win-
dow. With a harvest target date, growers can ad-
just input applications and quantities to allow
for proper plant and tuber maturity. Having
knowledge of the intended harvest date is also
important for pesticide applications, as most plant
protection products require a post-application
preharvest interval after each application. Tuber
maturity is an important factor during harvest.
Whether the vines senesce naturally or prema-
turely, the process sets the stage for tuber skin set
and physiological maturity. Proper skin set at
harvest is important for crop storage and mar-
keting (especially when marketed fresh). Imma-
ture tubers are highly susceptible to shatter
bruise and skinning during harvest (Thornton
and Bohl, 1998). Mature tubers may wound
heal faster and resist weight loss associated with
dehydration (McGee et al ., 1985). Soil type, tem-
perature, moisture, variety, and management of
fertilizer, irrigation, desiccation timing, and season-
long cultural management practices contribute
to the speed of skin set. Poor skin set results in
tuber skinning and wounding, which may lead
to pathogen infection and premature tuber
dehydration. Proper skin set may take up to
22 days or longer following vine death (Bowen
et al ., 1996).

Harvest

Skinning and bruising are perhaps the most
common impact-related defects occurring dur-
ing and after harvest. In 1996, Brook estimated
that a 1% decline in tuber impact-related defects
was valued at US$7.5 million (Brook, 1996).
Mature tubers resist skinning, but may be more
susceptible to blackspot bruise than immature
tubers (Corsini et al ., 1999). Moreover, a variety's
genetic background can impact its susceptibility
to bruising greatly (Pavek et al ., 1993), as can
soil temperature and moisture, and tractor and
harvester chain speed (Smittle et al ., 1974). To
minimize bruising, available soil water at har-
vest should be 60- 80% to allow for good soil and
tuber separation, and harvester soil load (Thornton
et al ., 1973).
In general, bruising increases as tuber pulp
temperature decreases, but tuber hydration also
plays a role. Dehydrated tubers are more suscep-
tible to blackspot bruise, and fully hydrated
tubers more susceptible to shatter bruise (Thorn-
ton e t al ., 1973). Depending on the variety's sus-
ceptibility to either bruise type, tuber hydration
must be balanced to reduce both. For moderately
hydrated tubers ( 60- 80% ASW), the ideal tuber
pulp temperature range at harvest is approxi-
mately 15- 18°C. Tuber decay may increase in
storage when at-harvest pulp temperatures are
above 18°C. Tubers with pulp temperatures
below 7°C during harvest and handling are
prone to tuber bruising. Harvester chains should
be padded and potential impact zones modified
to prevent tuber damage. Harvester chain con-
veyor and ground speed should be adjusted to
allow for a full, uniform flow of potatoes through
the whole machine (Thornton and Bohl, 1998).
Blades on harvesters and windrowers should
be adjusted to prevent tubers from colliding
with the primary chain on pickup. In addition,

Defoliation

Due to variety maturity differences and early
harvest dates, potato foliage may need to be
killed mechanically or chemically to induce
tuber maturity and skin set prior to harvest and
storage. Mechanical defoliation methods include
flaming, pulling, cutting, undercutting, rolling,
flailing, and beating vines. Chemical vine desic-
cation products include sulfuric acid and
non-selective herbicides such as diquat dibro-
mide, paraquat dichlor, or endothall. Growers
may incorporate mechanical and chemical
methods, especially if vines are large and imma-
ture and/or weeds are prevalent.
In addition to skin set and tuber maturity,
defoliation is also used to control tuber size,
avoid insect or disease infestations, and facilitate