HORTSCIENCE 45(12):1779–1787. 2010.
Evaluation of Roses from the
Earth-KindÒ Trials: Black Spot
(Diplocarpon rosae Wolf )
Resistance and Ploidy
David C. Zlesak1
University of Wisconsin–River Falls, 410 S. 3rd Street, River Falls, WI 54022
Vance M. Whitaker
University of Florida Horticultural Sciences, Gulf Coast Research and
Education Center, 14625 CR 672, Wimauma, FL 33598
Steve George
AgriLife Research and Extension, Texas A&M, 17360 Coit Road, Dallas, TX
75252
Stan C. Hokanson
Department of Horticultural Science, University of Minnesota, 1970 Folwell
Avenue, St. Paul, MN 55108
Additional index words. rose chromosome count, rose disease, fungal isolate, Diplocarpon
rosae pathogenic race
Abstract. Regional, replicated cultivar trials of landscape roses are an ongoing component of the Earth-KindÒ program, which was started at Texas A&M University in the
1990s to support environmental landscape stewardship. The rose trials within the EarthKind program identify and promote the most regionally adapted rose cultivars and are
conducted without fertilizers or pesticides and greatly reduced irrigation. Black spot
(caused by Diplocarpon rosae Wolf) is the most serious disease of outdoor-grown roses
worldwide as a result of the potential for rapid leaf yellowing and defoliation. Earth-Kind
designated cultivars for the south–central United States and roses under trial in other
regions or considered for future Earth-Kind trials (n = 73 roses) and two susceptible
control cultivars were challenged with North American Races 3, 8, and 9 of D. rosae,
which were previously characterized at the University of Minnesota. Young expanded
leaves were inoculated using detached leaf assays. Lesion length (LL) was measured for
susceptible reactions and cultivar ploidy was determined using root tip squashes. Diploid,
triploid, and tetraploid cultivars (n = 20, 30, and 23, respectively) were identified, and
race-specific resistances and partial resistances were also identified. Race-specific
resistance was generally more prevalent in newer rose cultivars and rose cultivars more
recently included in Earth-Kind trials. Nine cultivars were resistant to all three races
(Brite Eyesä, ‘Grouse’, Home RunÒ, Knock OutÒ, Paprikaä, Peachy Creamä, Pink
Knock OutÒ, Rainbow Knock OutÒ, and Yellow Submarineä). Blushing Knock OutÒ,
a sport of Knock OutÒ, was susceptible to Race 8. Partial resistance rank for LL was
generally consistent across races for roses susceptible to multiple races. The application
of these data includes: characterizing the minimum resistance level needed for roses to
warrant inclusion in Earth-Kind field trials, the identification of additional race-specific
resistance genes, identifying resistance-breaking isolates of D. rosae, understanding race
composition in field trials based on infection patterns of key cultivars, selection of parents
for resistance breeding efforts, and continued comparisons between LL and growing
bodies of Earth-Kind field resistance data.
Garden roses (Rosa sp.) are among the
most popular flowering shrubs in the world.
Diversity for traits such as form, color, and
fragrance of flowers, plant habit, size, environmental adaptability, and extended season
of flowering all contribute to their widespread
cultivation and versatility (Zlesak, 2006). Lower
maintenance cultivars that can tolerate regional
environmental conditions without routine dependence on pesticides and excessive care
are especially increasing in popularity (Harp
et al., 2009; Lonnee, 2005). Factors fueling
this trend include negative consumer attitude
HORTSCIENCE VOL. 45(12) DECEMBER 2010
toward pesticides, emerging legislation putting greater limits on pesticide availability
and use, busy lifestyles, and greater availability of lower maintenance rose cultivars
(Harp et al., 2009).
Earth-KindÒ Rose Trials are a component
of the overall Earth-Kind program, started at
Texas A&M (the term Earth-KindÒ and associated logo are trademarks of the Texas AgriLife Extension Service, Texas A&M System)
and help to serve the horticulture community
by identifying the most adaptable landscape
roses through regional cultivar trials (Harp
et al., 2009). Pesticides are not used during
the trials and cultural management practices
use techniques that support environmental
stewardship. This includes pre-plant incorporation of compost, maintenance of a 7.6- to
10.2-cm layer of organic mulch, and irrigation
methods that conserve water. Before a rose
can earn regional Earth-Kind designation, it
must exhibit consistent, superior performance
across multiyear and multilocation trials representing different soil types and other environmental conditions typical in the region. A
large number of cultivars are advertised as low
maintenance by nurseries wanting to capitalize on the popularity of lower maintenance
roses. It is difficult for consumers to know
which roses truly possess the highest levels
of pest and environmental tolerances. EarthKind designation gives consumers confidence
that they are choosing roses that will have
a high likelihood of success when basic plant
care is provided.
Disease susceptibility poses a major challenge to roses and limits their success as
low-maintenance landscape shrubs. Fungi
that attack roses include Diplocarpon rosae
(Wolf) (causal agent of black spot), Podosphaera pannosa (Wallr.: Fr.) de Bary (causal
agent of powdery mildew), and Cercospora
puderi B.H. Davis (one of the causal agents
of rose leaf spot) (Horst and Cloyd, 2007). Of
these, black spot is the most serious in the
outdoor landscape across most regions as a result of the potential for rapid disease development that typically leads to leaf yellowing and
defoliation (Dobbs, 1984). Black spot has been
the most prevalent and widespread disease in
the Earth-Kind rose trials (Mackay et al.,
2008). Plants repeatedly defoliated from black
spot become weakened and quickly fall out of
contention for Earth-Kind designation.
Diplocarpon rosae is capable of infecting
only the genus Rosa. Asexual spores (condia)
overwinter on stems and fallen leaves and are
transported to new growth in the spring through
water droplets. If free water remains present,
the conidia form germ tubes that penetrate the
leaf epidermis. Lesions may appear in as little
as 4 d as sub-cuticular mycelia radiate from the
point of infection. Condia-bearing acervuli
burst through the leaf cuticle followed by leaf
abscission in susceptible cultivars (Horst and
Cloyd, 2007).
Multiple studies have been conducted to
characterize the pathogenic race structure of
D. rosae and then to use the characterized
races to identify genes conferring host resistance (Debener et al., 1998; Whitaker et al.,
2007a, 2007b, 2010a; Yokoya et al., 2000).
Isolates are distinguished from one another
based on their differential ability to infect a
common set of rose genotypes. Those isolates
with the same host infection pattern are designated as a race. Collections that have been
preserved for continued research include six
races discovered in Germany (Debener et al.,
1998), four from Great Britain (Yokoya et al.,
2000), and three races from a genetically diverse set of 50 isolates originating in eastern North America (Whitaker et al., 2007a,
2007b). Whitaker et al. (2010b) evaluated
1779
this international collection, identified 11
unique races among them, and standardized
the race nomenclature. These studies and others
have uncovered genetic resistance within rose
species and cultivars that is race-specific. Such
race-specific resistance can only be characterized with controlled inoculations of roses with
individual races as opposed to a field setting
where the presence and prevalence of specific
races are not known. Using characterized races
for inoculations under controlled conditions is
also advantageous when surveying for partial
resistance (Whitaker and Hokanson, 2009a;
Xue and Davidson, 1998).
Detached leaf assays have been an efficient tool to characterize the resistance of
rose seedling populations, and they have been
found to be strongly correlated with whole
plant inoculations (Hattendorf et al., 2004;
Von Malek and Debener, 1998; Whitaker
and Hokanson, 2009a). Detached leaf assays
are preferable as a result of greater ease in
controlling humidity and inoculum levels
(Whitaker and Hokanson, 2009b). To the best
of our knowledge, single-spore isolates of D.
rosae have only been used for race characterization, comparison of pathogenicity of isolates, and to study the segregation of resistance
to particular races in genetic studies and the
characterization of partial resistance components. Single-spore isolates representing different races to the best of our knowledge have
not been used to individually challenge commercial cultivars for widespread race-specific
and horizontal resistance characterization.
Schulz et al. (2009) challenged rose accessions uninfected with black spot in two field
locations with a mixture of single-spore D.
rosae isolates using detached leaf assays.
However, the number of races represented
by these isolates is not known. Recently,
Whitaker and Hokanson (2009a) reported
using detached leaf assays to characterize
the partial resistance to black spot of segregating populations using the measurement of
LL, defined as the diameter of the lesion at its
widest point. LL was chosen by the authors
because of the ease of measurement and its
significant correlations with three other measures of partial resistance (Whitaker et al.,
2007b; Xue and Davidson, 1998).
Received for publication 23 July 2010. Accepted
for publication 9 Oct. 2010.
This research was supported by the Minnesota
Agricultural Experiment Station and grants from
the Minnesota Garden Calendar Fund and the
North Central Region Sustainable Agriculture Research and Education Program.
We thank Michelle Grabowski for her insightful
comments and technical assistance and Brandi
Miatke, Jolyne Pomeroy, and Amber Halberg for
their help caring for plants in the greenhouse and
helping to perform inoculations in the laboratory.
Plants for this research were generously donated or
provided at a reduced cost by Bailey Nurseries,
Chamblee’s Rose Nursery, Sam Kedem Nursery,
Spring Meadow Nursery, and The Conard-Pyle
Company (StarÒ Roses).
1
To whom reprint requests should be addressed;
e-mail zlesak@rocketmail.com.
1780
Using a collection of races to characterize
roses marketed for low-maintenance landscape use would be helpful beyond highlighting the resistance of roses for growers and
consumers. Knowledge of the genetic resistance of these roses would guide the selection
of cultivars that could serve as controls to help
differentiate the presence of known races in
field trials where race composition is not
known. Additionally, when cultivars with resistance to all known races become infected,
pathologists may characterize the infective
isolate(s) in search of new pathogenic races.
A widespread cultivar screen with known
races would also allow breeders to identify
desirable genetic resistances among a core set
of highly resistant cultivars. If knowledge of
resistance could be coupled with knowledge
of rose ploidy level, this would help breeders
develop disease-resistant cultivars more efficiently because of a greater likelihood of
reproductive success. Preferential crossability and fertility as a consequence of ploidy
level has been well documented in roses (El
Mokadem et al., 2001; Leus, 2005; Rowley,
1960; Shahare and Shastry, 1963).
The objectives of this study were to
characterize roses within or being considered
for the Earth-Kind rose program according to
1) resistance to three North American races
of D. rosae using detached leaf assays; and
2) ploidy through direct chromosome counts.
Materials and Methods
Plant material. Seventy-five rose cultivars
were challenged for race-specific and partial resistance and characterized for ploidy
(Table 1). These roses represent 17 EarthKind-designated cultivars for the south–
central United States (the only region currently
where roses have completed the Earth-Kind
trialing process and winners are designated),
30 cultivars under trial in the Earth-Kind
Brigade (mid-U.S. Earth-Kind rose trials), 20
cultivars under trial in the Northern EarthKind Rose Trials (north–central U.S. EarthKind rose trials; Harp et al., 2009), and two
controls. A limited number of cultivars are
represented in multiple Earth-Kind regional
groups such as Carefree Beautyä (‘Bucbi’)
and ‘Sea Foam’, which are in all three groups.
In addition, roses were included that are developing reputations as being highly resistant
to black spot and are being considered for
future Earth-Kind trials (Alba Meidilandä,
Candy Ohä Vivid Red, Carefree Celebrationä,
Carefree Marvelä, Double Knock OutÒ, Fragrant Spreaderä, Home RunÒ, Paprikaä,
Peachy Creamä, Rainbow Knock OutÒ, Strawberry Crushä, and Sunny Knock OutÒ). Also
included are parents or offspring of very resistant roses (‘Grouse’ and ‘Plaisanterie’). The
two susceptible controls were ‘Chorale’ and
‘Pariser Charme’.
At least two potted plants of each cultivar
were acquired from multiple nursery sources
and plants were all propagated on their own
roots (not grafted). Own root plants were
necessary to allow for cultivar ploidy determination using root tip squashes. Potted plants
of cultivars were obtained in the summer of
2007 and maintained on an outdoor gravel
pad. They were transferred to a cold room and
provided with a 4 C vernalization treatment
for 10 weeks before being pruned, repotted
as necessary, and brought to the greenhouse
(23 ± 4.0 C; St. Paul, MN; lat. 45 N) in midJan. 2008. Day extension with supplemental
light was used until 2200 HR (400-W metal
halide lamps; 150 mmolm–2.s–1 at plant
level) to encourage strong, vigorous growth.
Sulfur was burned in the greenhouse from
22:00 to 02:00 HR nightly to discourage
powdery mildew. Sulfur burners were turned
off a minimum of 3 d before leaves were
collected for detached leaf assays.
In late May 2008, a subset of roses needing
further characterization as a result of missing
data or to better understand inconsistent results
was given an 8-week vernalization (4 C)
treatment to reinvigorate growth. After vernalization, roses were cut back, repotted as needed,
and brought to the St. Paul greenhouse in midJuly and grown as described previously. There
was a final, yet smaller, subset of roses forced
for the same reasons. They were provided with
a 12-week vernalization treatment in midOct. 2008 (4 C; the longer vernalization treatment was the result of greenhouse availability
and logistics) and brought to the University
of Minnesota Horticulture Research Center
greenhouse in Chanhassen, MN, in mid-Jan.
2009 and grown under similar conditions as
previously described.
Isolate preparation and inoculations.
Fungal isolates ACT (Race 8; collected from
Brenham, TX), GVH (Race 3; collected from
Hastings, MN), and IGWA (Race 9; collected
from Appleton, WI) described in Whitaker
et al. (2010b) were retrieved from liquid
nitrogen storage. To obtain sufficient inoculum, aqueous suspensions of asexual spores
(conidia) were pipetted onto leaves of susceptible cultivars. Leaves were placed adaxial side up on a folded 15 · 28-cm sheet of
paper towel (BountyÒ; Proctor and Gamble,
Cincinnati, OH) moistened with distilled
water and sealed in an airtight, transparent
ClearpacÒ deli container (22.9 cm length ·
18.7 cm width · 6 cm height; cat. # C48DER;
Dart Corporation, Mason, MI). Containers
(subsequently referred to as boxes) were incubated in the laboratory at room temperature
(21 to 24 C). After the development of sporulating lesions in 10 to 14 d, the infected leaves
were stored in sealed polyethylene bags at –20
C for up to 6 weeks before inoculation. Spores
from the frozen leaves were collected by washing the lesions with distilled, deionized water
and quantified using a hemacytometer.
The two primary inoculation experiments
were performed on 28 Feb. 2008 and 3 Apr.
2008. The third, fourth, and fifth inoculations
were performed on 3 Aug. and 9 Sept. 2008
and 11 Mar. 2009, respectively. These additional inoculations over time were performed
for those host/isolate combinations having
missing data resulting from leaf degradation
or inconsistent results across the first two inoculation dates. A minimum of four inoculation
boxes of leaves and a maximum of 10 were
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
Alba Meidilandä (MEIflopan)
Alexander Mackenzie
Amiga Mia
April Moon
Barn Dance
Belindas Dream
Blushing Knock OutÒ (RADyod)
Brite Eyesä (RADbrit)
Caldwell Pink
Candy Ohä Vivid Red (ZLEmartincipar)
Carefree Beautyä (BUCbi)
Carefree Celebrationä (RADral)
Carefree Marvelä (MEIrameca)
Carefree Wonderä (MEIpitac)
Chorale
Chuckles
Climbing Pinkie
Country Dancer
Double Knock OutÒ (RADtko)
Dublin Bayä (MACdub)
Ducher
Duchesse de Brabant
Earth Song
Else Poulsen
Flora Dora
Folksinger
Fragrant Spreaderä (CHEWground)
Frontenac
Georgetown Tea
George Vancouver
Grouseä (KORimro)
John Cabot
John Davis
Home RunÒ (WEKcisbako)
Knock OutÒ (RADrazz)
Lena (BAIlena)
Marie Daly
Mme. Antoine Mari
Morden Blush
Mutabilis
New Dawn
Ole (BAIole)
Paprikaä (CHEWmaytime)
Pariser Charme
Peachy Creamä (HORcoherent)
Pearlie Mae
Penelope
Perle d Or
Pink Knock OutÒ (RADcon)
Plaisanterieä (LENtrimera)
Polar Joyä (BAIore)
Polonaise
Prairie Breeze
Prairie Harvest
Prairie Joy
Prairie Princess
Princess Verona
Quadra
Quietness
Rainbow Knock OutÒ (RADcor)
Ramblin Red
Rosarium Uetersenä (KORtersen)
Sea Foam
Spice
Square Dancer
Strawberry Crushä (HORmeteorie)
Summer Wind
Sunny Knock OutÒ (RADsunny)
Sunrise Sunsetä (BAIset)
Sven (BAIsven)
The Fairy
The Gift
William Baffin
Winter Sunset
Yellow Submarineä (BAIine)
Categoryx
4
3
2
2
2
1, 2
2
3
1
4
1, 2, 3
4
4
2
5
2
1
2
4
2
1
1
2
1
2
2
4
3
1
3
4
3
3
4
1, 2
3
1
1
3
1
1, 2
3
4
5
4
2
2
1
2
4
3
2
2
2
3
2
2
3
2
4
3
2
1, 2, 3
1
2
4
2, 3
4
3
3
1, 2
4
3
2
3
Commercial class
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Large-flowered climber
Polyantha
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Floribunda
Climbing polyantha
Shrub
Shrub
Climbing floribunda
China
Tea
Shrub
Floribunda
Shrub
Shrub
Shrub
Shrub
Tea
Shrub
Shrub
Hybrid kordesii
Hybrid kordesii
Shrub
Shrub
Shrub
Polyantha
Tea
Shrub
China
Large-flowered climber
Shrub
Shrub
Floribunda
Shrub
Shrub
Hybrid musk
Polyantha
Shrub
Hybrid musk
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Hybrid kordesii
Shrub
Shrub
Large-flowered climber
Large-flowered climber
Shrub
China
Shrub
Shrub
Shrub
Shrub
Shrub
Shrub
Polyantha
Polyantha
Hybrid kordesii
Shrub
Shrub
Yr of introduction and breederw
19851
19852
19783
19843
19753
19924
20045
20065
Unknown6
20087
19773
20085
20038
19908
19783
19589
195210
19733
20045
197511
186912
185713
19753
192414
Unknown6
19853
200215
19922
Unknown6
19942
198216
19782
19862
200617
19995
200718
Unknown6
190119
198820
<18946
193021
200718
200615
196522
200323
19813
192424
188425
20055
199626
200427
19843
19783
19853
199028
19723
19843
19942
20033
20075
20015
197716
196429
Unknown6
19723
200823
19753
20085
200427
200718
193230
198131
19832
19973
200427
Ploidy
2x
3x
4x
3x
3x
3x
3x
4x
2x
2x
4x
3x
3x
4x
4x
4x
2x
4x
3x
4x
2x
2x
4x
3x
3x
4x
2x
4x
2x
4x
2x
4x
3x
3x
3x
2x
2x
2x
4x
2x
3x
2x
3x
3x
3x
3x
3x
2x
3x
2x
3x
3x
4x
3x
3x
4x
3x
4x
3x
3x
4x
4x
3x
2x
4x
3x
4x
3x
3x
2x
2x
2x
4x
4x
4x
Race 3
Sz
S
S
S
R
S
R
R
S
S
S
S
S
S
S
S
S
S
R
S
S
S
S
S
S
S
S*
S
S
S
R
S
S*
R
R
S
S
S
S
S
S
S
R
S
R
S
S
S
R
S
S
S
S
S
S
S
S
S
S
R
S
S
S
S
S
S
S
S
S
S
S
S
S
S
R
Race 8
S*
R
S
S
S
S
S
R
R
S
S
S
S
S
S
S
S
S
S*
S
S
S
S
S
S
R
S
R
S
R
R
S
S*
R
R
S
S
S
S
S
S
S
R
Sv
R
S
S
S*
R
S
R
S
S
R
S
S
S
R
R
R
R
S
S
S
R
S
S
R
S
S
S
S
R
S
R
Race 9
S*
R
S*
S
R
S
R
R
R
R
S
S*
R
S
S
S
S
R
R
S
S
S
S
S
S*
S
R
R
S
S
R
S
R
R
R
S
S
S
S
S
S*
S
R
S
R
S
S
R
R
S
R
S
S
S
S
S
S
R
R
R
R
S
S
S
R
S
R
S
S*
S
S
S*
R
R
R
z
S = susceptible; R = resistant; S* = more than one but less than half of the replicates had susceptible reactions.
Cultivar name or widely known trademark name is listed followed by registered cultivar name, if different.
x
1 = 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.
w
Breeder or discoverer: 1Marie-Louise Meilland; 2Felicitas Svejda; 3Griffith Buck; 4Robert Basye; 5William Radler; 6Unknown; 7David Zlesak; 8Alain Meilland;
9
Roy Shepherd; 10E.P. Dering; 11Samuel McGredy IV; 12Jean-Claude Ducher; 13H. Bernède; 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;
26
Louis 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).
y
HORTSCIENCE VOL. 45(12) DECEMBER 2010
1781
prepared for each cultivar · race combination over the course of this study. For each
cultivar · race combination assessed at an inoculation date, two boxes of leaves were prepared. ‘Chorale’ was the universal susceptible
control cultivar for all inoculations except for
9 Sept. 2008 when it was not available in
sufficient quantity and ‘Pariser Charme’, also
susceptible to all three races (Whitaker and
Hokanson, 2009b), was substituted.
Conidial concentrations of the inoculum
varied with availability on each date and
ranged between 30,000 and 100,000 conidia/
mL. These concentrations were within the
range of inoculum concentrations used in
other studies (Whitaker et al., 2007b, 2010b;
Whitaker and Hokanson, 2009a). At each of
the five inoculation dates one susceptible
control cultivar was used. A limited group
of cultivars in this study were previously
characterized for resistance to these same
races (‘Folksinger’, ‘John Cabot’, ‘Lena’,
‘Morden Blush’, ‘Ole’, ‘Sea Foam’, and
‘Sven’) and served as additional, internal
controls (Hokanson et al., 2007; Whitaker
and Hokanson, 2009a; Whitaker et al.,
2007b). A replicate consisted of two or three
young fully expanded leaves (each composed
of three to seven leaflets) placed adaxial side
up on a moistened paper towel within a box
as described previously. A handheld spray
bottle was calibrated to deliver 0.75 mL of
inoculum per spray, and each box was sprayed
twice (1.5 mL per box). After 48 h, the boxes
were briefly opened to blot off inoculum
droplets from the leaf surfaces.
In a rare instance, detached leaves of
Polar Joyä (‘BAIore’) repeatedly deteriorated within 2 to 3 d after inoculation. To
understand the race resistance of this cultivar,
attached leaf inoculations were conducted
according to Whitaker et al. (2007b). Young
fully expanded leaves were sprayed with the
same inoculum preparations as for detached
leaf assays. Plants were brought to the laboratory for inoculation so that ambient temperatures were the same for the attached leaves
and the detached leaves. Inoculated leaves of
Polar Joyä were individually sealed in plastic
bags to keep humidity high for 48 h. Bags
were removed and the plants were brought
back to the greenhouse where resistance or
susceptibility was noted on the same day as the
detached leaves inoculated at the same time.
Disease rating. Ratings were performed
12 to 16 d post-inoculation with the exact
number of days before rating per inoculation
over time dependent on the progression of
disease. Replicates inoculated on a common
date were also rated on a common date. Leaves
with lesions that contained spore-bearing acervuli were rated as susceptible. For susceptible
reactions, the partial resistance component of
LL (see Xue and Davidson, 1998) was evaluated by measuring the diameter of the largest
lesion on each leaf using a digital caliper.
Leaves bearing lesions with no or few acervuli
during the first rating were incubated for 2
more days and examined again for the presence
of acervuli using a 40· dissecting microscope,
although LL was not reassessed at that time.
1782
Ploidy assessment. Root tip squashes
were used to isolate cells in metaphase for
chromosome counts for rose cultivars in
which ploidy level was not already determined by this direct method (Zlesak, 2009).
Chromosomes of five or more well-spread
metaphase cells were observed and counted
per genotype. Ploidy level was determined by
dividing the number of observed chromosomes by seven (for roses x = 7). Actively
growing root tips were harvested from potted
roses in the greenhouse and stored in vials of
water on ice for up to 24 h. Root tips were
subsequently fixed in Farmer’s fixative [3:1
(v/v), 95% ethanol: glacial acetic acid] and
refrigerated until the day of observation.
Water was replaced with 6 N HCl for hydrolization of cells for 90 min at room temperature, just before squashing, and acetocarmine
was used for staining.
Statistical analysis. Lesion length data for
susceptible cultivars were analyzed independently for each race using analysis of variance (ANOVA) to compare factors and to
compare infected cultivars for relative partial
resistance. The mean LL was calculated for
each inoculation box and that single value
was used for the analyses. The factors in the
ANOVA were: cultivar, time of inoculation,
and the cultivar · time interaction. Mean
separations were performed using Tukey’s
honestly significant difference (HSD) (P #
0.05). ANOVA was also calculated for roses
clearly susceptible to all races (50% or more
boxes were infected) with the following
factors: cultivar, race, time, and their interactions. Pearson’s correlation was calculated
between mean LL (across all infected boxes)
and mean overall landscape performance rating (Mackay et al., 2008) and mean LL and the
2-year mean black spot field defoliation rating
(Colbaugh et al., 2005) for south–central U.S.
Earth-Kind winning roses. The landscape
performance data and defoliation data were
taken at a common trial site in Dallas, TX
(Colbaugh et al., 2005; Mackay et al., 2008).
All statistics were computed using SPSS 12.0
software (SPSS Inc., Chicago, IL).
Results
Race-specific resistance was found among
rose cultivars within the Earth-Kind program to
Races 3, 8, and 9 of D. rosae (Table 1). Of the
eight possible race-specific resistant/susceptible
classes for the three races, six were represented (Table 2). Nine roses were not infected
by any of the three races (Brite Eyesä,
‘Grouse’, Home RunÒ, Knock OutÒ, Paprikaä,
Peachy Creamä, Pink Knock OutÒ, Rainbow
Knock OutÒ, and Yellow Submarineä).
Twelve roses were resistant to two pathogenic
races and 11 cultivars were resistant to only
one pathogenic race. The majority of roses
(n = 41), however, were susceptible to all
three races.
Determining race-specific resistance of
a limited group of cultivars was challenging
as a result of inconsistent infection patterns,
which was likely complicated by high partial resistance that delayed or prevented the
Table 2. Number of cultivars in the Earth-Kind
program (not including controls) in each of the
eight possible combinations for resistant (R)
and susceptible (S) reactions for the three races
of Diplocarpon rosae.
No. of
cultivars
9
9
0
4
3
7
0
41
Percent
cultivars
11.1
12.5
0.0
5.6
4.2
9.7
0.0
56.9
Race 3
R
S
R
S
R
S
R
S
Race 8
R
R
R
R
S
S
S
S
Race 9
R
R
S
S
R
R
S
S
formation of acervuli. Some roses had infection
in only a single or small subset of inoculated
boxes. When this occurred, additional boxes
beyond the original four (minimum of six and
up to 10 total) were inoculated with the race in
question over additional inoculation dates. If
leaves in only one box produced a sporulating
lesion out of the six (or more) boxes inoculated,
the cultivar was rated as resistant to that race. If
more than one but less than half of the boxes
contained infected leaves, the rose was classified as susceptible and is designated with an
asterisk in Table 1. There were 13 susceptible
rose/race combinations out of the 159 total
susceptible rose/race combinations using detached leaf assays in which more than one but
less than half of the boxes had infected leaves.
Typically, susceptible cultivar/race combinations resulted in leaves in all boxes being
infected.
The roses that were previously characterized for their resistance to these three races
displayed race-specific infection patterns in
this study that were consistent with previous
results (‘Folksinger’, ‘John Cabot’, ‘Lena’,
‘Morden Blush’, ‘Ole’, ‘Sea Foam’, and ‘Sven’;
Hokanson et al., 2007; Whitaker et al., 2007b;
Whitaker and Hokanson, 2009a).
Polar Joyä (‘BAIore’) was unique in that
its detached leaves consistently deteriorated
after 2 to 3 d of incubation. Leaves of Polar
Joyä differed from those of other cultivars in
that they were noticeably pubescent. Inoculations of intact leaves were performed as
described in the ‘‘Materials and Methods.’’
General race-specific reactions were recorded
on the same date as the detached leaf assays
inoculated at the same time and reactions were
consistent across inoculations (susceptible to
Race 3 only). LL data were not gathered for
comparison with other cultivars susceptible to
Race 3 because of the altered infection and
incubation environments.
Variation was found for lesion size among
susceptible cultivar/race combinations, indicating variability for partial resistance (Tables
3 and 4). Separate ANOVAs were performed
for each race (Table 3). Cultivars are ranked
based on mean lesion size for each race in
Table 4 with significance groups highlighted
(based on Tukey’s HSD, P # 0.05). Generally,
roses susceptible to multiple races tended to
have similar rank positions. For instance, Alba
Meidilandä (‘MEIflopan’) was susceptible to
all three races and was among rose cultivars
HORTSCIENCE VOL. 45(12) DECEMBER 2010
Table 3. Mean squares from analysis of variance
for effects resulting from cultivar (susceptible
only), time of inoculation, and their interaction
on lesion length calculated for Diplocarpon
rosae Races 3, 8, and 9.
Factors
Race 3
Race 8
Race 9
Cultivar
2.48**
2.05**
1.98**
Time
6.58**
6.34**
2.36**
Cultivar · time
1.01**
0.43*
0.81**
Error
0.53
0.29
0.33
**
Significant at P = 0.01; *significant at P = 0.05.
within the 15% smallest LL for all three races.
‘Belinda’s Dream’, on the other hand, was
consistently among rose cultivars with the
15% largest LL for all three races. Strikingly,
‘The Fairy’ and ‘Lena’ ranked first and second
across all three races for smallest LL.
There were 34 roses susceptible to all
three races with 50% or more of the boxes
containing infected leaves. The mean squares
from ANOVA and their significance for the
factors cultivar, race, time of inoculation, and
their interactions are presented in Table 5. All
factors and interactions were significant at
P # 0.01, except for cultivar · race, which
was significant at P # 0.05, and the race ·
time and cultivar · race · time interactions,
which were not significant.
A trend was observed for the prevalence
of race-specific resistance across the four
different groups of cultivars within the EarthKind program. The groups differ based on how
long the cultivars have been in the program
such that the longer they have been in the
program, the fewer race-specific resistance
reactions were observed. Cultivars that have
been in the program the longest are those that
have earned Earth-Kind designation in the
south–central United States. This group displayed the fewest number of cultivars with
race-specific resistance (18%) followed by
the Earth-Kind Brigade (37%), the Northern
Earth-Kind Rose Trials (50%), and finally
roses entering or being considered for EarthKind trialing (64%).
For the 17 south–central U.S. Earth-Kinddesignated roses, overall mean performance
ratings over a 3-year field evaluation trial in
Dallas, TX, have been reported (Mackay et al.,
2008) as well as defoliation ratings on the same
plants over 2 years (Colbaugh et al., 2005).
The landscape performance scale ranged from
0 to 10 with 10 being best and the defoliation
scale ranged from 0 to 5 with 5 being completely defoliated. The Pearson’s correlation
value between mean LL (averaged over races
and boxes) and overall performance rating had
a significant, inverse correlation (r = –0.642;
P < 0.01) and LL and overall 2-year mean
defoliation rating had a significant, positive
correlation (r = 0.618; P = 0.01). Therefore,
larger LLs were associated with lower mean
overall performance ratings and larger LLs
were associated with greater plant defoliation.
Knock OutÒ was omitted from the correlations
because LL was not available because it was
not infected by any of these three races.
The breeding programs with the most
roses represented in this study include those
led by the late Dr. Griffith Buck at Iowa State
HORTSCIENCE VOL. 45(12) DECEMBER 2010
University in Ames, IA (17 cultivars; marketed
as Buck roses) and by Dr. Felicitas Svedja
from Agriculture Canada in Ottawa, Ontario
(seven cultivars; marketed as Explorerä
roses). Trends were not detected among the
Buck roses for race resistance. There was documented susceptibility and resistance across
roses for each of the races and no clear pattern
of susceptibility or resistance based on introduction date. All seven of the Explorerä cultivars, on the other hand, were susceptible to
Race 3, whereas resistances to Races 8 and 9
were variable and did not appear to be associated with introduction date.
Chromosome counts revealed that most
(n = 30) roses were triploid (Table 1). In
addition, half of the 10 commercial classes of
roses were represented by multiple ploidy
levels (Table 6). After triploid, the next most
prevalent ploidy level was tetraploid and
lastly diploid (n = 23 and 20, respectively;
Table 6). One susceptible control was tetraploid (‘Chorale’) and the other was triploid
(‘Pariser Charme’).
Discussion
In this study, race-specific resistances were
discovered that were consistent among replications and with previous results (Hokanson
et al., 2007; Whitaker et al., 2007b; Whitaker
and Hokanson, 2009a). Moreover, differences
in partial resistance were discovered among
susceptible genotypes and relative rank was
generally consistent for cultivars susceptible
to multiple races. Comparing results from the
present study with results of previous EarthKind field trials indicates that data generated
from laboratory-detached leaf assays are significantly correlated with both field defoliation
as a result of black spot and overall cultivar
performance ratings. Based on these results,
further use of this technique in the Earth-Kind
program would be warranted to characterize
roses entered in the program and to consider
using this assay to pre-screen future entrants
for inclusion into the program.
Attempting to predict which roses will
possess durable black spot resistance is a
challenging prospect when relying solely on
field observations. Cultivars may display
strong field resistance when first released into
commerce, leading to their wide distribution.
Later, however, they may succumb to new or
migrating pathogenic races, especially when
they possess low levels of partial resistance.
This scenario has been documented with the
roses ‘Baby Love’ and ‘Martin Frobisher’
(Bolton and Svejda, 1979; Yokoya et al.,
2000). Consumers become confused and disappointed when bold marketing claims surrounding new landscape roses are not realized.
In addition, the relative amount of black spot
infection on susceptible cultivars can change
from season to season in a single planting.
Colbaugh et al. (2005) reported defoliation
data resulting from black spot for 107 rose
cultivars over two growing seasons, and although the overall Pearson correlation across
years of their study was significant and positive (r = 0.627; P < 0.01; calculated from their
reported data), variable relative resistance was
found across years for many cultivars.
Our data show a high frequency of racespecific resistance. Race-specific resistances
in some cultivars (including ‘Folksinger’ and
‘George Vancouver’, which are included in this
study) are controlled by single loci (Whitaker
et al., 2010a). Such resistances are not likely to
be durable. Mutations in an avirulence gene in
the pathogen and/or migration of pathogenic
isolates can soon lead to the loss of such resistances. Patterns of genetic diversity of D.
rosae isolates from eastern North America are
consistent with widespread geographical mixing of isolates of D. rosae, possibly through
the transportation of commercially sold roses
(Whitaker et al., 2007a). Using multiple, characterized races of D. rosae to challenge landscape roses in controlled settings would be very
valuable to help ascertain the relative partial
resistance of a cultivar once race-specific resistance(s) have been compromised.
Obtaining a set of D. rosae isolates that
can overcome race-specific resistance(s) of
any cultivar (thereby exposing underlying
partial resistance) is now possible. In the
host–isolate differential array used to establish
international nomenclature for D. rosae races,
none of the 15 roses used as hosts, including the
highly resistant Knock OutÒ rose, had racespecific resistance to all 11 races (Whitaker
et al., 2010b). As more races are identified and
added to the international race collection, it will
become an even stronger resource for identifying cultivar resistance as well as aiding in the
breeding of durably resistant cultivars. This
current study serves as an initial effort for
widespread screening of cultivars and uses
characterized North American D. rosae races
to aid in describing the resistance of roses in
the North American Earth-Kind trials. Future
cultivar screens would be possible using
a subset of races from the international race
collection for even greater applicability (i.e.,
a subset of races can be chosen that collectively infects all roses challenged to date).
Detached leaf assays are a controlled
high-throughput method for characterizing
resistance, and LL data from these assays
have been positively correlated with whole
plant inoculation assays (Jenkins, 1955;
Whitaker and Hokanson, 2009a). One of the
challenges with controlled inoculations on
whole plants or detached leaves is that susceptibility can vary based on leaf age (Horst
and Cloyd, 2007). Very young foliage has
greater susceptibility than more mature foliage, although in the field, black spot tends to
appear first on older foliage as a result of
increased humidity within the lower canopy of
the plant (Horst and Cloyd, 2007). Although
care was taken to try to standardize leaf age in
this study by using only recently, fully expanded leaves, leaf maturity likely introduced
some variability. Replication of cultivar · race
combinations over multiple boxes and over
time is important to gain a more complete and
accurate understanding of a cultivar’s resistance. When considering LL of only roses susceptible to all three races, the ANOVA reveals
the main factors (cultivar, race, and time of
1783
Table 4. Black spot lesion length for rose cultivars susceptible to one or more of Races 3, 8, and 9 of Diplocarpon rosae.
z
Means 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
were significant (P # 0.05), except for the
race · time interaction (Table 5). Among other
factors, the significance of time of inoculation
and its interactions may be the result of leaf
maturity differences.
In addition to the potential for very young
leaves having atypical susceptibility to a race,
strong partial resistance seemed to create
difficulty in classification of some roses for
race susceptibility (Table 1). Whitaker et al.
(2007b) reported difficulty characterizing
race specific versus partial resistance for
‘Sea Foam’ and attribute this to strong partial
resistance. Inconsistent results led to added
inoculations over time and the distinction
within susceptible roses displaying infection
in at least two boxes (Table 1) but less than
half of the overall boxes (at least six boxes in
such incidences were inoculated).
Roses enter the Earth-Kind program because of their reputation for strong performance
among nursery and landscape professionals
and rose society members (Harp et al., 2009).
Cultivars are then planted and evaluated in
replicated, randomized Earth-Kind regional
field trials and the best performers are awarded
with Earth-Kind regional designation. The
goal is to designate any deserving rose with
Earth-Kind status no matter where it was
developed or how long it has been on the
market. The trend for rose groups having been
in the Earth-Kind program longer to have a
lower frequency of cultivars with race-specific
resistance is worthy of note. In the process of
cultivar development, breeders select the
healthiest roses with the least amount of black
spot symptoms and do so typically in a relatively short period of time and in a fairly
Table 5. Mean squares from analysis of variance
for effects resulting from cultivar, race, time of
inoculation, and their interactions on lesion
length calculated using only roses susceptible
to Diplocarpon rosae Races 3, 8, and 9.
Factor
Mean square
Cultivar
4.64**
Race
4.90**
Time
7.19**
Cultivar · race
0.65*
Cultivar · time
1.20**
Race · time
0.70
Cultivar · race · time
0.40
Error
0.44
**
Significant at P # 0.01; * significant at P # 0.05.
restricted locale. As such, breeders may be
selecting strongly for race-specific resistance.
Over longer periods of time and over multiple
locations, roses that maintain useful resistance
in the landscape and earn Earth-Kind designation seem to primarily be those that possess
strong partial resistance.
In this study, nine roses proved resistant to
all three races of D. rosae. The question may
be raised whether these roses possess racespecific resistance or, in a very unlikely
possibility, are universally resistant to all
races. We are making the assumption that
roses resistant to all three races possess racespecific resistance. However, until a singlespore isolate is shown to infect such a rose,
race-specific resistance cannot be differentiated from the very unlikely event of universal
resistance. Race-specific resistance has recently been documented for three of the nine
cultivars resistant to Races 3, 8, and 9,
because infected plants have been identified
in landscapes (Brite Eyesä and Home RunÒ,
unpublished data; Knock OutÒ, Whitaker and
Hokanson, 2009b). Finding resistance-breaking pathogen isolates on cultivars characterized as resistant to all previously known races
is a valuable tool in the identification of new
races. Inoculating the remaining six of the
nine rose cultivars with additional characterized races from the international collection
(Whitaker et al., 2010b) and looking for
infected plants in landscapes will help to
identify infective isolates and confirm the
presence of race-specific resistance.
Using laboratory assays to gain an understanding of partial resistance of cultivars
may be the most immediate and predictive
tool for identifying roses that have greater
potential for durable field resistance to black
spot. Finding similar rank order positions for
LL for cultivars susceptible to multiple races
is overall very promising. Whitaker et al.
(2007b) found that among susceptible cultivars only, the D. rosae isolate · cultivar
interaction was not significant for LL but was
significant for other measures of partial resistance, including leaf area with symptoms
and incubation period. In this study, however,
the cultivar · race interaction for LL among
cultivars susceptible to all races was significant (Table 5). This suggests that relative
partial resistance across cultivars may change
to some extent depending on which race (and
possibly which isolate within a race) is en-
Table 6. Ploidy levels represented within the different commercial classes of roses challenged in this study
(not including controls).
Class
China
Climbing floribunda
Climbing polyantha
Floribunda
Hybrid kordesii
Hybrid musk
Large-flowered climber
Polyantha
Shrub
Tea
Total cultivars
No. Diploid
3
No. Triploid
No. Tetraploid
1
1
1
5
7
3
20
HORTSCIENCE VOL. 45(12) DECEMBER 2010
1
1
1
1
1
3
3
26
15
30
23
countered. Finding a positive correlation (r =
0.618; P = 0.01) between overall mean LL
across races and the 2-year mean defoliation
rating of Earth-Kind winning roses for the
south–central United States is very promising. Comparing the rank of these 16 roses for
LL and defoliation (Knock OutÒ was not
included because it was not infected in this
study and LL is not available), most cultivars
were within only a few positions in rank
across data types. The two most divergent
roses were ‘Caldwell Pink’ and ‘Ducher’,
which shifted in rank seven positions; ranks
of 2 and 8 in field defoliation to 9 and 15 for
LL, respectively. As long as cultivars do not
change rank widely, characterization of a set
of cultivars for partial resistance should still
be informative, even as the races encountered
in the field or additional laboratory inoculations change.
Many of the roses that have earned EarthKind designations in the south–central
United States were susceptible to all three
races (15 of the 17 roses) and displayed LLs
that were consistently among the largest
found (i.e., ‘Belinda’s Dream’ and ‘Climbing
Pinkie’). In the field, such cultivars routinely
become infected with black spot but consistently continue to flower well and retained
much of their foliage throughout the growing
season. It has been noted that some cultivars
defoliated under lower disease pressure than
others, an observation that has been confirmed by showing that LL is not necessarily
related to frequency of defoliation (Whitaker
et al., 2007b). A deficiency of the detached
leaf inoculation method is its inability to
measure the defoliation response, which can
be influenced by rose cultivar response to
ethylene and other factors (Whitaker and
Hokanson, 2009b).
‘Chorale’ and ‘Pariser Charme’ were
selected as controls for this study based on
susceptibility to all three races and not for
producing exceptionally large lesions after
infection. ‘Chorale’ had consistently smaller
LLs than ‘Pariser Charme’, and ‘Pariser
Charme’ had the largest LLs of all in this study
for the two races for which it was included
as a control (Races 3 and 9) (Table 4). Future
work could identify additional control cultivars
with known susceptibility that can also serve as
benchmarks for partial resistance levels. These
control cultivars could be used both in laboratory 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
threshold of minimum black spot field resistance acceptable for a rose to earn EarthKind designation. More work can be done to
correlate LL and a growing body of field
black spot defoliation data to determine
a minimum level of partial resistance based
on laboratory assays for roses with EarthKind potential. Laboratory assays would be
a very helpful tool to objectively eliminate
those roses that clearly do not possess the
minimum black spot resistance necessary to
earn Earth-Kind designation to better allocate
limited trialing resources. Besides possessing
1785
a minimum level of black spot resistance,
a rose also needs to grow and flower reliably
throughout the trialing region. Finding a significant, negative correlation (r = –0.642; P <
0.01) between our laboratory-based LL data
and overall landscape performance ratings in
a 3-year evaluation of the roses that eventually earned south–central U.S. Earth-Kind
designation highlights the strong impact of
black spot resistance on overall landscape
performance of roses (Mackay et al., 2008).
Another approach to developing durably
resistant cultivars would be for breeders to
continue to identify race-specific black spot
resistance genes and pyramid them into single
cultivars (Whitaker and Hokanson, 2009b).
To date, three race-specific resistance genes
have been identified (Whitaker et al., 2010a).
To more effectively pursue this approach, it will
be necessary to identify more resistance genes
and develop affordable molecular markers for
marker-assisted selection. Although a cultivar
may possess multiple race-specific resistances,
the threat still exists for D. rosae isolates to
emerge that possess the necessary combination
of virulence alleles to overcome the resistance.
The race-specific resistances described here for
cultivars to Races 3, 8, and 9 provide a valuable
starting point from which to begin to characterize additional race-specific resistance genes.
Blushing Knock OutÒ is a flower color
sport of Knock OutÒ, and finding a difference
in race-specific resistance to Race 8 between
the two opens up research opportunities to
characterize the genetic differences between
these two closely allied cultivars (Table 1;
Fig. 1). Blushing Knock OutÒ routinely
sports back to Knock OutÒ and we isolated
such a flower color reversion. We challenged
the plant of Blushing Knock OutÒ from
which the reversion was isolated and the
Knock OutÒ reversion to Race 8 and confirmed susceptibility of Blushing Knock OutÒ
and discovered that the reversion was re-
sistant to Race 8, just like the original Knock
OutÒ (unpublished data). Perhaps the change
leading to the color sport in Blushing Knock
OutÒ is also associated with a change in resistance to Race 8. One possibility to explore is
if Blushing Knock OutÒ is a Layer I periclinal
chimera of Knock OutÒ, which can explain the
common reversion to Knock OutÒ by meristem
layer rearrangement. The epidermal layer in
this case (derived from Layer I) may be housing
some changes that have led to a change in
anthocyanin pigmentation for the lighter color
in petal epidermal cells and leaf surface
changes leading to Race 8 susceptibility.
With black spot resistance being a high
priority in the Explorerä rose breeding program (Svejda, 2008), finding all of the roses
evaluated in the series susceptible to Race 3 is
of note. Uniform susceptibility to Race 3
across cultivars along with the prioritization
of black spot resistance during the selection
process together suggest that an isolate containing the virulence allele(s) present in Race 3
may not have been present at the trial locations
during the selection of those cultivars.
A predominance of triploid roses was
found in this study (Table 1). It has generally
been accepted that most commonly sold modern commercial classes of roses (typically
hybrid teas and floribundas) are tetraploid
(Krüssmann, 1981). A high frequency of triploidy among popular landscape roses often
sold as shrub roses has recently been reported
by Zlesak (2009), and this study includes
additional cultivars and helps to confirm this
trend in landscape roses. Although some
breeders may consciously be selecting for
triploidy, it is more likely that this is an
unintentional outcome of phenotypic selection. Triploidy can be associated with traits
favorable in landscape roses, and triploidy
has been documented to arise from any cross
combination among diploid, triploid, and
tetraploid parents (Zlesak, 2009).
Fig. 1. Representative leaflets displaying response to Races 3 and 8 of Diplocarpon rosae for three roses
within the Knock OutÒ series of cultivars.
1786
It is known that ploidy level can affect plant
morphology and fertility levels (Kermani et al.,
2003; Leus, 2005; Rowley, 1960; Shahare and
Shastry, 1963; Zlesak et al., 2005). In addition,
changes in ploidy have recently been associated with changes in race-specific resistance
between an induced tetraploid rose and the
original diploid (Allum et al., 2010). Triploidy
is typically associated with reduced fertility
(Leus, 2005; Rowley, 1960), which can aid in
reduced fruit set, freeing metabolic resources
for faster regrowth and rebloom. Triploidy
appears to offer a nice balance between traits
typically associated with the diploid level
(faster growth rates and greater branching)
and tetraploid level (thicker, larger plant parts)
(Zlesak, 2009; Zlesak et al., 2005). Documenting the ploidy levels of roses characterized for
black spot resistance in this study can aid
breeders in parental selection. Parents can be
chosen not only for their resistance, but also
fertility and anticipated ploidy level of their
offspring.
Finding significant correlations of LL
with field black spot ratings as well as overall
performance ratings in south–central U.S.
Earth-Kind rose field trials supports the use
of such screens for objectively eliminating
roses that do not meet a minimum level of
resistance for inclusion in future Earth-Kind
rose field trials. This large cultivar screen of
diverse germplasm for resistance to D. rosae
races and ploidy also opens doors for future
research, including the identification of additional race-specific resistance genes, identifying new races of D. rosae, gaining insight
to race composition in field trials based on
cultivar infection patterns, selection of parents for resistance breeding efforts, and
continued comparisons of laboratory LL data
and a growing body of field resistance data.
Literature Cited
Allum, J.F., D.H. Bringloe, and A.V. Roberts.
2010. Interactions of four pathotypes of Diplocarpon 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 Rosa rugosa 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.
Floriculture and Ornamental Biotech. 3(Special
Issue 1):166–175.
HORTSCIENCE VOL. 45(12) DECEMBER 2010
Hattendorf, A., M. Linde, L. Mattiesch, H. Kaufmann, and T. Debener. 2004. Genetic. analysis
of rose resistance genes and their localization in
the rose genome. Acta Hort. 651:123–130.
Hokanson, S.C., K. Zuzek, V.M. Whitaker, and S.
McNamara. 2007. Three new cold-hardy polyantha rose cultivars from the University of
Minnesota woody landscape plant breeding
program. HortScience 42:986–987 (Abstract).
Horst, R.K. and R. Cloyd. 2007. Compendium of
rose diseases and pests. 2nd Ed. APS Press, St.
Paul, MN.
Jenkins, W.R. 1955. Variability of pathogenicity and
physiology of Diplocarpon rosae Wolf, the rose
blackspot fungus. Am. Rose Annu. 40:92–97.
Kermani, M.J., V. Sarasan, A.V. Roberts, K.
Yokoya, J. Wentworth, and V.K. Sieber. 2003.
Oryzalin-induced chromosome doubling in Rosa
and its effect on plant morphology and pollen
viability. Theor. Appl. Genet. 107:1195–1200.
Krüssmann, G. 1981. The complete book of roses.
Timber Press, Portland, OR.
Leus, L. 2005. Resistance breeding for powdery
mildew (Podosphaera pannosa) and black spot
(Diplocarpon rosae) in roses. PhD thesis,
Ghent University, Ghent, Belgium.
Lonnee, D. 2005. A rosier outlook. Northern
Gardener 133:36–41.
Mackay, W.A., S.W. George, C. McKenney, J.J.
Sloan, R.I. Cabrera, J.A. Reinert, P. Colbaugh,
L. Lockett, and W. Crow. 2008. Performance of
garden roses in north–central Texas under
minimal input conditions. HortTechnology 18:
417–422.
HORTSCIENCE VOL. 45(12) DECEMBER 2010
Rowley, G.D. 1960. Triploid garden roses. Am.
Rose Annu. 45:108–113.
Schulz, D.F., M. Linde, O. Blechert, and T.
Debener. 2009. Evaluation of genus Rosa
germplasm for resistance to black spot, downy
mildew and powdery mildew. Europ. J. Hort.
Sci. 74:1–9.
Shahare, M.L. and S.V.S. Shastry. 1963. Meiosis in
garden roses. Chromosoma 13:702–724.
Svejda, F. 2008. The Canadian explorer roses.
National-Roses-Canada, London, Canada.
Von Malek, B. and T. Debener. 1998. Genetic
analysis of resistance to blackspot (Diplocarpon
rosae) in tetraploid roses. Theor. Appl. Genet.
96:228–231.
Whitaker, V.M., J. Bradeen, T. Debener, A. Biber,
and S.C. Hokanson. 2010a. Rdr3, a novel locus
conferring black spot disease resistance in
tetraploid roses: Genetic analysis, LRR profiling, and SCAR marker development. Theor.
Appl. Genet. 120(3):573–585.
Whitaker, V.M., T. Debener, A.V. Roberts, and
S.C. Hokanson. 2010b. A standard set of host
differentials and unified nomenclature for an
international collection of Diplocarpon rosae
races. Plant Pathol. 59:745–752.
Whitaker, V.M., J. Bradeen, and S.C. Hokanson.
2007a. Distribution of rose black spot. (Diplocarpon rosae Wolf) genetic diversity in
eastern North America using AFLP and implications for resistance screening. J. Amer. Soc.
Hort. Sci. 132:534–540.
Whitaker, V.M., K. Zuzek, and S.C. Hokanson.
2007b. Resistance of twelve rose genotypes to
fourteen isolates of Diplocarpon rosae (rose
blackspot) collected from eastern North America. Plant Breed. 126:83–88.
Whitaker, V.M. and S.C. Hokanson. 2009a. Partial
resistance to black spot disease in diploid and
tetraploid roses: General combining ability
and implications for breeding and selection.
Euphytica 169:421–429.
Whitaker, V.M. and S.C. Hokanson. 2009b. Breeding roses for disease resistance, p. 277–324. In:
Janick, J. (ed.). Plant breed. rev. Vol. 31. John
Wiley and Sons, New York.
Xue, A.G. and C.G. Davidson. 1998. Components
of partial resistance to black spot disease
(Diplocarpon rosae Wolf) in garden roses.
HortScience 33:96–99.
Yokoya, K., K.I. Kandasamy, S. Walker, Z.
Mandegaran, and A.V. Roberts. 2000. Resistance of roses to pathotypes of Diplocarpon
rosae. Ann. Appl. Biol. 136:15–20.
Zlesak, D.C. 2006. Rosa · hybrida L., p. 695–738.
In: Anderson, N.O. (ed.). Flower breeding and
genetics: Issues, challenges, and opportunities
for the 21st century. Springer, Dordrecht, The
Netherlands.
Zlesak, D.C. 2009. Pollen diameter and guard cell
length as predictors of ploidy in diverse rose
cultivars, species, and breeding lines. In: Zlesak,
D.C. (ed.). Roses. Floriculture and Ornamental
Biotech. 3(Special Issue 1):53–70.
Zlesak, D.C., C.A. Thill, and N.O. Anderson. 2005.
Trifluralin-mediated polyploidization of Rosa
chinensis minima (Sims) Voss seedlings.
Euphytica 141:281–290.
1787
