Introduction

General Cultivar Information

Peter Wilcox is a dark purple-skinned, yellow-fleshed cultivar suitable for fresh market. It also chips well although it is not being released as a chipping cultivar. Tubers are oblong and have low to moderate specific gravity. The yellow-flesh of Peter Wilcox is slightly darker than Yukon Gold, hence the carotenoid content of Peter Wilcox is slightly higher than Yukon Gold. Peter Wilcox is named in honor of a Professor at Loyola University in Maryland.

Pedigree Information

Peter Wilcox, evaluated under the pedigree B1816-5, was selected from a cross of B0810-1 (♀) x B0918-5 (♂) made in Beltsville, MD by K.G. Haynes (Fig. 1). B0810-1, the tan-skin female parent, was selected because of its good appearance and yellow-flesh. B0918-5, the purple-skin male parent, was selected because of its good appearance, high male fertility, and good general combining ability for color and appearance.

Fig. 1
figure 1

Pedigree of Peter Wilcox (B1816-5). Dates in parentheses indicate year of first field generation (i.e., single hill generation)

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Cultivar Development Program

Breeding and seedling tuber production of Peter Wilcox were done at the Beltsville Agricultural Research Center (BARC), Beltsville, MD, by K.G. Haynes. Seedlings were transplanted into 10.2 cm plastic pots in the greenhouse in Beltsville in the fall of 1995, and the largest mini-tuber from each pot was harvested and bulked by family. Mini-tubers were transplanted into the field on Chapman Farm, Presque Isle, ME in 1996, and B1816-5 was selected at harvest. Clonal selection and field performance evaluations were done on Chapman, Echo Lake, Lawrence Park, and Aroostook Farms in Presque Isle, Maine by K.G. Haynes. Foliar and soil-borne disease evaluations were conducted by K.G. Haynes, X.S. Qu, B.J. Christ, and S.H. Jansky. Preliminary evaluations were undertaken with cooperators in North Carolina, New Jersey, Pennsylvania, Maine, Florida, and New York beginning in 1998. Interregional performance trials through the NE-1014 Project began in 2005.

Release

Peter Wilcox was released by The United States Department of Agriculture, the North Carolina Agricultural Research Service, the Agricultural Experiment Stations of New Jersey, Pennsylvania, Florida and New York, and the Maine Agricultural and Forest Experiment Station in 2007. Plant Variety Protection has not been requested for Peter Wilcox; it is a publicly released cultivar.

Cultivar Description

Plants

Peter Wilcox has a semi-erect plant habit with intermediate (between stems clearly visible and stems hardly visible) foliage (Fig. 2a). Maturity is medium-early (about 100 days). Stems and petioles have medium-strong anthocyanin coloration. Stem wings are absent. Leaves are an olive-green (Royal Horticultural Society Color Chart, 2nd ed. - RHSCC 137B) with medium density and short pubescence. Leaf silhouette is open with an average of four primary leaflet pairs and seven secondary and tertiary leaflet pairs (Fig. 2b). The primary leaflets are medium sized with a medium ovate shape, a cuspidate leaflet tip shape, and a cordate leaflet base. The terminal leaflet is medium ovate, with a cuspidate leaflet tip, and a cordate leaflet base, with weak leaflet margin waviness. Stipule leaf size is small.

Fig. 2
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Pictures of Peter Wilcox: a plant, b leaf, c flowers, d tubers, and e light sprouts

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Flowers (Fig. 2c)

The shape of Peter Wilcox’s corolla is pentagonal and the color is blue-violet (RHSCC 91A) with a slightly lighter halo (RHSCC 91B-C). Average number of inflorescences per plant is 1.2 with a range of 1–2. Average number of florets per inflorescence is 5.3 with a range of 4–6. There is medium anthocyanin on the calyx. Anthers are yellow-orange (RHSCC 13A) and form a narrow cone. Pollen production is low to moderate and it has been successfully used as a male parent in hybridizations. Stigma color is yellow-green (RHSCC 147A) and the shape is clavate. Berry production in the field is moderate.

Tubers (Fig. 2d)

Tubers of Peter Wilcox are smooth with a dark purple skin (RHSCC 79B). Secondary skin color (RHSCC 92C) is present around the eyes. Tuber flesh color is yellow (RHSCC 11B) usually with no secondary flesh color, although a few purple streaks have occasionally been found in tubers grown in Maine. Tubers are oval to oblong and medium thick. The average length is 85 mm (range 68–101, standard deviation = 10.4), the average width is 68 mm (range 62–81, standard deviation = 4.6), and the average thickness is 57 mm (range 49–67, standard deviation = 4.9). Tuber eye depth is shallow and eyebrows have a slight prominence. Lateral eyes are shallow. Tubers have an average of 12.6 eyes (range 10–16). Distribution of eyes is predominantly apical. Tubers have a long dormancy (about 5 months).

Light Sprouts (Fig. 2e)

Sprouts grown in the presence of light are ovoid with the tip closed. Short root initials are present on the sprout and there is medium pubescence on the sprout tip. The sprout tip is a strong blue-violet in color.

Agronomic Performance

Total marketable yields of U.S. No. 1 grade potatoes (United States Standards for Grades of Potatoes 1997) of Peter Wilcox in comparison to Atlantic have averaged 73 % in four trials, 81 % of Chieftain in 21 trials, 96 % of Dark Red Norland in 19 trials, 81 % of Red LaSoda in six trials, 92 % of Red Pontiac in two trials, 83 % of Superior in 12 trials, and 101 % of Yukon Gold in four trials (Table 1).

Table 1 U.S. Number 1 yield and specific gravity of Peter Wilcox in comparison with standard cultivars at USDA/ARSa and cooperating state test plots from 2000 to 2006
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Tuber Quality Characteristics and Usage

Specific Gravity

Across all 68 trials along the east coast, specific gravity in Peter Wilcox has averaged 0.005 lower than Atlantic, 0.007 higher than Chieftain, 0.014 higher than Dark Red Norland and Red Pontiac, 0.005 higher than Red LaSoda, 0.004 lower than Superior and Yukon Gold (Table 1).

Internal Defects

Peter Wilcox has a low incidence of hollow heart and internal heat necrosis (Table 2) in comparison to standard cultivars. Purple-streaks have occasionally been observed. It is susceptible to blackspot bruising (Table 3).

Table 2 Incidence of hollow heart and internal heat necrosis in Peter Wilcox in comparison to standard cultivars at USDA/ARSa and cooperating state test plots from 2000 to 2006
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Table 3 Bruise test scores of Peter Wilcox and some standard cultivars from Presque Isle, ME
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Tuber Dormancy

In 3 years of evaluation, tubers of Peter Wilcox (and Chieftain) started to sprout on average 164 days after harvest, which is a longer dormancy period than Superior (140 days), Atlantic (135 days) and Dark Red Norland (129 days) (Table 4).

Table 4 Weight loss in storage and days to sprouting for tubers grown in Presque Isle, ME
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Weight Loss in Storage

Peter Wilcox is less susceptible to weight loss in storage than standard commercial varieties (Table 4). In 3 years of evaluation, Peter Wilcox has averaged 5.6 % weight loss when stored at 3.3 °C for 6–7 months and 17.1 % weight loss when stored at 10 °C for a similar period. In contrast, Dark Red Norland has averaged 7.3 % weight loss when stored at 3.3 °C for 6–7 months and 43.2 % weight loss when stored at 10 °C for a similar period.

Chip Color

Chip color has generally been satisfactory when processed from 10 or 7 °C in January (Table 5). However, it is being released primarily as a specialty, fresh market cultivar because of its skin and flesh colors.

Table 5 Chip processing color of Peter Wilcox in comparison to Yukon Gold harvested in late September and stored at indicated temperature until processed in month indicated from USDA/ARS test plots from 2000–2006
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Cooking Evaluations

Boil, bake and microwave color scores of Peter Wilcox have been good with no discoloration observed due to cooking. Texture has ranged from dry to moist, but never soggy (Table 6).

Table 6 Boil, bake and microwave cooking results of Peter Wilcox in comparison to some standard cultivars from Rock Springs, PA
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Disease Evaluations

Late Blight

Peter Wilcox was evaluated for 4 years (2005–2008) and found to be susceptible to foliar late blight (Table 7). Experiments were planted in early to mid June each year at the Russell E. Larson Agricultural Research Center at Rock Springs, PA on a Hagerstown silty clay loam. The experimental design was a randomized complete block with four replications from 2005 to 2007 and three replications in 2008. Plots consisted of five hills per clone spaced at 23 cm with 122 cm between plots within the row. Each treatment row had one adjacent spreader row. Seven cultivars (Atlantic, Chieftain, Katahdin, Kennebec, Russet Burbank, Snowden, Superior) were included as checks, with Kennebec considered the moderately resistant check. Commercial rates of fertilizer were banded in the row at planting. At hilling, additional liquid N was applied at commercial rates. Spreader rows were inoculated with a mixture of several isolates of the US-8 genotype of P. infestans at a concentration of 4 × 104 sporangia ml−1 in mid-August to promote uniform spread of the pathogen to all treatment plots. Plants were evaluated for percent diseased foliage three to four times from late August to early September and from these ratings, area under the disease progress curve (AUDPC) was calculated (Shaner and Finney 1977). The mean AUDPC was calculated by clone. Mean AUDPCs were separated by Fisher’s protected least significant difference test (P = 0.05), where the letter ‘a’ indicates most susceptible. In 3 years of evaluation (2005–2007), Peter Wilcox was rated susceptible to late blight. In 2008, Peter Wilcox was rated as moderately susceptible (Table 7).

Table 7 Mean area under the disease progress curve (AUDPC) for foliar late blight of Peter Wilcox in comparison to seven cultivars evaluated at Rock Springs, PA from 2005 to 2008
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Early Blight

Peter Wilcox was evaluated for 4 years (2005–2008) and found to be susceptible to foliar early blight (Table 8). Experiments were planted in May each year at the Russell E. Larson Agricultural Research Center at Rock Springs, PA on a Hagerstown silty clay loam. The experimental design was a randomized complete block with four replications from 2005 to 2007 and three replications in 2008. Plots consisted of five hills per clone spaced at 23 cm with 122 cm between plots within the row. Each treatment row had one adjacent spreader row of Norland (2005) or Dark Red Norland (2006–2008). Seven cultivars (Atlantic, Chieftain, Katahdin, Kennebec, Russet Burbank, Snowden, Superior) were included as checks, with Katahdin, Kennebec and Russet Burbank considered moderately resistant checks. Commercial rates of fertilizer were banded in the row at planting. Each year, natural infection with Alternaria solani occurred in the spreader rows. Plants were evaluated for percent diseased foliage four times from late July to the end of August and from these ratings, area under the disease progress curve (AUDPC) was calculated (Shaner and Finney 1977). The mean AUDPC was calculated by clone. Mean AUDPCs were separated by Fisher’s protected least significant difference test (P = 0.05), where the letter ‘a’ indicates most susceptible. In 3 years of evaluation (2005–2007), Peter Wilcox was rated susceptible to early blight. In 2008, Peter Wilcox was rated as moderately susceptible.

Table 8 Mean area under the disease progress curve (AUDPC) for foliar early blight of Peter Wilcox in comparison to seven cultivars evaluated at Rock Springs, PA from 2005 to 2008
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Common Scab

Peter Wilcox is susceptible to common scab. It was evaluated in 2005 in the National Common Scab Trials (Table 9; Haynes et al. 2010b). The experimental design was a randomized complete block at all locations. Three replications of five plants per clone were planted at each location. At harvest, all tubers were harvested and rated on a 0 to 6 scale for percent surface area covered with scab lesions (0 = 0 %; 1 = <2 %; 2 = 2 % to <5 %; 3 = 5 % to <10 %; 4 = 10 % to <25 %; 5 = 25 % to <50 %; 6 = >50 %) and rated on a 0 to 6 scale for type of lesion (0 = none; 1 = superficial, discrete; 2 = superficial, coalescing; 3 = raised, discrete; 4 = raised, coalescing; 5 = pitted, discrete; 6 = pitted, coalescing). Three variables were analyzed: an area index (AI), a lesion index (LI), and the proportion of scabby tubers (PS). For each plot, AI and LI were calculated as the sum of all the individual tuber surface area ratings, or individual tuber lesion ratings, respectively, divided by six times the number of tubers. Thus, AI and LI could range from zero (no scab) to one (very severe scab). At all three locations, AI and LI of Peter Wilcox tubers were similar to the susceptible check cultivars Atlantic and Ranger Russet.

Table 9 Mean area index (AI), type lesion index (LI), and proportion scabby tubers (PS) for Peter Wilcox, two standard resistant cultivars (Russet Burbank, Superior) and two standard susceptible cultivars (Atlantic, Ranger Russet) evaluated for resistance to common scab in Idaho, Maine and Minnesota in 2005
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Powdery Scab

Peter Wilcox was evaluated for 4 years (2005–2008) and found to be resistant to powdery scab (Table 10). Experiments were planted in late May each year in a naturally infested field in Potter County, PA. The experimental design was a randomized complete block with four replications from 2005 to 2007 and three replications in 2008. Plots consisted of 15 hills per clone spaced at 20 cm. Seven cultivars (Atlantic, Chieftain, Katahdin, Kennebec, Russet Burbank, Snowden, Superior) were included as checks, with Kennebec considered the susceptible check. Commercial rates of fertilizer were banded in the row at planting. At harvest, from late September to late October, the number of tubers with powdery scab was determined from the total number of tubers per plot. Clonal mean disease incidences were separated by Fisher’s protected least significant difference test (P = 0.05), where the letter ‘a’ indicates most susceptible. In all 4 years of evaluation Peter Wilcox was rated moderately resistant to powdery scab.

Table 10 Mean incidence of powdery scab on tubers of Peter Wilcox in comparison to seven cultivars evaluated at Rock Springs, PA from 2005 to 2008
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Verticillium Wilt

Peter Wilcox was evaluated in the National Verticillium Wilt Trial in 2007 and found to be highly susceptible to Verticillium wilt (Table 11). The experiment was planted on 3 May in an inoculated field near Hancock, Wisconsin. Plots consisted of four hills per clone spaced at 30.5 cm. Cultivars (Atlantic, Boulder, Dakota Diamond, Freedom Russet, MegaChip, Ranger Russet, Red Norland, Russet Norkotah, Stampede Russet, Superior, White Pearl, and Yukon Gem) were included as checks, with Russet Norkotah considered the most susceptible check and Ranger Russet the most resistant check. Visual wilt symptoms were rated three times during the growing season and from these ratings, area under the disease progress curve (AUDPC) was calculated (Shaner and Finney 1977). The mean AUDPC was calculated by clone. Mean AUDPCs were separated by Fisher’s protected least significant difference test (P = 0.05), where the letter ‘a’ indicates most susceptible. Peter Wilcox was as susceptible to Verticillium wilt as Russet Norkotah and more susceptible than Superior.

Table 11 Mean area under the disease progress curve (AUDPC) for foliar Verticillium wilt symptoms evaluated at Hancock, WI in 2007
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Viral Diseases

Peter Wilcox was tested in the National PVY Trial in 2003 and PVYOC was detected in infected plants (data not shown). Symptoms were readily visible. Peter Wilcox is also susceptible to PVS. It has not been evaluated for other viral diseases.

Tuber Chemistries

Carotenoids

Tuber carotenoid concentrations of Peter Wilcox and Yukon Gold were evaluated 2 years (2004–2005) from field grown tubers in Hastings, Florida and Presque Isle, Maine (Haynes et al. 2010a). Peter Wilcox tubers were 22 % higher in lutein, 40 % higher in zeaxanthin, and 39 % higher in total carotenoids than Yukon Gold (Table 12).

Table 12 Mean tuber carotenoid content of Peter Wilcox as compared to Yukon Gold grown in Presque Isle, ME and Hastings, FL in 2004–2005
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Glycoalkaloids

Total glycoalkaloid content of tubers averaged 11.99 mg per 100 g FW in 5 years of testing as compared to 7.22 mg for Red Pontiac (2 year average) and 7.58 mg for Yukon Gold (2 year average).

Cultivar Identification

DNA was assayed for SNP (single nucleotide polymorphism) marker genotyping based upon the SolCAP 8303 Infinium Potato SNP Array (Hirsch et al. 2013). For the similarity analysis, the samples were evaluated and compared based upon 3,702 SNPs with tetraploid (five cluster, dosage-specific) marker genotypes. The SNPs were filtered from the 3,763 reported by Hirsch et al. (2013) to 3,702 based on the new potato genome sequence pseudomolecule v. 4.03. The samples were compared in a pairwise fashion and with 221 different potato clones, including reference check varieties. Peter Wilcox was not very similar to any of the varieties in the database: it most similar to Dakota Russet (45.3 %), Strawberry Paw (45.2 %), Lenape (43.5 %), Superior (42.7 %) and Nicolet (42.6 %) (Table 13).

Table 13 Percent similarity of pairwise SNP genotype comparisons of the most similar samples and two reference varieties to Peter Wilcox, sorted by percent similarity
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Plant Variety Protection

Plant Variety Protection has not been requested for Peter Wilcox. It is a publicly released cultivar. As a result, it is sometimes marketed under the synonymous names ‘Purple Sun’ or ‘Blue Gold’.

Seed Availability

Seed of Peter Wilcox is available from certified seed growers in Maine and New York. Tissue culture plantlets may be requested from Dr. Keith Perry, Cornell University. Small amounts of seed for research purposes may be requested from the senior author.