Evaluation location: Minnesota, United States
Abstract. Thirty-two wild strawberry genotypes and two commercial cultivars were obtained from the US Department of Agriculture (USDA), Agricultural Research Service (ARS), National Clonal Germplasm Repository-Corvallis (NCGR) and planted in the field to test cold hardiness and foliar disease resistance at the University of Minnesota North Central Research and Outreach Center at Grand Rapids, MN. This station is located in USDA plant hardiness zone 3b. Among the 34 genotypes tested, Fragaria iinumae was sensitive to powdery mildew, leaf scorch, frost and mid-winter injury while F. nipponica was resistant to powdery mildew and leaf scorch. Fragaria iinumae suffered severe injury after winter 2009-10, while F. nipponica and F. orientalis survived that winter well, producing dense and vigorous plants in 2010. These species could be potentially useful diploid or tetraploid parents in breeding programs developing strawberries for the mid-western United States. Fragaria chiloensis PI 637983, was similar to the standard cultivars for winter hardiness and disease resistance. Wild strawberries bloomed earlier and had softer, smaller fruit than did the commercial F ×ananassa 'Jewel' and 'Mesabi'. Fragaria nipponica had the softest fruit. Fragaria orientalis PI 637933 and PI 637939 were notable for their pleasing taste and aroma, while most Fragaria nipponica, F itrupensis, and F. virginiana were very sour. The feral F. ×ananassa PI 641087 and PI 641088 may have the greatest immediate utility, with a combination of winter hardiness and excellent disease resistance, for octoploid breeding programs.
Material and methods
The evaluation was conducted at the University of Minnesota North Central Research and Outreach Center (NCROC) at Grand Rapids, MN, in USDA plant hardiness zone 3b. Field trials at this location have been useful in evaluating strawberry winter injury over the past three decades and selecting cultivars for northern climates based on winter hardiness and other important traits in a collaborative breeding program between the USDA Agriculture Research Service (ARS) and the University of Minnesota. The evaluation included 34 genotypes of
Fragaria iinumae, F. orientalis, F. nipponica, F. iturupensis, F vesca, F. chiloensis, F. virginiana, and wild accessions and standard cultivars ('Jewel' and 'Mesabi') of F ×ananassa Duchesne ex Rozier. 'Mesabi' was selected at Grand Rapids, MN, and is well adapted there for winter survival and fruiting. 'Jewel,' selected in New York, is widely grown in the eastern and mid-western US but" is not reliably winter hardy for commercial production at Grand Rapids.
Wild strawberries were shipped as runners from NCGR-Corvallis and propagated in the greenhouse and in the field at NCROC in 2008. In mid-October 2008, plants were dug and potted to 10 cm pots and grown in a heated greenhouse until January 2009, when they were moved to a cellar to spend their dormancy. Potted plants were moved out of the cellar on May 10, 2009, and stayed outdoors until planting.
On 28 May 2009, two, two-plant plots of each genotype were established in each of four blocks in a split block design. In each block, there were two sub-blocks with identical planting plans. One was overwintered with straw mulch, and the adjacent sub-block was overwintered without mulch. fn addition to the 34 replicated entries, two other wild genotypes, PI 637954 and Pl 641089, with insufficient plants for complete replication, were planted in a border row for observation. Starter fertilizer (11-52-0), monoammonium phosphate at rate of 80 g·114 L-1 rate and 500 ml solution per plant was used at planting and no additional fertilizer was applied after planting. Drip irrigation was installed as one T-tape per row (emitters spaced at 305 mm, 1.7 L·min-1, 102 L·h-1 for 30 mat 55.6 k Pa, John Deere Water, San Marcos, CA) and the field was irrigated once or twice per week as a supplement to precipitation. Weeds were manually removed and the space between rows was tilled as necessary to control weeds and runners. Straw mulch of 10-15 cm was applied on mulched plots in early November 2009, and removed to between rows in early April 2010. Straw was also added between the rows of the non-mulched plots in April so that all plots had surrounding straw during the 2010 growing season.
2009 Evaluations
Plants in each plot were initially spaced 0.6 m apart in rows 1.3 m apart. Plants were allowed to runner in 2010 to form short matted row plots. Some genotypes with excessive runners were trimmed manually to maintain them within their plot. Runners per plot were rated on 6 Aug. 2010 from 1 = 1-5 runners per plot; 2 = 6-10; 3 = 11-20; 4 = 21-30; and 5 => 30. Powdery mildew (Podosphaera aphanis (Wallr.) U. Braun & S. Takamatsu) and fungal leaf spot (Mycosphaerella fragariae (Tul.) Lindau) and leaf scorch (Diplocarpon earliana Ell. et Ev. (Wolf)) infections were rated on 6 Aug. and 25 September 2009, from 1 = no disease to 9 = severe infection. Frost resistance was evaluated on 13 October 2009, after several hard frosts from 9 to 13 October 2009, on a scale from I = no damage to 9 = all leaves fully desiccated.
2010 Evaluations
In 2010, genotypes were evaluated for several plant and fruit traits. The stand (% coverage of the plot) was estimated on 15 May during flowering and again at the early stage fruiting on 18 June. Winter injury was rated on 4 June from 1 (= all plants surviving , and vigorously growing) to 9 (= all plants dead) based on visual estimation of survival of the plants and the health and regrowth of the surviving plants. Plant vigor was rated on 18 June from 0 (= dead) to 9 (= highly vigorous) based primarily on the number and size of leaves produced. Growth habit was rated on 28 June from 1 (= prostrate) to 5 (= erect). Productivity was rated from 0 (no fruit) to 9 (heavily fruiting) when approximately 50% of the fruit appeared to be ripe. Using the same rating scale as in 2009, powdery mildew, fungal leaf spotting (leaf scorch/blight/spot) severity were rated on 7 July and 27 July. Fungal leaf spotting diseases appeared to include leaf scorch, leaf blight and leaf spot in 2010. As all three could be observed on one genotype, and necrotic lesions often coincided, a single fungal leaf disease score was given for each plot.
Berry weight was estimated based on random samples of 20 berries from a midseason harvest date (approximately 50% ripe fruit) from plots that fruited. Fruit shape was described as oblate, globose, globose conic, conic, long conic, necked, long wedge or short wedge according to the University of Florida key. External and internal fruit colors were described. Skin toughness was rated from 1 (very tender) to 9 (very tough) based on resistance to thumb abrasion when rubbed between thumb and forefinger. Firmness was rated from 1 (very soft) to 9 (very firm) when squeezed between thumb and forefinger. Flavor was characterized with descriptors and rated hedonically by JJL from 1 (very poor) to 9 (excellent).
Ratings were performed by SY and JJL in 2009 and 2010, respectively. Data for plant and fruit traits in each year were analyzed, where appropriate, using ANOVA with Statistix Software (Analytical Software, Tallahassee, FL). Mean separations were based on Fisher's protected LSD (P<=0.05).