HORTSCIENCE 45(12):1779–1787. 2010. et al., 2009). Pesticides are not used during the trials and cultural management practices use techniques that support environmental Evaluation of from the stewardship. This includes pre-plant incorpo- Ò ration of compost, maintenance of a 7.6- to Earth-Kind Trials: Black Spot 10.2-cm layer of organic mulch, and irrigation methods that conserve water. Before a ( rosae Wolf) can earn regional Earth-Kind designation, it must exhibit consistent, superior performance across multiyear and multilocation trials rep- Resistance and Ploidy resenting different soil types and other envi- David C. Zlesak1 ronmental conditions typical in the region. A University of Wisconsin–River Falls, 410 S. 3rd Street, River Falls, WI 54022 large number of cultivars are advertised as low maintenance by nurseries wanting to capital- Vance M. Whitaker ize on the popularity of lower maintenance University of Florida Horticultural Sciences, Gulf Coast Research and roses. It is difficult for consumers to know which roses truly possess the highest levels Education Center, 14625 CR 672, Wimauma, FL 33598 of pest and environmental tolerances. Earth- Steve George Kind designation gives consumers confidence that they are choosing roses that will have AgriLife Research and Extension, Texas A&M, 17360 Coit Road, Dallas, TX a high likelihood of success when basic plant 75252 care is provided. Disease susceptibility poses a major chal- Stan C. Hokanson lenge to roses and limits their success as Department of Horticultural Science, University of Minnesota, 1970 Folwell low-maintenance landscape shrubs. Fungi Avenue, St. Paul, MN 55108 that attack roses include Diplocarpon rosae (Wolf) (causal agent of black spot), Podos- Additional index words. rose chromosome count, rose disease, fungal isolate, Diplocarpon phaera pannosa (Wallr.: Fr.) de Bary (causal rosae pathogenic race agent of powdery mildew), and Cercospora Abstract. Regional, replicated cultivar trials of landscape roses are an ongoing compo- puderi B.H. Davis (one of the causal agents nent of the Earth-KindÒ program, which was started at Texas A&M University in the of rose leaf spot) (Horst and Cloyd, 2007). Of 1990s to support environmental landscape stewardship. The rose trials within the Earth- these, black spot is the most serious in the Kind program identify and promote the most regionally adapted rose cultivars and are outdoor landscape across most regions as a re- conducted without fertilizers or pesticides and greatly reduced irrigation. Black spot sult of the potential for rapid disease develop- (caused by Diplocarpon rosae Wolf) is the most serious disease of outdoor-grown roses ment that typically leads to leaf yellowing and worldwide as a result of the potential for rapid leaf yellowing and defoliation. Earth-Kind defoliation (Dobbs, 1984). Black spot has been designated cultivars for the south–central United States and roses under trial in other the most prevalent and widespread disease in regions or considered for future Earth-Kind trials (n = 73 roses) and two susceptible the Earth-Kind rose trials (Mackay et al., control cultivars were challenged with North American Races 3, 8, and 9 of D. rosae, 2008). Plants repeatedly defoliated from black which were previously characterized at the University of Minnesota. Young expanded spot become weakened and quickly fall out of leaves were inoculated using detached leaf assays. Lesion length (LL) was measured for contention for Earth-Kind designation. susceptible reactions and cultivar ploidy was determined using root tip squashes. Diploid, Diplocarpon rosae is capable of infecting triploid, and tetraploid cultivars (n = 20, 30, and 23, respectively) were identified, and only the genus Rosa. Asexual spores (condia) race-specific resistances and partial resistances were also identified. Race-specific overwinter on stems and fallen leaves and are resistance was generally more prevalent in newer rose cultivars and rose cultivars more transported to new growth in the spring through recently included in Earth-Kind trials. Nine cultivars were resistant to all three races water droplets. If free water remains present, (Brite Eyesä, ‘Grouse’, Home RunÒ, Knock OutÒ, Paprikaä, Peachy Creamä, Pink the conidia form germ tubes that penetrate the Knock OutÒ, Rainbow Knock OutÒ, and Yellow Submarineä). Blushing Knock OutÒ, leaf epidermis. Lesions may appear in as little a sport of Knock OutÒ, was susceptible to Race 8. Partial resistance rank for LL was as 4 d as sub-cuticular mycelia radiate from the generally consistent across races for roses susceptible to multiple races. The application point of infection. Condia-bearing acervuli of these data includes: characterizing the minimum resistance level needed for roses to burst through the leaf cuticle followed by leaf warrant inclusion in Earth-Kind field trials, the identification of additional race-specific abscission in susceptible cultivars (Horst and resistance genes, identifying resistance-breaking isolates of D. rosae, understanding race Cloyd, 2007). composition in field trials based on infection patterns of key cultivars, selection of parents Multiple studies have been conducted to for resistance breeding efforts, and continued comparisons between LL and growing characterize the pathogenic race structure of bodies of Earth-Kind field resistance data. D. rosae and then to use the characterized races to identify genes conferring host resis- tance (Debener et al., 1998; Whitaker et al., Garden roses (Rosa sp.) are among the toward pesticides, emerging legislation put- 2007a, 2007b, 2010a; Yokoya et al., 2000). most popular flowering shrubs in the world. ting greater limits on pesticide availability Isolates are distinguished from one another Diversity for traits such as form, color, and and use, busy lifestyles, and greater avail- based on their differential ability to infect a fragrance of flowers, plant habit, size, envi- ability of lower maintenance rose cultivars common set of rose genotypes. Those isolates ronmental adaptability, and extended season (Harp et al., 2009). with the same host infection pattern are des- of flowering all contribute to their widespread Earth-KindÒ Rose Trials are a component ignated as a race. Collections that have been cultivation and versatility (Zlesak, 2006). Lower of the overall Earth-Kind program, started at preserved for continued research include six maintenance cultivars that can tolerate regional Texas A&M (the term Earth-KindÒ and asso- races discovered in Germany (Debener et al., environmental conditions without routine de- ciated logo are trademarks of the Texas Agri- 1998), four from Great Britain (Yokoya et al., pendence on pesticides and excessive care Life Extension Service, Texas A&M System) 2000), and three races from a genetically di- are especially increasing in popularity (Harp and help to serve the horticulture community verse set of 50 isolates originating in east- et al., 2009; Lonnee, 2005). Factors fueling by identifying the most adaptable landscape ern North America (Whitaker et al., 2007a, this trend include negative consumer attitude roses through regional cultivar trials (Harp 2007b). Whitaker et al. (2010b) evaluated

HORTSCIENCE VOL. 45(12) DECEMBER 2010 1779 this international collection, identified 11 Using a collection of races to characterize of cultivars were obtained in the summer of unique races among them, and standardized roses marketed for low-maintenance land- 2007 and maintained on an outdoor gravel the race nomenclature. These studies and others scape use would be helpful beyond highlight- pad. They were transferred to a cold room and have uncovered genetic resistance within rose ing the resistance of roses for growers and provided with a 4 C vernalization treatment species and cultivars that is race-specific. Such consumers. Knowledge of the genetic resis- for 10 weeks before being pruned, repotted race-specific resistance can only be character- tance of these roses would guide the selection as necessary, and brought to the greenhouse ized with controlled inoculations of roses with of cultivars that could serve as controls to help (23 ± 4.0 C; St. Paul, MN; lat. 45 N) in mid- individual races as opposed to a field setting differentiate the presence of known races in Jan. 2008. Day extension with supplemental where the presence and prevalence of specific field trials where race composition is not light was used until 2200 HR (400-W metal races are not known. Using characterized races known. Additionally, when cultivars with re- halide lamps; 150 mmolÁm–2.s–1 at plant for inoculations under controlled conditions is sistance to all known races become infected, level) to encourage strong, vigorous growth. also advantageous when surveying for partial pathologists may characterize the infective Sulfur was burned in the greenhouse from resistance (Whitaker and Hokanson, 2009a; isolate(s) in search of new pathogenic races. 22:00 to 02:00 HR nightly to discourage Xue and Davidson, 1998). A widespread cultivar screen with known powdery mildew. Sulfur burners were turned Detached leaf assays have been an effi- races would also allow breeders to identify off a minimum of 3 d before leaves were cient tool to characterize the resistance of desirable genetic resistances among a core set collected for detached leaf assays. rose seedling populations, and they have been of highly resistant cultivars. If knowledge of In late May 2008, a subset of roses needing found to be strongly correlated with whole resistance could be coupled with knowledge further characterization as a result of missing plant inoculations (Hattendorf et al., 2004; of rose ploidy level, this would help breeders data or to better understand inconsistent results Von Malek and Debener, 1998; Whitaker develop disease-resistant cultivars more effi- was given an 8-week vernalization (4 C) and Hokanson, 2009a). Detached leaf assays ciently because of a greater likelihood of treatment to reinvigorate growth. After vernal- are preferable as a result of greater ease in reproductive success. Preferential crossabil- ization, roses were cut back, repotted as needed, controlling humidity and inoculum levels ity and fertility as a consequence of ploidy and brought to the St. Paul greenhouse in mid- (Whitaker and Hokanson, 2009b). To the best level has been well documented in roses (El July and grown as described previously. There of our knowledge, single-spore isolates of D. Mokadem et al., 2001; Leus, 2005; Rowley, was a final, yet smaller, subset of roses forced rosae have only been used for race character- 1960; Shahare and Shastry, 1963). for the same reasons. They were provided with ization, comparison of pathogenicity of iso- The objectives of this study were to a 12-week vernalization treatment in mid- lates, and to study the segregation of resistance characterize roses within or being considered Oct. 2008 (4 C; the longer vernalization treat- to particular races in genetic studies and the for the Earth-Kind rose program according to ment was the result of greenhouse availability characterization of partial resistance compo- 1) resistance to three North American races and logistics) and brought to the University nents. Single-spore isolates representing dif- of D. rosae using detached leaf assays; and of Minnesota Horticulture Research Center ferent races to the best of our knowledge have 2) ploidy through direct chromosome counts. greenhouse in Chanhassen, MN, in mid-Jan. not been used to individually challenge com- 2009 and grown under similar conditions as mercial cultivars for widespread race-specific Materials and Methods previously described. and horizontal resistance characterization. Isolate preparation and inoculations. Schulz et al. (2009) challenged rose acces- Plant material. Seventy-five rose cultivars Fungal isolates ACT (Race 8; collected from sions uninfected with black spot in two field were challenged for race-specific and par- Brenham, TX), GVH (Race 3; collected from locations with a mixture of single-spore D. tial resistance and characterized for ploidy Hastings, MN), and IGWA (Race 9; collected rosae isolates using detached leaf assays. (Table 1). These roses represent 17 Earth- from Appleton, WI) described in Whitaker However, the number of races represented Kind-designated cultivars for the south– et al. (2010b) were retrieved from liquid by these isolates is not known. Recently, central United States (the only region currently nitrogen storage. To obtain sufficient inocu- Whitaker and Hokanson (2009a) reported where roses have completed the Earth-Kind lum, aqueous suspensions of asexual spores using detached leaf assays to characterize trialing process and winners are designated), (conidia) were pipetted onto leaves of sus- the partial resistance to black spot of segre- 30 cultivars under trial in the Earth-Kind ceptible cultivars. Leaves were placed adax- gating populations using the measurement of Brigade (mid-U.S. Earth-Kind rose trials), 20 ial side up on a folded 15 · 28-cm sheet of LL, defined as the diameter of the lesion at its cultivars under trial in the Northern Earth- paper towel (BountyÒ; Proctor and Gamble, widest point. LL was chosen by the authors Kind Rose Trials (north–central U.S. Earth- Cincinnati, OH) moistened with distilled because of the ease of measurement and its Kind rose trials; Harp et al., 2009), and two water and sealed in an airtight, transparent significant correlations with three other mea- controls. A limited number of cultivars are ClearpacÒ deli container (22.9 cm length · sures of partial resistance (Whitaker et al., represented in multiple Earth-Kind regional 18.7 cm width · 6 cm height; cat. # C48DER; 2007b; Xue and Davidson, 1998). groups such as Carefree Beautyä (‘Bucbi’) Dart Corporation, Mason, MI). Containers and ‘Sea Foam’, which are in all three groups. (subsequently referred to as boxes) were in- In addition, roses were included that are de- cubated in the laboratory at room temperature veloping reputations as being highly resistant (21 to 24 C). After the development of spor- Received for publication 23 July 2010. Accepted to black spot and are being considered for ulating lesions in 10 to 14 d, the infected leaves for publication 9 Oct. 2010. future Earth-Kind trials (Alba Meidilandä, were stored in sealed polyethylene bags at –20 This research was supported by the Minnesota Candy Ohä Vivid Red, Carefree Celebrationä, C for up to 6 weeks before inoculation. Spores Agricultural Experiment Station and grants from Carefree Marvelä, Double Knock OutÒ,Fra- from the frozen leaves were collected by wash- the Minnesota Garden Calendar Fund and the Ò North Central Region Sustainable Agriculture Re- grant Spreaderä, Home Run ,Paprikaä, ing the lesions with distilled, deionized water Ò search and Education Program. Peachy Creamä, Rainbow Knock Out ,Straw- and quantified using a hemacytometer. We thank Michelle Grabowski for her insightful berry Crushä, and Sunny Knock OutÒ). Also The two primary inoculation experiments comments and technical assistance and Brandi included are parents or offspring of very re- were performed on 28 Feb. 2008 and 3 Apr. Miatke, Jolyne Pomeroy, and Amber Halberg for sistant roses (‘Grouse’ and ‘Plaisanterie’). The 2008. The third, fourth, and fifth inoculations their help caring for plants in the greenhouse and two susceptible controls were ‘Chorale’ and were performed on 3 Aug. and 9 Sept. 2008 helping to perform inoculations in the laboratory. ‘Pariser Charme’. and 11 Mar. 2009, respectively. These addi- Plants for this research were generously donated or At least two potted plants of each cultivar tional inoculations over time were performed provided at a reduced cost by Bailey Nurseries, Chamblee’s Rose Nursery, Sam Kedem Nursery, were acquired from multiple nursery sources for those host/isolate combinations having Spring Meadow Nursery, and The Conard-Pyle and plants were all propagated on their own missing data resulting from leaf degradation Company (StarÒ Roses). roots (not grafted). Own root plants were or inconsistent results across the first two inoc- 1To whom reprint requests should be addressed; necessary to allow for cultivar ploidy deter- ulation dates. A minimum of four inoculation e-mail [email protected]. mination using root tip squashes. Potted plants boxes of leaves and a maximum of 10 were

1780 HORTSCIENCE VOL. 45(12) DECEMBER 2010 Table 1. Rose cultivars with their Earth-Kind group classification, commercial class, year of introduction, ploidy, and Diplocarpon rosae race-specific reaction. Cultivary Categoryx Commercial class Yr of introduction and breederw Ploidy Race 3 Race 8 Race 9 Alba Meidilandä (MEIflopan) 4 Shrub 19851 2x Sz S* S* Alexander Mackenzie 3 Shrub 19852 3x SRR Amiga Mia 2 Shrub 19783 4x SSS* April Moon 2 Shrub 19843 3x SSS Barn Dance 2 Shrub 19753 3x RSR Belindas Dream 1, 2 Shrub 19924 3x SSS Blushing Knock OutÒ (RADyod) 2 Shrub 20045 3x RSR Brite Eyesä (RADbrit) 3 Large-flowered climber 20065 4x RRR Caldwell Pink 1 Polyantha Unknown6 2x SRR Candy Ohä Vivid Red (ZLEmartincipar) 4 Shrub 20087 2x SSR Carefree Beautyä (BUCbi) 1, 2, 3 Shrub 19773 4x SSS Carefree Celebrationä (RADral) 4 Shrub 20085 3x SSS* Carefree Marvelä (MEIrameca) 4 Shrub 20038 3x SSR Carefree Wonderä (MEIpitac) 2 Shrub 19908 4x SSS Chorale 5 Shrub 19783 4x SSS Chuckles 2 Floribunda 19589 4x SSS Climbing Pinkie 1 Climbing polyantha 195210 2x SSS Country Dancer 2 Shrub 19733 4x SSR Double Knock OutÒ (RADtko) 4 Shrub 20045 3x RS*R Dublin Bayä (MACdub) 2 Climbing floribunda 197511 4x SSS Ducher 1 China 186912 2x SSS Duchesse de Brabant 1 Tea 185713 2x SSS Earth Song 2 Shrub 19753 4x SSS Else Poulsen 1 Floribunda 192414 3x SSS Flora Dora 2 Shrub Unknown6 3x SSS* Folksinger 2 Shrub 19853 4x SRS Fragrant Spreaderä (CHEWground) 4 Shrub 200215 2x S* S R Frontenac 3 Shrub 19922 4x SRR Georgetown Tea 1 Tea Unknown6 2x SSS George Vancouver 3 Shrub 19942 4x SRS Grouseä (KORimro) 4 Shrub 198216 2x RRR John Cabot 3 Hybrid kordesii 19782 4x SSS John Davis 3 Hybrid kordesii 19862 3x S* S* R Home RunÒ (WEKcisbako) 4 Shrub 200617 3x RRR Knock OutÒ (RADrazz) 1, 2 Shrub 19995 3x RRR Lena (BAIlena) 3 Shrub 200718 2x SSS Marie Daly 1 Polyantha Unknown6 2x SSS Mme. Antoine Mari 1 Tea 190119 2x SSS Morden Blush 3 Shrub 198820 4x SSS Mutabilis 1 China <18946 2x SSS New Dawn 1, 2 Large-flowered climber 193021 3x SSS* Ole (BAIole) 3 Shrub 200718 2x SSS Paprikaä (CHEWmaytime) 4 Shrub 200615 3x RRR Pariser Charme 5 Floribunda 196522 3x SSv S Peachy Creamä (HORcoherent) 4 Shrub 200323 3x RRR Pearlie Mae 2 Shrub 19813 3x SSS Penelope 2 Hybrid musk 192424 3x SSS Perle d Or 1 Polyantha 188425 2x SS*R Pink Knock OutÒ (RADcon) 2 Shrub 20055 3x RRR Plaisanterieä (LENtrimera) 4 Hybrid musk 199626 2x SSS Polar Joyä (BAIore) 3 Shrub 200427 3x SRR Polonaise 2 Shrub 19843 3x SSS Prairie Breeze 2 Shrub 19783 4x SSS Prairie Harvest 2 Shrub 19853 3x SRS Prairie Joy 3 Shrub 199028 3x SSS Prairie Princess 2 Shrub 19723 4x SSS Princess Verona 2 Shrub 19843 3x SSS Quadra 3 Hybrid kordesii 19942 4x SRR Quietness 2 Shrub 20033 3x SRR Rainbow Knock OutÒ (RADcor) 4 Shrub 20075 3x RRR Ramblin Red 3 Large-flowered climber 20015 4x SRR Rosarium Uetersenä (KORtersen) 2 Large-flowered climber 197716 4x SSS Sea Foam 1, 2, 3 Shrub 196429 3x SSS Spice 1 China Unknown6 2x SSS Square Dancer 2 Shrub 19723 4x SRR Strawberry Crushä (HORmeteorie) 4 Shrub 200823 3x SSS Summer Wind 2, 3 Shrub 19753 4x SSR Sunny Knock OutÒ (RADsunny) 4 Shrub 20085 3x SRS Sunrise Sunsetä (BAIset) 3 Shrub 200427 3x SSS* Sven (BAIsven) 3 Shrub 200718 2x SSS The Fairy 1, 2 Polyantha 193230 2x SSS The Gift 4 Polyantha 198131 2x SSS* William Baffin 3 Hybrid kordesii 19832 4x SRR Winter Sunset 2 Shrub 19973 4x SSR Yellow Submarineä (BAIine) 3 Shrub 200427 4x RRR zS = susceptible; R = resistant; S* = more than one but less than half of the replicates had susceptible reactions. yCultivar name or widely known trademark name is listed followed by registered cultivar name, if different. x1 = Earth-Kind-designated rose for the south–central United States; 2 = Earth-Kind Brigade; 3 = Northern Earth-Kind Rose Trials; 4 = under consideration for future Earth-Kind rose trials; 5 = susceptible control cultivars. wBreeder or discoverer: 1Marie-Louise Meilland; 2Felicitas Svejda; 3Griffith Buck; 4Robert Basye; 5William Radler; 6Unknown; 7David Zlesak; 8Alain Meilland; 9Roy Shepherd; 10E.P. Dering; 11Samuel McGredy IV; 12Jean-Claude Ducher; 13H. Berne`de; 14Svend Poulsen; 15Christopher Warner; 16Reimer Kordes; 17Tom Carruth; 18Kathy Zuzek; 19Antoine Mari; 20Lynn Collicutt; 21Henry Bosenberg; 22Mathias Tantau, Jr.; 23Colin Horner; 24Joseph Pemberton; 25Joseph Rambaux; 26Louis Lens; 27Ping Lim; 28Henry Marshall; 29Ernest Schwartz; 30Ann Bentall; 31Joyce Demits. v‘Parisier Charme’ was used as a susceptible substitute control cultivar in this study for one inoculation over time for Races 3 and 9 as a result of its previously characterized susceptibility to these races (Whitaker et al., 2010b).

HORTSCIENCE VOL. 45(12) DECEMBER 2010 1781 prepared for each cultivar · race combina- Ploidy assessment. Root tip squashes Table 2. Number of cultivars in the Earth-Kind tion over the course of this study. For each were used to isolate cells in metaphase for program (not including controls) in each of the cultivar · race combination assessed at an in- chromosome counts for rose cultivars in eight possible combinations for resistant (R) oculation date, two boxes of leaves were pre- which ploidy level was not already deter- and susceptible (S) reactions for the three races pared. ‘Chorale’ was the universal susceptible mined by this direct method (Zlesak, 2009). of Diplocarpon rosae. control cultivar for all inoculations except for Chromosomes of five or more well-spread No. of Percent 9 Sept. 2008 when it was not available in metaphase cells were observed and counted cultivars cultivars Race 3 Race 8 Race 9 sufficient quantity and ‘Pariser Charme’, also per genotype. Ploidy level was determined by 9 11.1 R R R susceptible to all three races (Whitaker and dividing the number of observed chromo- 9 12.5 S R R 0 0.0 R R S Hokanson, 2009b), was substituted. somes by seven (for roses x = 7). Actively 4 5.6 S R S Conidial concentrations of the inoculum growing root tips were harvested from potted 3 4.2 R S R varied with availability on each date and roses in the greenhouse and stored in vials of 7 9.7 S S R ranged between 30,000 and 100,000 conidia/ water on ice for up to 24 h. Root tips were 0 0.0 R S S mL. These concentrations were within the subsequently fixed in Farmer’s fixative [3:1 41 56.9 S S S range of inoculum concentrations used in (v/v), 95% ethanol: glacial acetic acid] and other studies (Whitaker et al., 2007b, 2010b; refrigerated until the day of observation. Whitaker and Hokanson, 2009a). At each of Water was replaced with 6 N HCl for hydro- formation of acervuli. Some roses had infection the five inoculation dates one susceptible lization of cells for 90 min at room tempera- in only a single or small subset of inoculated control cultivar was used. A limited group ture, just before squashing, and acetocarmine boxes. When this occurred, additional boxes of cultivars in this study were previously was used for staining. beyond the original four (minimum of six and characterized for resistance to these same Statistical analysis. Lesion length data for up to 10 total) were inoculated with the race in races (‘Folksinger’, ‘John Cabot’, ‘Lena’, susceptible cultivars were analyzed indepen- question over additional inoculation dates. If ‘Morden Blush’, ‘Ole’, ‘Sea Foam’, and dently for each race using analysis of vari- leaves in only one box produced a sporulating ‘Sven’) and served as additional, internal ance (ANOVA) to compare factors and to lesion out of the six (or more) boxes inoculated, controls (Hokanson et al., 2007; Whitaker compare infected cultivars for relative partial the cultivar was rated as resistant to that race. If and Hokanson, 2009a; Whitaker et al., resistance. The mean LL was calculated for more than one but less than half of the boxes 2007b). A replicate consisted of two or three each inoculation box and that single value contained infected leaves, the rose was classi- young fully expanded leaves (each composed was used for the analyses. The factors in the fied as susceptible and is designated with an of three to seven leaflets) placed adaxial side ANOVA were: cultivar, time of inoculation, asterisk in Table 1. There were 13 susceptible up on a moistened paper towel within a box and the cultivar · time interaction. Mean rose/race combinations out of the 159 total as described previously. A handheld spray separations were performed using Tukey’s susceptible rose/race combinations using de- bottle was calibrated to deliver 0.75 mL of honestly significant difference (HSD)(P # tached leaf assays in which more than one but inoculum per spray, and each box was sprayed 0.05). ANOVA was also calculated for roses less than half of the boxes had infected leaves. twice (1.5 mL per box). After 48 h, the boxes clearly susceptible to all races (50% or more Typically, susceptible cultivar/race combina- were briefly opened to blot off inoculum boxes were infected) with the following tions resulted in leaves in all boxes being droplets from the leaf surfaces. factors: cultivar, race, time, and their inter- infected. In a rare instance, detached leaves of actions. Pearson’s correlation was calculated The roses that were previously character- Polar Joyä (‘BAIore’) repeatedly deterio- between mean LL (across all infected boxes) ized for their resistance to these three races rated within 2 to 3 d after inoculation. To and mean overall landscape performance rat- displayed race-specific infection patterns in understand the race resistance of this cultivar, ing (Mackay et al., 2008) and mean LL and the this study that were consistent with previous attached leaf inoculations were conducted 2-year mean black spot field defoliation rating results (‘Folksinger’, ‘John Cabot’, ‘Lena’, according to Whitaker et al. (2007b). Young (Colbaugh et al., 2005) for south–central U.S. ‘Morden Blush’, ‘Ole’, ‘Sea Foam’, and ‘Sven’; fully expanded leaves were sprayed with the Earth-Kind winning roses. The landscape Hokanson et al., 2007; Whitaker et al., 2007b; same inoculum preparations as for detached performance data and defoliation data were Whitaker and Hokanson, 2009a). leaf assays. Plants were brought to the labo- taken at a common trial site in Dallas, TX Polar Joyä (‘BAIore’) was unique in that ratory for inoculation so that ambient temper- (Colbaugh et al., 2005; Mackay et al., 2008). its detached leaves consistently deteriorated atures were the same for the attached leaves All statistics were computed using SPSS 12.0 after 2 to 3 d of incubation. Leaves of Polar and the detached leaves. Inoculated leaves of software (SPSS Inc., Chicago, IL). Joyä differed from those of other cultivars in Polar Joyä were individually sealed in plastic that they were noticeably pubescent. Inocu- bags to keep humidity high for 48 h. Bags Results lations of intact leaves were performed as were removed and the plants were brought described in the ‘‘Materials and Methods.’’ back to the greenhouse where resistance or Race-specific resistance was found among General race-specific reactions were recorded susceptibility was noted on the same day as the rose cultivars within the Earth-Kind program to on the same date as the detached leaf assays detached leaves inoculated at the same time. Races3,8,and9ofD. rosae (Table 1). Of the inoculated at the same time and reactions were Disease rating. Ratings were performed eight possible race-specific resistant/susceptible consistent across inoculations (susceptible to 12 to 16 d post-inoculation with the exact classes for the three races, six were repre- Race 3 only). LL data were not gathered for number of days before rating per inoculation sented (Table 2). Nine roses were not infected comparison with other cultivars susceptible to over time dependent on the progression of by any of the three races (Brite Eyesä, Race 3 because of the altered infection and disease. Replicates inoculated on a common ‘Grouse’, Home RunÒ, Knock OutÒ,Paprikaä, incubation environments. date were also rated on a common date. Leaves Peachy Creamä,PinkKnockOutÒ, Rainbow Variation was found for lesion size among with lesions that contained spore-bearing acer- Knock OutÒ, and Yellow Submarineä). susceptible cultivar/race combinations, indi- vuli were rated as susceptible. For susceptible Twelve roses were resistant to two pathogenic cating variability for partial resistance (Tables reactions, the partial resistance component of races and 11 cultivars were resistant to only 3 and 4). Separate ANOVAs were performed LL (see Xue and Davidson, 1998) was evalu- one pathogenic race. The majority of roses for each race (Table 3). Cultivars are ranked ated by measuring the diameter of the largest (n = 41), however, were susceptible to all based on mean lesion size for each race in lesion on each leaf using a digital caliper. three races. Table 4 with significance groups highlighted Leaves bearing lesions with no or few acervuli Determining race-specific resistance of (based on Tukey’s HSD, P # 0.05). Generally, during the first rating were incubated for 2 a limited group of cultivars was challenging roses susceptible to multiple races tended to more days and examined again for the presence as a result of inconsistent infection patterns, have similar rank positions. For instance, Alba of acervuli using a 40· dissecting microscope, which was likely complicated by high par- Meidilandä (‘MEIflopan’) was susceptible to although LL was not reassessed at that time. tial resistance that delayed or prevented the all three races and was among rose cultivars

1782 HORTSCIENCE VOL. 45(12) DECEMBER 2010 Table 3. Mean squares from analysis of variance University in Ames, IA (17 cultivars; marketed reported data), variable relative resistance was for effects resulting from cultivar (susceptible as Buck roses) and by Dr. Felicitas Svedja found across years for many cultivars. only), time of inoculation, and their interaction from Agriculture Canada in Ottawa, Ontario Our data show a high frequency of race- on lesion length calculated for Diplocarpon (seven cultivars; marketed as Explorerä specific resistance. Race-specific resistances rosae Races 3, 8, and 9. roses). Trends were not detected among the in some cultivars (including ‘Folksinger’ and Factors Race 3 Race 8 Race 9 Buck roses for race resistance. There was doc- ‘George Vancouver’, which are included in this Cultivar 2.48** 2.05** 1.98** umented susceptibility and resistance across study) are controlled by single loci (Whitaker Time 6.58** 6.34** 2.36** roses for each of the races and no clear pattern et al., 2010a). Such resistances are not likely to Cultivar · time 1.01** 0.43* 0.81** of susceptibility or resistance based on intro- be durable. Mutations in an avirulence gene in Error 0.53 0.29 0.33 ** duction date. All seven of the Explorerä cul- the pathogen and/or migration of pathogenic Significant at P = 0.01; *significant at P = 0.05. tivars, on the other hand, were susceptible to isolates can soon lead to the loss of such re- Race 3, whereas resistances to Races 8 and 9 sistances. Patterns of genetic diversity of D. within the 15% smallest LL for all three races. were variable and did not appear to be associ- rosae isolates from eastern North America are ‘Belinda’s Dream’, on the other hand, was ated with introduction date. consistent with widespread geographical mix- consistently among rose cultivars with the Chromosome counts revealed that most ing of isolates of D. rosae, possibly through 15% largest LL for all three races. Strikingly, (n = 30) roses were triploid (Table 1). In the transportation of commercially sold roses ‘The Fairy’ and ‘Lena’ ranked first and second addition, half of the 10 commercial classes of (Whitaker et al., 2007a). Using multiple, char- across all three races for smallest LL. roses were represented by multiple ploidy acterized races of D. rosae to challenge land- There were 34 roses susceptible to all levels (Table 6). After triploid, the next most scape roses in controlled settings would be very three races with 50% or more of the boxes prevalent ploidy level was tetraploid and valuable to help ascertain the relative partial containing infected leaves. The mean squares lastly diploid (n = 23 and 20, respectively; resistance of a cultivar once race-specific re- from ANOVA and their significance for the Table 6). One susceptible control was tetra- sistance(s) have been compromised. factors cultivar, race, time of inoculation, and ploid (‘Chorale’) and the other was triploid Obtaining a set of D. rosae isolates that their interactions are presented in Table 5. All (‘Pariser Charme’). can overcome race-specific resistance(s) of factors and interactions were significant at any cultivar (thereby exposing underlying P # 0.01, except for cultivar · race, which Discussion partial resistance) is now possible. In the was significant at P # 0.05, and the race · host–isolate differential array used to establish time and cultivar · race · time interactions, In this study, race-specific resistances were international nomenclature for D. rosae races, which were not significant. discovered that were consistent among repli- none of the 15 roses used as hosts, including the A trend was observed for the prevalence cations and with previous results (Hokanson highly resistant Knock OutÒ rose, had race- of race-specific resistance across the four et al., 2007; Whitaker et al., 2007b; Whitaker specific resistance to all 11 races (Whitaker different groups of cultivars within the Earth- and Hokanson, 2009a). Moreover, differences et al., 2010b). As more races are identified and Kind program. The groups differ based on how in partial resistance were discovered among added to the international race collection, it will long the cultivars have been in the program susceptible genotypes and relative rank was become an even stronger resource for identi- such that the longer they have been in the generally consistent for cultivars susceptible fying cultivar resistance as well as aiding in the program, the fewer race-specific resistance to multiple races. Comparing results from the breeding of durably resistant cultivars. This reactions were observed. Cultivars that have present study with results of previous Earth- current study serves as an initial effort for been in the program the longest are those that Kind field trials indicates that data generated widespread screening of cultivars and uses have earned Earth-Kind designation in the from laboratory-detached leaf assays are sig- characterized North American D. rosae races south–central United States. This group dis- nificantlycorrelatedwithbothfielddefoliation to aid in describing the resistance of roses in played the fewest number of cultivars with as a result of black spot and overall cultivar the North American Earth-Kind trials. Future race-specific resistance (18%) followed by performance ratings. Based on these results, cultivar screens would be possible using the Earth-Kind Brigade (37%), the Northern further use of this technique in the Earth-Kind a subset of races from the international race Earth-Kind Rose Trials (50%), and finally program would be warranted to characterize collection for even greater applicability (i.e., roses entering or being considered for Earth- roses entered in the program and to consider a subset of races can be chosen that collec- Kind trialing (64%). using this assay to pre-screen future entrants tively infects all roses challenged to date). For the 17 south–central U.S. Earth-Kind- for inclusion into the program. Detached leaf assays are a controlled designated roses, overall mean performance Attempting to predict which roses will high-throughput method for characterizing ratings over a 3-year field evaluation trial in possess durable black spot resistance is a resistance, and LL data from these assays Dallas, TX, have been reported (Mackay et al., challenging prospect when relying solely on have been positively correlated with whole 2008) as well as defoliation ratings on the same field observations. Cultivars may display plant inoculation assays (Jenkins, 1955; plants over 2 years (Colbaugh et al., 2005). strong field resistance when first released into Whitaker and Hokanson, 2009a). One of the The landscape performance scale ranged from commerce, leading to their wide distribution. challenges with controlled inoculations on 0to10with10beingbestandthedefoliation Later, however, they may succumb to new or whole plants or detached leaves is that sus- scale ranged from 0 to 5 with 5 being com- migrating pathogenic races, especially when ceptibility can vary based on leaf age (Horst pletely defoliated. The Pearson’s correlation they possess low levels of partial resistance. and Cloyd, 2007). Very young foliage has value between mean LL (averaged over races This scenario has been documented with the greater susceptibility than more mature fo- and boxes) and overall performance rating had roses ‘Baby Love’ and ‘Martin Frobisher’ liage, although in the field, black spot tends to a significant, inverse correlation (r =–0.642; (Bolton and Svejda, 1979; Yokoya et al., appear first on older foliage as a result of P < 0.01) and LL and overall 2-year mean 2000). Consumers become confused and dis- increased humidity within the lower canopy of defoliation rating had a significant, positive appointed when bold marketing claims sur- the plant (Horst and Cloyd, 2007). Although correlation (r = 0.618; P = 0.01). Therefore, rounding new landscape roses are not realized. care was taken to try to standardize leaf age in larger LLs were associated with lower mean In addition, the relative amount of black spot this study by using only recently, fully ex- overall performance ratings and larger LLs infection on susceptible cultivars can change panded leaves, leaf maturity likely introduced were associated with greater plant defoliation. from season to season in a single planting. some variability. Replication of cultivar · race Knock OutÒ was omitted from the correlations Colbaugh et al. (2005) reported defoliation combinations over multiple boxes and over because LL was not available because it was data resulting from black spot for 107 rose time is important to gain a more complete and not infected by any of these three races. cultivars over two growing seasons, and al- accurate understanding of a cultivar’s resis- The breeding programs with the most though the overall Pearson correlation across tance. When considering LL of only roses sus- roses represented in this study include those years of their study was significant and posi- ceptible to all three races, the ANOVA reveals led by the late Dr. Griffith Buck at Iowa State tive (r = 0.627; P < 0.01; calculated from their the main factors (cultivar, race, and time of

HORTSCIENCE VOL. 45(12) DECEMBER 2010 1783 Table 4. Black spot lesion length for rose cultivars susceptible to one or more of Races 3, 8, and 9 of Diplocarpon rosae.

zMeans within column under race connected by a common vertical line do not differ significantly using Tukey’s honestly significant difference (P = 0.05).

1784 HORTSCIENCE VOL. 45(12) DECEMBER 2010 inoculation) and the two-way interactions restricted locale. As such, breeders may be countered. Finding a positive correlation (r = were significant (P # 0.05), except for the selecting strongly for race-specific resistance. 0.618; P = 0.01) between overall mean LL race · time interaction (Table 5). Among other Over longer periods of time and over multiple across races and the 2-year mean defoliation factors, the significance of time of inoculation locations, roses that maintain useful resistance rating of Earth-Kind winning roses for the and its interactions may be the result of leaf in the landscape and earn Earth-Kind desig- south–central United States is very promis- maturity differences. nation seem to primarily be those that possess ing. Comparing the rank of these 16 roses for In addition to the potential for very young strong partial resistance. LL and defoliation (Knock OutÒ was not leaves having atypical susceptibility to a race, In this study, nine roses proved resistant to included because it was not infected in this strong partial resistance seemed to create all three races of D. rosae. The question may study and LL is not available), most cultivars difficulty in classification of some roses for be raised whether these roses possess race- were within only a few positions in rank race susceptibility (Table 1). Whitaker et al. specific resistance or, in a very unlikely across data types. The two most divergent (2007b) reported difficulty characterizing possibility, are universally resistant to all roses were ‘Caldwell Pink’ and ‘Ducher’, race specific versus partial resistance for races. We are making the assumption that which shifted in rank seven positions; ranks ‘Sea Foam’ and attribute this to strong partial roses resistant to all three races possess race- of 2 and 8 in field defoliation to 9 and 15 for resistance. Inconsistent results led to added specific resistance. However, until a single- LL, respectively. As long as cultivars do not inoculations over time and the distinction spore isolate is shown to infect such a rose, change rank widely, characterization of a set within susceptible roses displaying infection race-specific resistance cannot be differenti- of cultivars for partial resistance should still in at least two boxes (Table 1) but less than ated from the very unlikely event of universal be informative, even as the races encountered half of the overall boxes (at least six boxes in resistance. Race-specific resistance has re- in the field or additional laboratory inocula- such incidences were inoculated). cently been documented for three of the nine tions change. Roses enter the Earth-Kind program be- cultivars resistant to Races 3, 8, and 9, Many of the roses that have earned Earth- cause of their reputation for strong performance because infected plants have been identified Kind designations in the south–central among nursery and landscape professionals in landscapes (Brite Eyesä and Home RunÒ, United States were susceptible to all three and rose society members (Harp et al., 2009). unpublished data; Knock OutÒ, Whitaker and races (15 of the 17 roses) and displayed LLs Cultivars are then planted and evaluated in Hokanson, 2009b). Finding resistance-break- that were consistently among the largest replicated, randomized Earth-Kind regional ing pathogen isolates on cultivars character- found (i.e., ‘Belinda’s Dream’ and ‘Climbing field trials and the best performers are awarded ized as resistant to all previously known races Pinkie’). In the field, such cultivars routinely with Earth-Kind regional designation. The is a valuable tool in the identification of new become infected with black spot but consis- goal is to designate any deserving rose with races. Inoculating the remaining six of the tently continue to flower well and retained Earth-Kind status no matter where it was nine rose cultivars with additional character- much of their foliage throughout the growing developed or how long it has been on the ized races from the international collection season. It has been noted that some cultivars market. The trend for rose groups having been (Whitaker et al., 2010b) and looking for defoliated under lower disease pressure than in the Earth-Kind program longer to have a infected plants in landscapes will help to others, an observation that has been con- lower frequency of cultivars with race-specific identify infective isolates and confirm the firmed by showing that LL is not necessarily resistance is worthy of note. In the process of presence of race-specific resistance. related to frequency of defoliation (Whitaker cultivar development, breeders select the Using laboratory assays to gain an un- et al., 2007b). A deficiency of the detached healthiest roses with the least amount of black derstanding of partial resistance of cultivars leaf inoculation method is its inability to spot symptoms and do so typically in a rela- may be the most immediate and predictive measure the defoliation response, which can tively short period of time and in a fairly tool for identifying roses that have greater be influenced by rose cultivar response to potential for durable field resistance to black ethylene and other factors (Whitaker and spot. Finding similar rank order positions for Hokanson, 2009b). Table 5. Mean squares from analysis of variance LL for cultivars susceptible to multiple races ‘Chorale’ and ‘Pariser Charme’ were for effects resulting from cultivar, race, time of is overall very promising. Whitaker et al. selected as controls for this study based on inoculation, and their interactions on lesion (2007b) found that among susceptible culti- susceptibility to all three races and not for length calculated using only roses susceptible to Diplocarpon rosae Races 3, 8, and 9. vars only, the D. rosae isolate · cultivar producing exceptionally large lesions after interaction was not significant for LL but was infection. ‘Chorale’ had consistently smaller Factor Mean square significant for other measures of partial re- LLs than ‘Pariser Charme’, and ‘Pariser Cultivar 4.64** sistance, including leaf area with symptoms Charme’ had the largest LLs of all in this study Race 4.90** and incubation period. In this study, however, for the two races for which it was included Time 7.19** Cultivar · race 0.65* the cultivar · race interaction for LL among as a control (Races 3 and 9) (Table 4). Future Cultivar · time 1.20** cultivars susceptible to all races was signif- work could identify additional control cultivars Race · time 0.70 icant (Table 5). This suggests that relative with known susceptibility that can also serve as Cultivar · race · time 0.40 partial resistance across cultivars may change benchmarks for partial resistance levels. These Error 0.44 to some extent depending on which race (and control cultivars could be used both in labora- **Significant at P # 0.01; * significant at P # 0.05. possibly which isolate within a race) is en- tory inoculations and in the field. ‘Belinda’s Dream’ has earned Earth-Kind designation in the south–central United States and is generally considered at the Table 6. Ploidy levels represented within the different commercial classes of roses challenged in this study (not including controls). threshold of minimum black spot field re- sistance acceptable for a rose to earn Earth- Class No. Diploid No. Triploid No. Tetraploid Kind designation. More work can be done to China 3 correlate LL and a growing body of field Climbing floribunda 1 black spot defoliation data to determine Climbing polyantha 1 Floribunda 1 1 a minimum level of partial resistance based Hybrid kordesii 1 3 on laboratory assays for roses with Earth- Hybrid musk 1 1 Kind potential. Laboratory assays would be Large-flowered climber 1 3 a very helpful tool to objectively eliminate Polyantha 5 those roses that clearly do not possess the Shrub 7 26 15 minimum black spot resistance necessary to Tea 3 earn Earth-Kind designation to better allocate Total cultivars 20 30 23 limited trialing resources. Besides possessing

HORTSCIENCE VOL. 45(12) DECEMBER 2010 1785 a minimum level of black spot resistance, sistant to Race 8, just like the original Knock It is known that ploidy level can affect plant a rose also needs to grow and flower reliably OutÒ (unpublished data). Perhaps the change morphology and fertility levels (Kermani et al., throughout the trialing region. Finding a sig- leading to the color sport in Blushing Knock 2003; Leus, 2005; Rowley, 1960; Shahare and nificant, negative correlation (r = –0.642; P < OutÒ is also associated with a change in re- Shastry, 1963; Zlesak et al., 2005). In addition, 0.01) between our laboratory-based LL data sistance to Race 8. One possibility to explore is changes in ploidy have recently been associ- and overall landscape performance ratings in if Blushing Knock OutÒ is a Layer I periclinal ated with changes in race-specific resistance a 3-year evaluation of the roses that eventu- chimera of Knock OutÒ, which can explain the between an induced tetraploid rose and the ally earned south–central U.S. Earth-Kind common reversion to Knock OutÒ by meristem original diploid (Allum et al., 2010). Triploidy designation highlights the strong impact of layer rearrangement. The epidermal layer in is typically associated with reduced fertility black spot resistance on overall landscape this case (derived from Layer I) may be housing (Leus, 2005; Rowley, 1960), which can aid in performance of roses (Mackay et al., 2008). some changes that have led to a change in reduced fruit set, freeing metabolic resources Another approach to developing durably anthocyanin pigmentation for the lighter color for faster regrowth and rebloom. Triploidy resistant cultivars would be for breeders to in petal epidermal cells and leaf surface appears to offer a nice balance between traits continue to identify race-specific black spot changes leading to Race 8 susceptibility. typically associated with the diploid level resistance genes and pyramid them into single With black spot resistance being a high (faster growth rates and greater branching) cultivars (Whitaker and Hokanson, 2009b). priority in the Explorerä rose breeding pro- and tetraploid level (thicker, larger plant parts) To date, three race-specific resistance genes gram (Svejda, 2008), finding all of the roses (Zlesak, 2009; Zlesak et al., 2005). Document- have been identified (Whitaker et al., 2010a). evaluated in the series susceptible to Race 3 is ing the ploidy levels of roses characterized for To more effectively pursue this approach, it will of note. Uniform susceptibility to Race 3 black spot resistance in this study can aid be necessary to identify more resistance genes across cultivars along with the prioritization breeders in parental selection. Parents can be and develop affordable molecular markers for of black spot resistance during the selection chosen not only for their resistance, but also marker-assisted selection. Although a cultivar process together suggest that an isolate con- fertility and anticipated ploidy level of their may possess multiple race-specific resistances, taining the virulence allele(s) present in Race 3 offspring. the threat still exists for D. rosae isolates to may not have been present at the trial locations Finding significant correlations of LL emerge that possess the necessary combination during the selection of those cultivars. with field black spot ratings as well as overall of virulence alleles to overcome the resistance. A predominance of triploid roses was performance ratings in south–central U.S. The race-specific resistances described here for found in this study (Table 1). It has generally Earth-Kind rose field trials supports the use cultivars to Races 3, 8, and 9 provide a valuable been accepted that most commonly sold mod- of such screens for objectively eliminating starting point from which to begin to charac- ern commercial classes of roses (typically roses that do not meet a minimum level of terize additional race-specific resistance genes. hybrid teas and floribundas) are tetraploid resistance for inclusion in future Earth-Kind Blushing Knock OutÒ is a flower color (Kru¨ssmann, 1981). A high frequency of trip- rose field trials. This large cultivar screen of sport of Knock OutÒ, and finding a difference loidy among popular landscape roses often diverse germplasm for resistance to D. rosae in race-specific resistance to Race 8 between sold as shrub roses has recently been reported races and ploidy also opens doors for future the two opens up research opportunities to by Zlesak (2009), and this study includes research, including the identification of ad- characterize the genetic differences between additional cultivars and helps to confirm this ditional race-specific resistance genes, iden- these two closely allied cultivars (Table 1; trend in landscape roses. Although some tifying new races of D. rosae, gaining insight Fig. 1). Blushing Knock OutÒ routinely breeders may consciously be selecting for to race composition in field trials based on sports back to Knock OutÒ and we isolated triploidy, it is more likely that this is an cultivar infection patterns, selection of par- such a flower color reversion. We challenged unintentional outcome of phenotypic selec- ents for resistance breeding efforts, and the plant of Blushing Knock OutÒ from tion. Triploidy can be associated with traits continued comparisons of laboratory LL data which the reversion was isolated and the favorable in landscape roses, and triploidy and a growing body of field resistance data. Knock OutÒ reversion to Race 8 and con- has been documented to arise from any cross firmed susceptibility of Blushing Knock OutÒ combination among diploid, triploid, and and discovered that the reversion was re- tetraploid parents (Zlesak, 2009). Literature Cited Allum, J.F., D.H. Bringloe, and A.V. Roberts. 2010. Interactions of four pathotypes of Dip- locarpon rosae with species and hybrids of Rosa. Plant Pathol. 59:516–522. Bolton, A.T. and F.J. Svejda. 1979. A new race of Diplocarpon rosae capable of causing severe black spot on hybrids. Can. Plant Dis. Surv. 59:38–40. Colbaugh, P.F., W.T. Crow, W.A. Mackay, and S.W. George. 2005. Rose variety reaction to blackspot disease under minimal input growing conditions, 2001. Biol. and Cultural Tests for Control of Plant Dis. 20:17. Debener, T., R. Drewes-Alvarez, and K. Rockstroh. 1998. Identification of five physiological races of blackspot, Diplocarpon rosae Wolf, on roses. Plant Breed. 117:267–270. Dobbs, R.B. 1984. Research battles blackspot in roses. Am. Rose Annu. 69:44–54. El Mokadem, H., L. Crespel, J. Meynet, S. Gudin, and Y. Jacob. 2001. Gametic behaviour of parthenogenetic plants of Rosa canina L. and R. hybrida L. Acta Hort. 547:289–296. Harp, D.A., D.C. Zlesak, G. Hammond, S. George, and W. Mackay. 2009. Earth-KindÒ rose trials— Identifying the world’s strongest, most beautiful landscape roses. In: Zlesak, D.C. (ed.). Roses. Fig. 1. Representative leaflets displaying response to Races 3 and 8 of Diplocarpon rosae for three roses Floriculture and Ornamental Biotech. 3(Special within the Knock OutÒ series of cultivars. Issue 1):166–175.

1786 HORTSCIENCE VOL. 45(12) DECEMBER 2010 Hattendorf, A., M. Linde, L. Mattiesch, H. Kauf- Rowley, G.D. 1960. Triploid garden roses. Am. fourteen isolates of Diplocarpon rosae (rose mann, and T. Debener. 2004. Genetic. analysis Rose Annu. 45:108–113. blackspot) collected from eastern North Amer- of rose resistance genes and their localization in Schulz, D.F., M. Linde, O. Blechert, and T. ica. Plant Breed. 126:83–88. the rose genome. Acta Hort. 651:123–130. Debener. 2009. Evaluation of genus Rosa Whitaker, V.M. and S.C. Hokanson. 2009a. Partial Hokanson, S.C., K. Zuzek, V.M. Whitaker, and S. germplasm for resistance to black spot, downy resistance to black spot disease in diploid and McNamara. 2007. Three new cold-hardy poly- mildew and powdery mildew. Europ. J. Hort. tetraploid roses: General combining ability antha rose cultivars from the University of Sci. 74:1–9. and implications for breeding and selection. Minnesota woody landscape plant breeding Shahare, M.L. and S.V.S. Shastry. 1963. Meiosis in Euphytica 169:421–429. program. HortScience 42:986–987 (Abstract). garden roses. Chromosoma 13:702–724. Whitaker, V.M. and S.C. Hokanson. 2009b. Breed- Horst, R.K. and R. Cloyd. 2007. Compendium of Svejda, F. 2008. The Canadian explorer roses. ing roses for disease resistance, p. 277–324. In: rose diseases and pests. 2nd Ed. APS Press, St. National-Roses-Canada, London, Canada. Janick, J. (ed.). Plant breed. rev. Vol. 31. John Paul, MN. Von Malek, B. and T. Debener. 1998. Genetic Wiley and Sons, New York. Jenkins, W.R. 1955. Variability of pathogenicity and analysis of resistance to blackspot (Diplocarpon Xue, A.G. and C.G. Davidson. 1998. Components physiology of Diplocarpon rosae Wolf, the rose rosae) in tetraploid roses. Theor. Appl. Genet. of partial resistance to black spot disease blackspot . Am. Rose Annu. 40:92–97. 96:228–231. (Diplocarpon rosae Wolf) in garden roses. Kermani, M.J., V. Sarasan, A.V. Roberts, K. Whitaker, V.M., J. Bradeen, T. Debener, A. Biber, HortScience 33:96–99. Yokoya, J. Wentworth, and V.K. Sieber. 2003. and S.C. Hokanson. 2010a. Rdr3, a novel locus Yokoya, K., K.I. Kandasamy, S. Walker, Z. Oryzalin-induced chromosome doubling in Rosa conferring black spot disease resistance in Mandegaran, and A.V. Roberts. 2000. Resis- and its effect on plant morphology and pollen tetraploid roses: Genetic analysis, LRR pro- tance of roses to pathotypes of Diplocarpon viability. Theor. Appl. Genet. 107:1195–1200. filing, and SCAR marker development. Theor. rosae. Ann. Appl. Biol. 136:15–20. Kru¨ssmann, G. 1981. The complete book of roses. Appl. Genet. 120(3):573–585. Zlesak, D.C. 2006. Rosa · hybrida L., p. 695–738. Timber Press, Portland, OR. Whitaker, V.M., T. Debener, A.V. Roberts, and In: Anderson, N.O. (ed.). Flower breeding and Leus, L. 2005. Resistance breeding for powdery S.C. Hokanson. 2010b. A standard set of host genetics: Issues, challenges, and opportunities mildew (Podosphaera pannosa) and black spot differentials and unified nomenclature for an for the 21st century. Springer, Dordrecht, The (Diplocarpon rosae) in roses. PhD thesis, international collection of Diplocarpon rosae Netherlands. Ghent University, Ghent, Belgium. races. Plant Pathol. 59:745–752. Zlesak, D.C. 2009. Pollen diameter and guard cell Lonnee, D. 2005. A rosier outlook. Northern Whitaker, V.M., J. Bradeen, and S.C. Hokanson. length as predictors of ploidy in diverse rose Gardener 133:36–41. 2007a. Distribution of rose black spot. (Dip- cultivars, species, and breeding lines. In: Zlesak, Mackay, W.A., S.W. George, C. McKenney, J.J. locarpon rosae Wolf) genetic diversity in D.C. (ed.). Roses. Floriculture and Ornamental Sloan, R.I. Cabrera, J.A. Reinert, P. Colbaugh, eastern North America using AFLP and impli- Biotech. 3(Special Issue 1):53–70. L. Lockett, and W. Crow. 2008. Performance of cations for resistance screening. J. Amer. Soc. Zlesak, D.C., C.A. Thill, and N.O. Anderson. 2005. garden roses in north–central Texas under Hort. Sci. 132:534–540. Trifluralin-mediated polyploidization of Rosa minimal input conditions. HortTechnology 18: Whitaker, V.M., K. Zuzek, and S.C. Hokanson. chinensis minima (Sims) Voss seedlings. 417–422. 2007b. Resistance of twelve rose genotypes to Euphytica 141:281–290.

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