Physiological effects of psyllid (Paratrioza cockerelli) on potato

Feeding by psyllid nymphs causes “Psyllid yellows,” a characteristic yellowing of shoots which results in a dramatic loss of tuber yield. If psyllid infestation is not controlled, the onset of shoot yellowing and growth reductions can occur within two weeks. When insecticides are applied after yellowing, recovery of plants often is not complete. In this study comparisons were made of the physiology and growth of tops and tubers of recovered and permanently injured potatoes of the cultivars ‘Denali,’ ‘Kennebec,’ and ‘Norgold Russet.’ Permanently injured plants senesced rapidly, developed less shoot growth, aerial tubers and shortened and thickened internodes. They had lower tuber yields than plants that recovered. Permanently injured ‘Denali’ developed shoots on old flowering stems. In permanently injured plants of all cultivars, the tubers were small and badly misshapen but numbers increased. In recovered ‘Kennebec’ plants tubers were also misshapen, but were of average size. Tubers of recovered ‘Denali’ and ‘Norgold Russet’ were of acceptable shape. In samplings made during tuber growth, dry weight percentages of total soluble sugars, sucrose and glucose of tubers from permanently injured plants were found to be the same as those of recovered plants. In all cultivars, starch percentages of tubers from permanently injured plants were higher than that of the recovered, but the pattern of carbohydrate content changes was similar for permanently injured and recovered cultivars.     

Short-Term exposure to low temperature in the fall affects field performance of seed tubers

Field trials were conducted in Alberta and Ontario with Norchip, Norland, Russet Burbank, Sangre and Superior potato cultivars to determine response to short-term exposure of seed tubers to below freezing temperatures in the fall. Exposure of the dormant seed tubers to ?-1 C for up to 48 hours did not affect the yield or growth characteristics of the cultivars. However exposure of seed tubers to ?-5 C temperatures for 6 or 12 hours caused marked (P < 0.05) reductions in marketable yield, marketable tuber number, maturity and vine size. Exposure to ?-5 C for periods longer than 12 hours resulted in severe freezing damage and subsequent decay of most of the tubers.     

Soil water stress and the growth and yield of potato plants grown from microtubers and conventional seed tubers

Tuber yields of potato plants grown from microtubers in fields are more variable than yields from conventional seed tubers (CT). One reason could be their higher susceptibility to water stress. This study clarified the effect of soil water stress from 1 month after emergence on the growth and yield of plants grown from conventional seed tubers and microtubers in fields. Microtubers (0.5–3 g) and conventional seed tubers (50 g) were grown in Hokkaido, Japan, over three field seasons. One month after emergence, poly-shelters were placed over the plots to prevent rainfall, and either irrigated (wet plot) or non-irrigated (dry plot) treatments were formed. At mid-flowering (about 50 days after emergence) leaf area index (LAI) in microtuber plants was decreased relatively more due to soil water stress than LAI in conventional seed tuber plants. However, at maximum shoot growth (about 80 days after emergence) both microtuber and conventional seed tuber plants had a similar relative decrease in LAI due to soil water stress. At mid-flowering and maximum shoot growth microtuber and conventional seed tuber plants had reduced stomatal conductance due to soil water stress, but the reduction in stomatal conductance was greater in conventional seed tuber plants than in microtuber plants. Microtuber and conventional seed tuber plants had similar root development at maximum shoot growth. Tuber production from mid-flowering until plant maturity was similarly affected by soil water stress in microtuber and conventional seed tuber plants. At harvest, plants affected by soil water stress had about 87% of the tuber dry weight of irrigated plants. We conclude, that the greater variation on tuber yield of microtuber plants cannot be attributed to soil water stress from 1 month after emergence.     

Morphological changes and yield enhancement of superior potatoes by AC 243,654

AC 243,654, 1-(m)-methoxybenzyl)-3-nitroguanidine, represents a new family of plant growth regulators with cytokinin-like activity. This compound was tested at 1 and 4 kg/ha on greenhouse-grown potatoes (Solanum tuberosum L.), cultivar Superior. The treatments were applied two weeks after emergence. Morphological and yield effects of AC 243,654 were determined at 4, 7, 11, 14, 21, 42 and 70 days after treatment (DAT). Shortly after treatment, AC 243,654 stimulated plant height, branch number and stolon number. Plant weights were initially stimulated. At the final harvest, 70 DAT, the dry weight of treated plants was greater than that of the check plants; this is attributed to the anti-senescence effect of this compound on vine tissue. AC 243,654 induced tuber initiation at 14 DAT, whereas checks did not have tubers until one week later. Between 14 and 70 DAT, treated plants produced greater yield than check plants; the yield increases were primarily due to larger tubers. The yield of smaller tubers was decreased by the treatments. Although the number of larger tubers increased, total tuber number was decreased by AC 243,654. Observations on tuber number over time suggested that the check plants initiated more tubers than treated plants but most of these aborted. In treated plants, less tubers were initiated.     

Growth and yield of potatoes as affected by severity of stem canker (Rhizoctonia solani)

Summary Different amounts ofRhizoctonia solani inoculum were applied to sprouted and non-sprouted seed tubers cultivar Désirée during planting in field experiments at Rothamsted in 1984 and 1985. Severity of stem canker and stolon infection increased with increasing amounts of inoculum; with small or medium amounts these infections were most severe on plants from sprouted seed. In 1985 plants grown from non-sprouted, seed had most stem canker when large amounts of inoculum were applied. Severe infection following high inoculum levels delayed shoot emergence, increased the variability in stem height and decreased the mean height of stems and weight of foliage but did not affect the number of stems per plant or final plant populations. Tuber initiation was delayed and tuber numbers slightly decreased by severe infection and at harvest the size distribution of tubers was altered. Tuber yields from severely infected plants were decreased by 15% (sprouted seed) and 19% (non-sprouted seed) in 1984 and by 17 and 28% respectively in 1985.     

Effect of cultivar maturity period on the growth and yield of potato plants grown from microtubers and conventional seed tubers

Potato plants of early cultivars grown from microtubers have been reported to have a much lower growth vigor and produce lower yields than microtubers of late cultivars. This study intended to clarify the field performance of plants grown from directly planted microtubers of cultivars with different maturity periods, with a special attention to early cultivars. The experiments were conducted at Hokkaido University, Japan, over four years. Microtubers and conventional seed tubers of the early cultivar Kitaakari, late cultivars Konafubuki and Norin 1, and very late breeding line IWA-1 were planted, and the plant growth and tuber yields were analyzed. The microtuber plants of Kitaakari had a lower initial increase in leaf area index than conventional seed tuber plants, but at the maximum shoot growth had the same leaf area index. This pattern was also observed in the other cultivars. Tuber initiation and tuber bulking occurred on average five days later in microtuber plants than in conventional seed tuber plants of cultivar Kitaakari. At maximum shoot growth, microtuber plants had on average 65% of tuber dry weight of conventional seed tuber plants, with small variation among cultivars. Irrespective of maturity period, microtuber plants showed a higher tuber increase after maximum shoot growth, achieving around 86% of tuber dry weight of conventional seed tuber plants at harvest. From the results of this study we conclude that microtuber plants of early and late cultivars have a similar yield potential relative to conventional seed tuber plants, and microtubers of both early and late cultivars might be used as an alternative seed tuber source for potato production, if necessary.     

Effect of weight of potato minitubers on sprout growth,emergence and plant characteristics at emergence

Summary The behaviour of minitubers in five weight classes, having mid-point values between 0.19 and 3.00 g, was studied during sprouting and emergence under controlled conditions. Lighter tubers took longer to produce sprouts of 2 mm, and their sprouts grew more slowly between 2 and 4 mm and 4 and 6 mm. As sprouts lengthened their rate of growth increased. The influence of tuber weight was less for heavier tubers and also decreased as the sprouts grew longer. When tubers with sprouts of the same length were planted in pots, sprouts from lighter tubers took longer to emerge. Emergence was later and differences between weight classes were larger when tubers were planted deeper (6 or 9 cm) or when they had shorter sprouts at planting (2 or 4 mm). At emergence, plants from lighter tubers had thinner stems and lower stem and root weights, but higher stem weights proportional to tuber weights and higher shoot:root ratios.     

The Influence of Plant Growth Regulators and Inflorescence Removal on Plant Growth,Yield, and Skin Color of Red LaSoda Tubers

Foliar applications of the synthetic auxin compound 2,4-dichlorophenoxyacetic acid (2,4-D) have been used to enhance skin color of red potato cultivars for many years. Relatively, little is known about the mode of action of this compound, but it has been proposed that 2,4-D stimulates ethylene production and enhances the activity of enzymes involved in anthocyanin biosynthesis. The objective of this study was to compare several other plant growth regulators to 2,4-D for their effect on plant growth, tuber yield, and skin color of the light red-skinned cultivar “Red Lasoda.” Trials were conducted in 2010 and 2011 to evaluate the effects of foliar applications of the ethylene-stimulating growth regulator ethephon, the ethylene-inhibitor 1-methylcyclopropene (1-MCP), and 2,4-D. Manual inflorescence removal was also included as a treatment to determine the influence of anthocyanin sink competition between tubers and flowers on tuber skin color. Ethephon application caused floral abscission in treated plants and reduced plant height compared to other treatments and the non-treated control. While 2,4-D caused foliar injury and ethephon slightly increased the yield of <56 g tubers, neither growth regulator affected total and marketable yield. Both ethephon and 2,4-D increased tuber skin color at harvest compared to the non-treated control. In contrast, 1-MCP and inflorescence removal had no effect on yield, tuber size, and skin color. The results indicate that ethephon and 2,4-D foliar applications have similar effects on skin color and appearance, supporting the hypothesis that 2,4-D works by stimulating ethylene production within the plant.     

Potato Stolon and Tuber Growth Influenced by Nitrogen Form

Abstract

Potato tuber initiation and its growth are key processes determining tuber yield, which are closely related to stolon growth, and are influenced by many factors including N nutrition. We investigated the influences of different forms of nitrogen (N) on stolon and tuber growth in sand culture with a nitrification inhibitor during 2010 – 2011, and using two potato cultivars. Plants supplied with NO₃-N (N as nitrate, NO₃^-) produced more and thicker stolons than those supplied with NH₄-N (N as ammonium, NH₄⁺) at tuber initiation stage. In the plants fed NO₃-N, the stolon tips swelled or formed tubers earlier and produced more tubers than in those fed with NH₄-N. However, no significant difference was observed among N forms in terms of tuber yield at harvest, this may have been because of the shoot growth rate at tuber initiation stage was lower in the plants fed NO₃-N. During the tuber bulking stage, the difference in shoot DWs among N forms began to decrease, and the shoot DW of plants fed NO₃-N was even heavier than those fed NH₄-N in some cases. The influence of N form on potato plant growth may therefore vary with the potato growth stage.     

Accumulation of dry matter and N,P, K,by Russet Burbank,Oromonte and Red McClure potatoes

Potato plants of Russet Burbank, Oromonte and Red McClure varieties were studied in the San Luis Valley of Colorado to determine their dry matter and nutrient accumulation. The following observations were made in the 1967 growing season.

1. Growth patterns for the three varieties were similar, but average growth rate of Oromonte and Red McClure was higher than that of Russet Burbank.
2. Top growth reached its peak in 61, 55, and 50 days after emergence for Russet Burbank, Oromonte and Red McClure, respectively.
3. In the 3 weeks before killing forst, tuber growth equalled 50% of maximum tuber dry weight for Russet Burbank and about 61% for Oromonte and Red McClure.
4. Total N uptake with a yield of 300 cwt per acre was about 127 lb for each variety, assuming a linear relationship between tuber yield and nitrogen uptake. The tubers contained about 71% of the total N uptake.
5. Total P uptake with a yield of 300 cwt per acre was about 16 1b for each variety, assuming a linear relationship between tuber yield and P uptake. The tubers contained about 83% of total P uptake.
6. Total K uptake with a yield of 300 cwt per acre was 132, 187, and 147 lb (not significantly different) for Russet Burbank, Oromonte and Red McClure, respectively, assuming a linear relationship between tuber yield and K uptake. The Russet Burbank tubers contained 63% of the total K uptake and Oromonte and Red McClure about 52%.
7. Tuber dry matter per cent increased during the season until 2 weeks before a killing frost (65 to 69 days after emergence). At harvest, the dry matter approximated 21% for the varieties studied.

     

Invertase activity as affected by cytokinin-like compounds during potato tuberizationin vitro

Acid invertase plays an important role in plants by hydrolyzing sucrose. Compounds exhibiting cytokinin activity were examined for their effects on acid invertase activity at three tuberization stages of potato,Solanum tuberosum L. Single nodal segments from the cultivar Atlantic were cultured on Murashige and Skoog (MS) medium with 6% sucrose and maintained under a 16-hr photoperiod. Media were supplemented with either 2 mg kinetin/1, 0.1 mg thidiazuron/1,1.0 mg AC243, 654/1 (a benzyl nitroguanidine), or 0.1 mg AC239, 604/1 (a phenyl nitroguanidine). Basal acid invertase activity was measured in stolon segments at three morphologically distinct tuberization stages: (1) “hook,” (2) “swelling,” and (3) “initiation.” The onset of tuber initiation was significantly advanced by kinetin and thidiazuron compared to controls. Stolons elongated during the hook and swelling stages until tubers were initiated. Basal acid invertase activity in stolons cultured on control medium significantly increased from hook stage to swelling stage and then decreased slightly when tubers were initiated. At the hook stage, highest basal acid invertase activity occurred when the segments were treated with kinetin and thidiazuron. Kinetin and thidiazuron induced basal acid invertase activity significantly decreased following the hook stage to tuber initiation. Basal acid invertase activity were significantly lower in stolons treated with nitroguanidines than the control activity following the hook stage of development. During the swelling and tuber initiation stages, nitroguanidines-treated segments showed reduction in basal acid invertase activity similar to segments treated with kinetin and thidiazuron treatments. Tuber initiation was preceded by a stimulation of basal acid invertase activity followed by a decrease prior to tuber initiation. Kinetin and thidiazuron stimulated enzyme activity early in the growth of stolons which may have resulted in faster stolon growth and in earlier tuber initiation. The two nitroguanidines, although able to mimic many cytokinin effects in bioassays, did not seem to act in the same way as kinetin.     

Growth and Tuberization of Hydroponically Grown Potatoes

Three hydroponic systems (aeroponics, aerohydroponics, and deep-water culture) were compared for the production of potato (Solanum tuberosum) seed tubers. Aerohydroponics was designed to improve the root zone environment of aeroponics by maintaining root contact with nutrient solution in the lower part of the beds, while intermittently spraying roots in the upper part. Root vitality, shoot fresh and dry weight, and total leaf area were significantly highest when cv. Superior, a medium early-maturing cultivar, was grown in the aeroponic system. This better plant growth in the aeroponic system was accompanied by rapid changes of solution pH and EC, and early tuberization. However, with cv. Atlantic, a mid-late maturing cultivar, there were no significant differences in shoot weight and leaf area among the hydroponic systems. The first tuberization was observed in aeroponics on 26–30 and 43–53 days after transplanting for cvs Superior and Atlantic, respectively. Tuberization in aerohydroponics and deep-water culture system occurred about 3–4 and 6–8 days later, respectively. The number of tubers produced was greatest in the deep-water culture system, but the total tuber weight per plant was the least in this system. For cv. Atlantic, the number of tubers <30 g weight was higher in aerohydroponics than in aeroponics, whereas there was no difference in the number of tubers >30 g between aerohydroponics and aeroponics. For cv. Superior, there was no difference in the size distribution of tubers between the two aeroponic systems. It could be concluded that deep-water culture system could be used to produce many small tubers (1–5 g) for plant propagation. However, the reduced number of large tubers above 5 g weight in the deep-water culture system, may favor use of either aeroponics or aerohydroponics. These two systems produced a similar number of tubers in each size group for the medium-early season cv. Superior, whereas aerohydroponics produced more tubers than aeroponics for the mid-late cultivar Atlantic.     

Effect of simulated hail damage on yield and quality of Russet Burbank potatoes

The yield and quality response of Russet Burbank potatoes to simulated hail damage was dependent upon the amount of damage and on the growth stage at which the damage occurred. Plots of Russet Burbank potato plants were subjected to 0%, 10%, 25%, 50%, 75%, or 100% defoliation at 3, 5, 7, 9, 11, or 14 weeks after emergence. A motorized flail was used to simulate the complex damage that occurs during a natural hail storm. Early season simulated hail damage, when the plants were mainly vegetative, caused yield losses proportional to the degree of damage. Simulated hail damage during the early phases of tuber growth caused minor yield losses (< 5%) at low defoliation levels but severe losses at the higher levels with proportional loss of tuber quality. Defoliation later in the season resulted in progressively less reduction of both yield and quality. At or near maturity, late season simulated hail damage caused insignificant losses unless the stems were damaged enough to prevent nutrition and carbohydrate translocation into the tubers.     

Utilization of potatoes for life support systems in space. I. Cultivar-photoperiod interactions

The productive potential of potatoes (Solanum tuberosum L. cvs. Norland, Superior, Norchip, and Kennebec) was assessed for life support systems being proposed for space stations and/or lunar colonies. Plants were grown in walk-in growth rooms for 15 weeks at 20 C under 12-, 16- and 20-h photoperiods of 400 μmol m^t-2s^t-1 photosynthetic photon flux (PPF). Norland yielded the greatest tuber fresh weight, producing 2.3, 2.4, and 2.9 kg/plant under 12-, 16-, and 20-h photoperiods, respectively. The respective yields for the other cultivars under 12-, 16-, and 20-h were: Superior, 1.9, 1.5, and 1.8 kg/plant; Norchip, 1.8, 1.4, and 2.0 kg/plant; and Kennebec, 2.3, 0.2, and 0.8 kg/plant. Shoot and total plant biomass increased with lengthening photoperiods except for Kennebec, which showed increased shoot growth but no change in total growth with the longer photoperiods. Kennebec shoot growth under the 20-h photoperiod, and to some extent under 16-h, was noticeably stunted with shortened internodes. In addition, leaves of these plants showed mild chlorosis with rusty “flecking” of the surfaces. The harvest index (ratio of tuber yield/total biomass) was highest for all cultivars under the 12-h photoperiod, with a maximum of 0.69 for Norland. Similarly, the tuber yield per input of irradiant energy also was highest under 12-h for all cultivars. The tuber yield expressed on an area basis for the highest yielding treatment (Norland under 20-h) equaled 2.2 kg dry matter m^t-2. Over 15 weeks this equates to a productivity of 20.7 g tuber dry matter m^t-2 day^t-1. Assuming 3.73 kcal per g tuber dry matter and a daily human dietary requirement of 2800 kcal, then 36 m² of potatoes could supply the daily energy requirement for one human. Potential for increasing productivity is discussed.     

种植密度与马铃薯块茎大小的分布 Ⅰ.密度与块茎生长的关系

种植密度是影响马铃薯块茎生长发育的主要因素之一。主栽品种 Mira 和783-1各3种密度的试验结果表明,随着单位面积播种密度的增加,单位面积上的主茎数及块茎数目均增加,而块茎的平均重量及大中薯率则减少。单位面积上的光能截获量与植株干重、块茎干重和块茎的平均重量均呈极显著的直线相关。Mira 和783-1的光能转化系数分别为1.444克干重/兆焦和1.106克干重/兆焦。分配到块茎的干物质比率随着块茎的生长而增加,最终两品种的干物质转化率均为85%左右。而密度对各品种的光能转化率及块茎干重分配率均无显著影响。因此,种植密度主要是通过对单位面积上块茎数量的控制来影响块茎所获得的屯合产物的数量及块茎的生长发育。     

Growth and yield of potato plants grown from microtubers in fields

Despite many reports of thein vitro production of microtubers, little is known about plant growth and yield from microtubers planted in the field. This study clarified differences in growth and yields between potato plants grown in the field from microtubers and from conventional seed tubers. The experiments were performed at Hokkaido University, Japan, over four years. Conventional seed tubers of about 50 g and microtubers of two sizes (0.5–1.0 g and 1.0–3.0 g) of the latematurity cultivar Norin 1 were planted, and plant growth and tuber yields were analyzed. The microtuber plants had a lower initial increase in root and leaf area index than conventional seed tuber plants, but had the same leaf area index after about 40 days from emergence. The first tuber formation in microtuber plants was about 7 days later than in conventional seed tuber plants, while tuber bulking occurred about 14 days later in microtuber plants. Consequently, the onset of tuber weight increase was later in microtuber plants, but the rate of increase thereafter was similar between conventional seed tuber and microtuber plants. At harvest the tuber fresh weight of microtuber plants was 82% that of conventional tuber plants, suggesting a potential for using microtubers for field planting.     

Effect of seed tuber and seed piece size on growth and incidence of hollow heart in Norgold Russet potatoes

Small seed pieces decreased yield and number of tubers. Both seed tuber and seed piece size affected the percentage of tubers with hollow heart. Hollow heart increased from 14 to 22% as the seed tuber size increased from 57 to 228 g and decreased from 27 to 19% as the seed piece size increased from 28 to 57 g. The least hollow heart (11%) occurred in tubers of plants grown from 57 g whole seed pieces and the most (26%) in tubers of plants grown from 228 g seed tubers cut into 28 g seed pieces. Hollow heart was positively correlated with the mean tuber size and negatively correlated with the number of mainstems per hill, total yield, and total number of tubers.     

Effect of supplemental nitrogen on true potato seed weight

The effect on true potato seed (TPS) weight of supplemental nitrogen (N) applied during seed development was investigated using crosses DTO-33 × R128.6 (“A” produced in the field) and Atzimba × R128.6 (“B” produced in the field and in a screenhouse). Dry weights of tops and tubers of the mother plants were also measured in the screenhouse. The response to supplemental N (0-240 kg/ha) in 100-TPS weight of cross A and B from the field was positive and linear. In the screenhouse, where higher total N (0-1200 kg/ha) was applied, the responses in 100-TPS weight and dry weight of tops and tubers were curvilinear, with maximum levels at 800, 1000 and 400 kg/ha, respectively. The 100-TPS weight of cross B was 40% higher in the field than in the screenhouse. In the field, increased frequency of supplemental N applications increased 100-TPS weight of large and medium berries of cross B, but had no effect on seed from small berries nor on seed from any berries of cross A. In the screenhouse, increased application frequency decreased tuber dry weight and increased dry weight of tops, but had no effect on 100-TPS weight. It was concluded that supplemental N must be applied during seed development and at higher total levels than those required for optimum tuber yields in order to maximize 100-TPS weight. The lower seed weight from the screenhouse suggests that other environmental factors (e.g., temperature) present during growth of the mother plant can affect the weight of the resultant TPS.     

Cultivar response to Fusarium storage rot as affected by two methods of seed origin propagation; Clonal selection andin vitro culture

Progeny tubers from seed potatoes originating from either a traditional clonal selection method or the more modernin vitro tissue culture method of propagation were tested for storage rot response following inoculation of three tuber sites with twoFusarium species. Significant differences were found among disease responses for the twoFusarium species and for the four cultivars tested. Disease symptoms were less severe forF. solani var.coeruleum than forF. sambucinum. Fusarium sambucinum caused less disease in Superior than Kennebec, Russet Burbank and Sebago. Seed propagation method did not significantly affect disease response except for one case;in vitro culture of Kennebec had less disease due toF. sambucinum at the tuber side inoculation site than clonal selection. Kennebec and Superior inoculatedF. sambucinum had significant differences with respect to the number of years of field propagation for the tuber side inoculation site and for the tuber mean rot index. Significant differences in number of years of field propagation were also found for eye-ends of Superior tubers and stem ends of Russet Burbank tubers inoculated withF. solani var.coeruleum.     

Tuber initiation and development in irrigated and non-irrigated potatoes

Tuber initiation and development are processes basic to potato production and are particularly critical in areas with short growing seasons. It is important to know how and to what extent management decisions affect these processes in order to maximize the yield of marketable tubers. A two-year field study, conducted in southcentral Alaska, examined top growth, tuber initiation, and tuber development in eight potato cultivars grown with and without irrigation. Plants of the cultivars Allagash Russet, Bake-King, Green Mountain, Kennebec, Lemhi Russet, Russet Burbank, Shepody, and Superior were harvested weekly throughout the growing season, and top dry weight, numbers of tubers, and individual tuber fresh weights were recorded. Top dry weight was reduced by moisture stress shortly after emergence in 1993, and about one month following emergence in 1994, when early-season soil moisture was greater. The weight of tubers was similarly affected within approximately 5 wk of emergence in 1993 and 6 weeks in 1994. Tuber weight at harvest was increased two-to three-fold by irrigation in all cultivars. The number of tubers each plant set was affected by irrigation in most, but not all, cultivars. Some varieties (Lemhi Russet in 1994, Allagash Russet both years) set more tubers than were maintained through the growing season. Tuber remnants found during sample collection indicated that tuber reabsorption had occurred. Irrigated Green Mountain had more than one tuber initiation period during the season, whereas other varieties such as Shepody maintained a relatively constant number of tubers following initial tuber set. Tuber size distribution at the end of the growing season showed that larger tubers were favored by irrigation.