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Ideas for Mitigation of the Effects of a Reduced Water Year
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POTATO
RESPONSE TO DROUGHT
The
extent tuber yield and quality are adversely affected by drought will
depend upon the severity, timing, and duration of water stress during the
growing season. It is critical to have an understanding of how water
stress at each growth stage influences tuber yield and quality if a grower
is to balance water supply with potential returns. Specific irrigation
management guidelines for each growth stage are presented below. Vegetative Growth - The
vegetative growth stage begins at seed piece sprouting and extends to
stolon formation. Water stress during the vegetative growth stage tends to
acclimate (harden) the plant to water stress, potentially reducing the
effect of water stress in later growth stages. Water stress during the
vegetative growth stage reduces leaf area, vine and root expansion, plant
height, and delays canopy development. Water deficits during vegetative
growth have also been shown to decrease the number of tubers set per
plant, which then results in fewer and larger tubers at harvest. Water
stress limited only to the vegetative growth stage of Russet Burbank
potatoes has been shown to decrease total yield but actually increase
yield of U.S. No. 1 quality tubers. A general management guideline is to
withhold irrigation until full emergence for silt loam soils, if winter
precipitation has been sufficient to fill the soil profile to near field
capacity. In drought years, however, irrigation may be needed before full
emergence to limit water stress, particularly on coarse-textured soils.
Under these conditions, it is usually best to apply irrigation before
planting to minimize disease development. As a general guide, soil water
depletion in the upper 12 inches of the root zone should be limited to 50%
available soil water during vegetative growth. However, caution must be
used to ensure that water stress is reduced or eliminated at the time of
tuber initiation and early bulking. This will not normally be a problem
with irrigation systems other than center pivots or linear-moves. However,
center pivots or linear-move systems with flow rates less than 7 to 7.5
gpm/acre may never be able to catch up on irrigation throughout the
remainder of the season. Tuberization - Tuberization
begins when stolon tips begin to swell and tubers begin to develop but are
not appreciably enlarging (<1/2 inch diameter). Although additional
tubers may continue to form on stolons during later stages of plant
development, tubers that contribute the most to marketable yield are
formed at this time. Water stress during tuber initiation can
substantially reduce tuber yield and quality. In a 1987 field study at the
Aberdeen Research and Extension Center, Russet Burbank potatoes exposed to
moderate (10 days) to severe (14 days) periods of water deficits during
different stages of tuber development had lower total and U.S. No. 1
yields when stress occurred during tuber initiation (Table 2). Tubers that
are stressed during tuberization often are severely misshapen with pointed
stem ends, multiple knobs, and other malformations. Tuber Bulking - The
tuber bulking growth stage extends from the time tubers are about one-half
inch in diameter to canopy senescence. Under ideal conditions, this growth
stage is characterized by a relatively constant rate of increase in tuber
size and weight. Interruptions in tuber growth by water stress often
result in misshapen tubers having knobs, growth cracks, and irregular
shapes characterized as “bottlenecks,” “dumbbells,” and other
irregular curved shapes. Thus, periods of water stress during tuber
bulking often decreases U.S. No. 1 yields by increasing the percentage of
U.S. No. 2 tubers. Sustained irrigation deficits during the tuber bulking
will reduce tuber size and marketable yields. The
effects of water stress during tuber bulking were evaluated in an Aberdeen
study in which the nine weeks immediately following tuberization were
divided into three 3-week intervals designated as early (E), mid (M), and
late (L) bulking. Constant water stress was
imposed during 2 or 3 of these periods by irrigating at the rate of 80% or
60% of normal crop water use. The greatest reductions in total yield
generally occurred when deficit irrigation was imposed during early-mid (EM),
and mid-late (ML) bulking, regardless of water stress intensity. Spreading
deficit irrigation evenly over the three periods or during the early- and
late-bulking periods resulted in less total yield reductions. Effects of
drought timing on U.S. No. 1 yield were similar to those for total yield.
Water stress during the tuber bulking increased the percentage of
undersized tubers with a corresponding decrease in 4 to 10 oz size tubers.
Water stress during the early and mid bulking periods resulted in the
lowest specific gravities and increased the percentages of dark ends. In
drought years, irrigation cutoffs during the month of August are highly
probable in fields dependent on surface-water supplies. In a 1992 study at
Aberdeen, water was cut off during the first or third week in August
either abruptly or gradually over a 2-week period. The same amount of
water was applied for both cutoff methods at a given cutoff date. The
mid-August cutoff reduced total and U.S. No. 1 yields by about 13%, while
cutting off water the first week in August reduced total yield by 30% and
U.S. No. 1 yield by 50%. There was little benefit from gradual irrigation
cutoff compared to abrupt irrigation cutoff. Late July and early August
are peak tuber bulking periods and severe water stress during this period
will substantially reduce tuber yield and grade, regardless of the manner
it is imposed. Over the past 20 years, research at the Aberdeen Research
and Extension Center has shown that total yield of Russet Burbank will
generally be reduced 25 cwt/acre for every inch crop water use is reduced
due to water stress during the tuber bulking growth stage. Maturation - Maturation
stage of growth begins with canopy senescence. Tuber growth rates
gradually decline with loss of canopy, although significant amounts of
carbohydrate are transported from the vines to the tubers during this
growth stage. Exceptionally dry soil conditions late in the growing season
not only reduce yield but can also shorten dormancy, reduce specific
gravity, and increase reducing sugar content. Dry soil conditions can also
dehydrate tubers, making them more susceptible to blackspot bruise.
Consequently, care should be taken after irrigation is cut off to make
sure the available soil water does not drop below 50% prior to vine kill.
It is also important to get complete vine kill or removal since remaining
live vines will continue to extract soil moisture. Under dry soil conditions, the tuber moisture content or hydration level will steadily decrease resulting in increased susceptibility to blackspot bruise. Tubers can be partially rehydrated by irrigating prior to harvest. For tubers maturing in dry soils (<50% available soil moisture), irrigation should be applied about 7 to 10 days prior to harvest to adequately rehydrate tubers. Tubers maturing in soils with adequate available moisture (>60%) can usually be rehydrated by irrigating two to three days prior to harvest. Growers and irrigation districts should always hold back adequate water to properly condition the soil and rehydrate the tubers before harvest. This is particularly important on medium and heavy textured soils where soil conditioning is necessary to soften clods to minimize tuber damage during harvest. |
