Blackspot info

jim1961 Zone 6a Central Pa.August 6, 2014

I consider Bill Radler to be tops with respect to the
roses he produces. He is extremely serious about screening for
disease resistance in his seedlings, going to the extreme of actually
collecting blackspot spores and inoculating his seedlings
with them in an effort to select for the best health possible. The
levels he takes his screening to extend far beyond what most
of us would consider. His roses stand out in testing with dense
foliage and vibrant good health during the establishment year
and two summers of formal testing.

Breeding for black spot resistance is a high priority for
many of us. Public demand for low-maintenance roses
has increased. Fewer people are willing to expose themselves
and their families to pesticides for the sake of having
healthy roses. Chemically-dependent roses have disappointed
gardeners for decades, and with the introduction of Knock
Out® and other very healthy roses in recent years, public expectation
for rose health has increased. Fortunately, there are
resources to help us become more strategic and efficient in the
development of highly resistant roses.

Commercially, it is not surprising to see rose introducers
that have traditionally paid little attention to prioritizing
blackspot resistance scramble to do so now in order to stay
in business. Those who sell roses realize they need to provide
their customers with the roses they want to buy. It is clearer
than ever that consumers want to buy disease-resistant roses.
True progress in increasing disease resistance in roses through
breeding takes time and strategy. Clever marketing and vague
promises of disease resistance can pacify customers for only
so long. A rose nursery owner recently summed up what his
customers are saying: “I already have and enjoy Knock Out®.
What do you have that grows like Knock Out® but comes in
other colors and has fragrance?” It is clear that consumers want
to buy roses that are fragrant, come in a wide range of colors,
and have attractive and variable flower and plant form. There
is a lot of opportunity for rose breeders!

Genes in old and new cultivars and in wild roses can be used
to develop new roses with significantly increased resistance
levels as well as commercial appeal.

As a crop, overall rose sales seem to be in a slump. Perhaps
this is due in part to the downturn in the economy and the
lingering perception that roses are susceptible to disease and
hard to grow. If it weren’t for Knock Out®, Drift®, Home Run®
and other healthy roses in the marketplace, rose sales probably
would have plummeted even more. People are leading busy
lives and want good value for their money. They expect plants
that perform well. I predict that as we continue to significantly
elevate pest resistance levels in roses across a broader array of
flower and plant types, we will see a resurgence of rose sales as
consumers realize that roses are a good investment.

My goal in assembling this article is to highlight strategies
we as breeders can use to develop disease resistant roses.
These strategies can also be applied to increasing resistance to
other rose diseases. First, I’ll set the stage describing some background
information about disease development and resistance
types and then showcase a data set that colleagues and I generated
that can be very useful in the selection of parents.

Setting the Stage for Understanding

There are two main categories of disease resistance: vertical
(aka race-specific) and horizontal (aka field or partial) resistance.
Savvy breeders combine both in new cultivars. Through understanding
these resistance categories and the typical inheritance
for each, breeders can consciously choose parents and
selection methods that lead to roses with greater resistance and
more durable forms of blackspot resistance.

I’ll describe these resistance categories in more detail, and
share data that colleagues and I generated about specific roses.
First, it is important to remember the factors that lead to increased
disease levels:

1. a more susceptible rose cultivar.

2. a more conducive environment (wet foliage, poor air
flow, optimum temperatures, etc.).

3. a more aggressive form of the pathogen (the pathogen
causing blackspot is the fungus Diplocarpon rosae).

4. a longer time of exposure with conditions favorable to
disease development.

Our goal should be to identify those roses that have relatively
less disease when all other factors are equal (i.e. similar air
circulation, moisture, sun, etc.). It is important to plan disease
evaluation so the differences we see between roses can be attributed
to the genetics of the roses, not to the other factors.

Definitions of resistance categories

Resistance to blackspot can be vertical or horizontal.

A rose with vertical resistance is resistant only to certain
races, or genetic forms, of a pathogen. When a specific race of
a pathogen comes in contact with a rose having vertical resistance
to it, visible blackspot lesions typically do not develop.
This type of resistance can be very effective when it works, because
roses will typically have no visible blackspot symptoms
at all. Vertical resistance can be thought of as a “yes” or “no”

A rose with horizontal resistance, in contrast, may be infected
with blackspot but its resistance limits some aspect of
the pathogen’s life cycle. Roses with horizontal resistance to
blackspot typically display visible blackspot symptoms. The
pathogen can be slowed in a number of ways including reduced
spore germination and penetration, slowed pathogen growth
once inside the rose, and reduced spore production to reinfect
more tissue. When all other factors are equal, roses with horizontal
resistance have significantly less disease development
than roses without this form of resistance.

The expression of horizontal resistance can vary greatly, its
variation depending on the extent and method by which the
rose limits disease development. Horizontal resistance is generally
effective against all races or genetic forms of the fungus
and is considered more durable than vertical resistance. It is far
more difficult for a pathogen to overcome horizontal resistance
than vertical resistance. To observe the horizontal resistance of
a rose, we must strip away vertical resistance factors by permitting
disease to develop on the rose. In this way we can compare
roses for their incremental differences in horizontal resistance.

Why is vertical resistance typically less durable than
horizontal resistance? If a new race or genetic form of the fungus
is present in your garden through sexual reproduction and
natural segregation of genes, mutation, or migration of a race
of fungus not previously found there, a once highly resistant
rose can quickly lose the effectiveness of its vertical resistance.
It is easy to see how races of blackspot are spread through
commerce. At local box stores I have frequently seen new
shipments of blackspot-infected roses with labels indicating
they were grown by a nursery a couple of states away. If you
don’t have these races of blackspot in your garden and you take
home one of these roses, a new race of blackspot will enter your
garden and attack your roses. Vertical resistance is typically
based on one or a few resistance genes and can be overcome

Figure 1. ‘Hansa’ routinely experiences significant defoliation from black spot in the upper

by the fungus. If these “weak spots” in the rose’s armor are
overcome, the performance of the rose is drastically decreased,
and its ornamental value is greatly reduced. No one knows how
long a vertical resistance gene will be effective before it breaks
down. In the next section I’ll describe how these resistance
genes in the rose and genes in the fungus typically interact.

There are multiple examples of failed vertical blackspot resistance
in roses. For instance, in Canada during the late 1970s,
the once highly blackspot-resistant cultivar ‘Martin Frobisher’
suddenly became susceptible (Bolton and Svejda, 1979). Unfortunately,
‘Martin Frobisher’ had weak underlying horizontal
resistance. In addition, ‘Hansa’ and most members of the
Pavement series of Rugosas have been reported as being very
blackspot-resistant. But for at least the past decade they have
suffered significant defoliation by fall in the upper Midwest
(Figure 1). Baby LoveTM (‘Scrivluv’) is a non-Rugosa example
of a rose that has been touted
for being extremely resistant
to blackspot, but a race that
has become more common
over the years has stripped its
vertical resistance and its underlying
horizontal resistance
appears limited. On a side
note, perhaps the strong powdery
mildew resistance Baby
LoveTM has and transmits is
also due to vertical resistance.
It will be very disappointing
if the powdery mildew resistance
in Baby LoveTM and
its many offspring suddenly
breaks down.

The best-case scenario is for us to develop roses with strong
vertical and horizontal resistance. When the vertical resistance
is stripped away, these roses will still be useful in typical landscapes/
gardens, and their appearance will not reinforce the
notion that roses must be sprayed and are incompatible with
environmentally responsible gardening.

Genetics of Resistance

Vertical (Race-specific) resistance

Vertical resistance relies on one gene in a host interacting or
battling with one gene in the pathogen. There can be multiple
“battles” based on how many different genes in the rose there
are each paired up and interacting with a gene in the fungus.
Either the rose or the fungus wins the overall war. If the fungus
wins, disease develops. In this section I’ll call vertical resistance
race-specific resistance because “race-specific” will help in understanding
the genetics. For a pathogen to be able to infect a
host and cause disease, it needs to have at least one effective
virulence gene (genes that enable the fungus to infect a rose).
An individual fungus can have multiple virulence genes. A
rose can also have multiple race-specific resistance genes. The
potential for the development of disease between each pathogen
and host depends on the specific forms or variants of these
genes (alleles) and their interaction (“battle”).

Here’s a generalized explanation of vertical resistance that
greatly helped me understand the concept. Every race-specific
resistance gene in a rose is like a door, and the doors are positioned
in a series. Before blackspot lesions can form, each door
must be opened so the fungus can get in. Alleles (forms of the
gene) that confer resistance are typically dominant. Each door
(resistance gene) has a lock on it and when the rose has the right
allele, the door is in the locked position.

In contrast the fungus has
virulence genes, genes that
allow them to attack roses.
These virulence genes can be
thought of as keys. If on the
fungus’s keychain there are
keys that unlock each door in
the rose’s series of doors, the
fungus can get in and cause
disease. If it can only unlock
some doors, but not all of
them, the fungus cannot get
all the way in and the race-specific
resistance is still effective.
Fungi that share the same set
of keys, and therefore are able
to cause disease on the same set of roses, are grouped together
as a common race. There is variability in the fungus causing
blackspot, just as there is variability among all the different
kinds of roses we enjoy growing. Fungal members of a race may
have vast differences in their other genes and growth rates, but
it is having the same set of keys (and therefore the ability to attack
the same roses) that groups them together.

Sexual reproduction in the fungus allows for new races to
develop. These new races may have a different combination
of keys on their keychain than either parent. Parental fungal
strains that independently cannot unlock all the locked doors
in a rose may have offspring that have the right set or combination
of keys that can unlock all the doors. Fortunately, the fungus
causing blackspot very seldom reproduces sexually. Most
of the time blackspot reproduces asexually, leading to more
fungus that is genetically the same as the original. Mutation
can also lead to differences that can lead to a new race. In the

Over multiple generations
of using parents with strong
horizontal resistance and
selecting the most resistant
offspring for further breeding
efforts, significant progress
increasing horizontal resistance
can be made.

international blackspot race array which is housed at the University
of Minnesota, there are at least 11 races, with additional
isolates awaiting testing to determine if they are new races or
are members of the races that have been previously characterized.
The stored races have been collected primarily from the
United States and Europe.

Race-specific resistance is very powerful because it typically
stops disease from developing. After all, what breeder
or rose grower doesn’t get excited when among a planting of
roses there are some that are completely clean next to severely
infected roses? Excited breeders and marketers often quickly
introduce such plants into the marketplace as highly resistant
roses. Until the race-specific resistance is broken down, it is
basically a guess how much underlying horizontal resistance a
rose possesses. It is fantastic when a rose with stripped vertical
resistance later proves to have strong horizontal resistance, but
more commonly the underlying horizontal resistance is less
than satisfactory.

Horizontal Resistance

Unlike race-specific resistance, horizontal resistance is generally
a result of the presence of several genes, each making a small
contribution toward overall resistance. Horizontal resistance is
typically effective against all races of the pathogen. The contributions
from all the alleles of the genes add up to the particular
level of horizontal resistance and the genetic inheritance
of horizontal resistance can typically be classified as additive.
This means that horizontal resistance of roses varies considerably,
depending on what the resistances of all these different
genes and alleles add up to. It is very unlikely for the pathogen
to mutate or change enough to overcome all genes involved, so
horizontal resistance is considered to be highly durable.

Due to the multiple genes contributing to horizontal resistance
and the different parts of a pathogen’s lifecycle that can
be affected, it is difficult to isolate and understand the mode
of action of each resistance gene involved. Although less is
known about the mode of action and specific genetics of partial
resistance compared to race-specific resistance, it can be
effectively selected for and improved by breeders. Some have
suggested that genes involved in horizontal resistance may just
be race-specific resistance alleles that have been overcome, but
we need more evidence before considering this point settled.

The more genes and the more copies of individual alleles that
contribute to horizontal resistance in a single plant, the greater
the horizontal resistance. In a cross of two parents that differ in
horizontal resistance, the average horizontal resistance across
all the seedlings is typically midway between the resistances of
the two parents. However, because of gene segregation, there
is a distribution of resistance between seedlings and the possibility
one can find some seedlings that have greater horizontal
resistance than the more resistant parent. Over multiple generations
of using parents with strong horizontal resistance and
selecting the most resistant offspring for further breeding efforts,
significant progress increasing horizontal resistance can
be made. The breeder is concentrating these additive genetic
factors through the process of selecting new parents, with each
generation having greater resistance. b

End of Part 1. Part 2 will be published in the next issue of the RHA

This post was edited by jim1961 on Wed, Aug 6, 14 at 16:08

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jim1961 Zone 6a Central Pa.

Lets do it this way... Anybody that's interested in all this Blackspot info please send your email address to and I'll send you all the info in a email... Thanks
Email i got from David:

Hi Jim!!

Attached are a lot of papers we've worked on over the years. The way the races are determined are described in the international standard array paper and the one in Plant Breeding. One has isolates of blackspot forms collected and individually innoculate them one at a time on a diverse set of roses. All the isolates that infect the same subset of roses should share the same genes to infect certain roses and then fall within a common race. Some isolates are more vigorous than others too.

The two part article in the RHA newsletter describes that process more and the gene for gene interaction between the rose and fungus and the difference between horizontal and vertical resistance. I'll send the second RHA newsletter in a different email because of the size.

The distribution article with general background DNA fingerprinting just highlights how variable the fungus is and that there isn't strong patterns or differences based on geography highlighting forms are likely moving kind of readily through commerce. This helps to showcase how important having strong horizontal resistance is and that vertical resistance can easily fail.

Take Care,

David C. Zlesak, Ph.D.
Associate Professor of Horticulture
UWRF Plant and Earth Science Department
410 S. 3rd Street
River Falls, WI 54022

This post was edited by jim1961 on Wed, Aug 6, 14 at 15:48

    Bookmark   August 6, 2014 at 3:39PM
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strawchicago(zone 5a)

Thank you, Jim, for the info, much appreciated.

    Bookmark   August 6, 2014 at 9:07PM
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jim1961 Zone 6a Central Pa.

I was reading from another source also that kordes and some other breeders are now using these ideas to create better disease resistant roses.
William Radler already took these steps to create knockout roses and some of his other roses.
Radler started breeding in the 1970's and it took him until 1989 to create knockout roses. Then years later they were marketed...
But like stated its a long slow process as you can see in the case of William Radler and the Knockouts.

So all we can do as consumers is try different roses and see how they work for us...provide them with the proper amount of direct sun daily, water, and depending on your soil fertilizer if needed.

Strawberryhill grows some great looking roses so I suggest if you like to experiment then try her ideas and see how it goes... :-)

    Bookmark   August 7, 2014 at 12:42PM
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jim1961 Zone 6a Central Pa.

Characterization of 74 Roses for Blackspot
Resistance and Ploidy
The goal of the Earth-Kind® rose trialing program is to
identify and promote regionally-adapted plants that
perform well in low-maintenance landscapes. Roses
are trialed for several years and locations throughout a region
before they earn the Earth-Kind® designation. Based on the
data, roses are then recommended that are highly likely to
grow and bloom successfully with only basic plant care. What
sets Earth-Kind apart from standard anecdotal recommendations
and marketing-driven promotions is that there is a strong
body of independently-generated data to back up the recommendations.
So far the only region in the country to have a fullfledged
list of Earth-Kind® roses is the South Central US. This
is the only region where roses have gone through the whole
Roses are being trialed in the Earth-Kind® manner in other
regions of the US. The tests involve designated spacing, nutrient
management, and irrigation parameters and consistent
evaluation of traits in order to find roses that merit Earth-Kind®
designation for other regions. Unfortunately, some have downplayed
the contribution of the Earth-Kind® program, saying
many of those roses do not do well in their region. The regional
basis of Earth-Kind® has not always been well-articulated, and
many have mistakenly misunderstood that Earth-Kind® roses
should do well in every climate. It is difficult to force garden
writers to emphasize the regionality of the program in their
articles. The 74 roses we characterized for blackspot resistance
and ploidy (all are listed in Table 1 in the appendix) are designated
Earth-Kind in the South Central US, are part of the
Earth-Kind® Brigade roses being trialed in the central US, or
are part of the North Central US Earth-Kind® trials��"or they
are being used as controls, or they are other roses being considered
for future Earth-Kind trials or are key parents of healthy
In the South Central US, blackspot is one of the most important
determiners of plant performance in the low-maintenance
landscape. Plants with significant blackspot infection
become weakened and compromised. Blackspot is also a key issue
impacting plant performance in other areas of the country
where summers are wet and humid. With the great resource of
the international race array at our fingertips at the University
of Minnesota, our goal was to characterize blackspot resistance
of these roses with defined races. Specific race/rose combinations
would allow us to better determine the type(s) of resistances
each rose had (vertical and horizontal) and understand
how well the laboratory tests paralleled relative horizontal resistance
levels we observed in the field.
We obtained and grew own-root plant of these roses in the
greenhouse. They were placed in a walk-in cooler in the fall and
brought to the greenhouse in January where they all uniformly
flushed out with vigorous growth. New, fully expanded leaves
were harvested for the pathogen inoculations in the laboratory.
The goal was to have leaves at a common stage of development
and to try to avoid differences in cuticle hardness due
to leaf age, etc. Younger leaves are actually more susceptible to
blackspot than older leaves. This may sound odd, but we see
blackspot on older leaves of plants in our gardens first because
these leaves are lower on the plant where humidity and avail-

able water are typically greater. It takes longer for water from
dew and rain to evaporate and the fungal spores needs several
hours with water on the tissue to enter the plant.
Harvested leaves from the greenhouse were placed on
moistened paper towels in clear, sealable salad boxes and each
box was sprayed with a common volume of solution containing
spores of a particular race. We used three races (races 3,
8, and 9 of the international race array) which were collected
from the eastern United States. We had different leaf harvesting
and inoculation dates over time. We had at least four boxes
of each rose by race combination with two leaves in each box
by the end of the study. Of the 74 roses, two roses (‘Chorale’
and ‘Pariser Charme’) were susceptible controls��"roses we
knew were susceptible to all three races and were expected
to produce blackspot to confirm that our inoculum and techniques
were effective. These control roses were chosen solely
because of their susceptibility to blackspot and not for being
contenders in the Earth-Kind
At the end of about 14 days
we rated the samples. The
threshold for a positive disease
reaction was the presence
of visible lesions with fruiting
structures of the fungus
penetrating the surface of the
leaves. In some rare situations
we had small discolored lesions
without fungal fruiting
structures and spore production.
Roses without visible lesions
or fruiting fungal structures
were classified as having
vertical resistance to the particular race. When we had fruiting
structures, we measured the three largest lesion diameters
across different leaflets in each box. Lesion diameter after this
common incubation period became what we used to rank and
compare roses for horizontal resistance.
Additionally, since we had own-root plants, I could perform
root tip squashes and chromosome counts. Ploidy determination
provides additional information to help support rose
breeders in their efforts toward developing resistant roses.
Two tables of data are presented from our experiment documenting
vertical resistance (Table 1) and relative horizontal
resistance (Table 2) to the three races of blackspot. Permission
has been granted to reprint them from HortScience. The full
reference is listed at the end (Zlesak et al., 2010).
Ploidy Data
Surprisingly, the ploidy level most commonly represented was
triploid. There may be some very good reasons for triploidy to
be so common as it is generally associated with traits people
typically value in roses for the landscape. As ploidy level increases,
typically plant parts are larger and thicker, with less
branching and slower growth rates. Triploidy is a nice balance
between the benefits of diploid and tetraploid levels and also
has the added benefit of generally lower fertility for natural
deadheading and quicker reflowering. Of course these traits
associated with higher and lower ploidy need to be thought of
in the context of comparisons made between plants of similar
genetic background (2x and 4x polyanthas, etc.), and there is
variation between individuals at any given ploidy level and genetic
background. For instance, a diploid rugosa would likely
have thicker leaves than a tetraploid hybrid tea. I have clearly
seen trends in these growth
characteristics associated with
ploidy as I compare my diploid
and induced tetraploid polyanthas
and species roses and
also tetraploid roses and the
diploid plants I was able to recover
out of them from regeneration
of sex cells. As I continue
to count even our classic
roses, it is surprising how common
triploids are (e.g. ‘Tropicana’
and ‘John F. Kennedy’).
As breeders we may want to
aim for triploid cultivars, especially
for landscape use where
we want relatively substantial
blooms on well-branched, vigorous plants that generally selfclean
and rebloom relatively fast.
Practical Interpretation and Use of
the Disease Information to Breed for
Blackspot-Resistant Roses
Nine of the roses in Table 1 displayed vertical resistance to all
three races (Brite EyesTM, ‘Grouse’, Home Run®, Knock Out®,
Pink Knock Out®, PaprikaTM, Peachy CreamTM, Rainbow
Knock Out®, and Yellow SubmarineTM). This does not mean
that no identified blackspot races can infect these roses. For instance,
in the lab I’m culturing an isolate of D. rosae I isolated
from Home Run®, and a more recent race of blackspot in the
international race array that was isolated from Knock Out®.
Fortunately, these two roses appear to have good underlying
horizontal resistance when infected with blackspot. For most
of the others, the underlying horizontal resistance is not clear.
As we compare and contrast the roses in the Earth-Kind®
pipeline, it is interesting to see the trend for percentage of
cultivars containing at least one race-specific resistance gene.
Those that have been in the Earth-Kind® program the longest
and have won designation in the South have the least at 18%,
those the next longest in the program (Earth-Kind® Brigade)
have 37%, those in the Northern Earth-Kind® Rose Trials have
50%, and the last group (primarily newer roses first entering
the program) have the greatest at 64%. This trend showcases
how valuable horizontal resistance is for long-term durability
and performance in the landscape. The trend for newer roses
typically having a greater chance at having race-specific genes
is likely a natural outcome of breeders interested in disease
resistance becoming excited about the roses they have not seen
any blackspot on.
Roses enter the Earth-Kind® program based on positive
anecdotal reputations for strong performance in the lowmaintenance
landscape. During the trials, races that can infect
each of the roses will emerge. The roses with strong horizontal
resistance plus desirable ornamental traits are the ones that
earn Earth-Kind® designation.
One strategy for breeding blackspot-resistant roses, although
risky, is to combine as many different race-specific resistance
alleles as possible in a single rose. One can strategically cross
roses with different vertical resistance factors in order to find
the seedlings that accomplish the goal. Stacked resistance
alleles will make it more challenging for a pathogen to have
all the right keys in its keychain to enter and cause blackspot.
For instance, one can cross ‘Barn Dance’ (vertical resistance to
races 3 and 9) and ‘Prairie Harvest’ (vertical resistance to race 8)
to try to obtain some seedlings with resistance to all three races.
Although this strategy could be pursued, it is nearly inevitable
that some form of D. rosae will eventually emerge that has all
the right keys to overcome all those alleles and cause disease.
Such a race can eventually move through commerce and
become widespread.
Since horizontal resistance has better durability, the data in
table 2 are especially helpful. Notice that the relative position
of most roses across races is quite similar, even though some of
the specific isolates of the races used are more aggressive than
others and generally can lead to larger lesions. There can be
a range of how aggressive individuals of a particular race are
(remember a race includes all the individuals that just share
the same “keychain” and they may have differences in their
background genetic makeup). For instance, the isolate of race
3 used produced larger lesions than the specific isolates of
races 8 and 9. ‘The Fairy’ and LenaTM ranked first and second
consistently for smallest lesions across all races! Other roses like
‘Alba Meidiland’ and ‘New Dawn’ consistently rank relatively
high for horizontal resistance as well. This data highlights that
there is good predictability for relative horizontal resistance
based on lesion diameter.
We have blackspot resistance field data for the roses that
are designated Earth-Kind® in the South Central US. As we
correlate the mean lesion size in the lab with the percentage
of defoliation due to blackspot in the field, r=0.62. R values
range from -1 to +1. A score of 0 means there is no predictive
association. The stronger the negative number, the stronger
the trend: the more one value increases, the more the other one
decreases. The stronger the positive number, the stronger the
trend that as one value increases so does the other. So in this
case there is a pretty strong trend that as lesion size goes up for
a rose in the lab, the more defoliation we see for that rose in
the field. Since different isolates of different races can be more
aggressive and lead to different lesion expansion rates, it is very
promising to see such a strong correlation in the midst of that
variability. As more blackspot resistance data are collected
from other ongoing Earth-Kind® trials involving some of
these 70+ roses, it will be interesting to revisit the correlation
between the laboratory and field data.
Clearly, horizontal resistance is the critical resistance type
for us to aim for. We can use parents with relatively strong
horizontal resistance to generate new seedlings that increase
resistance and hopefully also combine desirable ornamental
qualities. In order to assess horizontal resistance, we need to
strip away vertical resistance so we can observe disease on
all the seedlings and select for increased levels of horizontal
Vance Whitaker and Stan Hokanson (2009) document
the horizontal resistance of progenies that differed based
on horizontal resistance of parents. They used both diploid
and tetraploid rose populations and found the same trends.
Their data demonstrated that parents with greater horizontal
resistance tend to produce offspring with greater horizontal
resistance��"an expected outcome if the genetics controlling the
trait are primarily additive. Figure 2 illustrates the distribution
of resistance in a seedling population one would typically find.
For instance, if one crosses parents that differ in horizontal
resistance, seedlings typically have a level of horizontal
resistance between the two parents. The distribution or
spread of the range of resistance among the seedlings can be
very narrow with most if not all seedlings falling between the
resistance levels of two parents. Sometimes the distribution is
more spread out with a small percentage of seedlings that are
more susceptible than the more susceptible parent and more
resistant than the more resistant parent.
When we cross a rose with high horizontal resistance with
one with low resistance (maybe to bring in a different color or
flower form from the more susceptible parent) it is clear why
we typically end up with lots of seedlings with resistance levels
not even close to that of our more resistant parent. In order
to combine all the valuable traits in a single plant, one has
to be patient and in many cases be willing to raise multiple
generations before enough of the desired resistance genes are
combined along with other desirable features in a single plant.
Unveiling Horizontal Resistance in Our
Breeding Programs
One method to increase different races in our gardens in order
to better strip away vertical resistance and view horizontal
resistance is to introduce diseased leaves from different places.
One challenge to this strategy is that in order to comply with
APHIS (Animal and Plant Heath Inspection Service) regulations,
we need to get permits to take pathogens like D. rosae
across state lines, and then we must keep them contained and
not let them loose in the environment. The international race
array therefore is not a resource that can be released in outdoor
If we keep our blackspotted leaves within state lines, our
consciences will be clear as we work with increasing blackspot
diversity in our gardens. On the surface, these APHIS
regulations seem like a good approach for helping to manage
disease-causing organisms. However, when I see roses at my
local box store trucked up to Minnesota from other states with
blackspot, I have to stop and scratch my head. One valuable
strategy to try to obtain a reflection of race diversity in our
own gardens is to plant indicator cultivars. For instance, one
can plant ‘George Vancouver’ (resistant to race 8), ‘Candy Oh!
Vivid Red’ (resistant to race 9), and Love and PeaceTM (resistant
to race 3; described in Whitaker et al., 2010). If all three roses are
infected with blackspot, that suggests there are likely to be a lot
of keys and key combinations present in your fungi populations
and you likely have good race diversity. Blackspot can take time
to spread from one side of your garden to another. One can also
try to take a mixture of blackspotted leaves from established
beds and scatter them through a new bed to establish a culture
of different blackspot races. An alternative would be to steep
the leaves in cool distilled water for up to a day, strain out leaves
and spray the solution over your new bed right before you’d
expect several hours of dew on the leaves.
Besides encouraging race diversity in our gardens, it is
important to have environmental conditions that support
blackspot and are as consistent as possible through our gardens.
We want to be able to attribute the differences we observe in
disease severity to differences in resistance in the roses. We
want to find that delicate balance of environmental conditions
which are conducive to the development of blackspot, but
not so conducive that in short order our beds are completely
defoliated and it is hard to detect the subtle differences in
resistance between seedlings.
An Example of Putting Theory into
My experience using Yellow SubmarineTM as a parent provides
an interesting case study. So far, I have not seen blackspot
on Yellow SubmarineTM in my region. Nor have I heard of
blackspot infecting this rose in any other region (it does get cercospora
though, a disease sometimes confused with blackspot).
Yellow SubmarineTM has developed a very good reputation for
blackspot resistance and logically so. I raised a population of

RHA member Bill Radler has faithfully
and strategically selected for higher and higher
horizontal resistance over generations. He has done so
by encouraging great race diversity in his testing beds
to strip away the vertical resistance factors during the
trialing phase. Bill was able to develop Knock Out®
and is continuing to breed many other roses with
health comparable that of to Knock Out®. As his roses
are planted across the nation, most gardens have less
race diversity than his yard. Hopefully the vertical
resistance factors will be effective against the local races
that do not have the right set of “keys” to get in. Even
when vertical resistance fails in some regions, strong
horizontal resistance kicks in to preserve the utility of such
a rose in the landscape. Bill is a pioneer demonstrating the
potential of what is possible in roses. As we use the tools before
us to continue to make advances in breeding healthy roses, it is
exciting to imagine what will be possible. b
Bolton, A.T. and Svejda F.J. 1979. “A new race of Diplocarpon rosae
capable of causing severe black spot in Rosa rugosa hybrids.” Can-
Plant-Dis-Surv 59(2): 38-40.
Whitaker, V.M, Debener T, Roberts A.V., and Hokanson, S.C.
2010. “A standard set of host differentials and unified nomenclature
for an international collection of Diplocarpon rosae races.” Plant
Pathology 59:745��"752.
Whitaker, V.M. and S.C. Hokanson. 2009. “Partial resistance to
black spot disease in diploid and tetraploid roses: General combining
ability and implications for breeding and selection.” Euphytica
Whitaker, V.M., Zuzek K., and Hokanson S.C. 2007. “Resistance
of twelve rose genotypes to fourteen isolates of Diplocarpon rosae
(rose blackspot) collected from eastern North America.”
Plant Breeding 126:83��"88.
Zlesak, D.C., Whitaker, V.M., George, S., and Hokanson S.C.
2010. “Evaluation of Roses from the Earth-Kind Trials: Blackspot
(Diplocarpon rosae Wolf) Resistance and Ploidy.” HortScience 45:1779��"
Figure 1 (printed in part 1, fall 2012 issue). ‘Hansa’ routinely
experiences significant defoliation from black spot in the upper
Figure 2. When crossing a parent with low horizontal resistance
(Parent A) and high horizontal resistance (Parent B) most of the
offspring typically have intermediate horizontal resistance.

    Bookmark   August 13, 2014 at 10:32PM
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strawchicago(zone 5a)

Thank you, Jim, that info. is interesting for a breeder-perspective. I see Kordes' success in DR-roses in their ability to utilize multiflora-genes (cluster root & cluster-flowering & less thorns) plus floribunda to produce larger blooms.

I prefer Kordes' Flower Carpet over Knock-out, since Kordes' Flower Carpet is a clustering & surface root, and blooms well with less rainfall. Knock-out root is deeper, thus require heavy rain to bloom. Kordes' Flower Carpet is almost thornless, typical of some multiflora. The cluster-blooming is impressive for landscape view.

    Bookmark   August 14, 2014 at 7:50AM
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jim1961 Zone 6a Central Pa.

Yes this is from a BREEDERS perspective!

I'll be trying two kordes roses next year... Brilliant Veranda & Plum perfect.
I'm excited to see how they do here...

Thomas Affleck had BS on almost all his leaves along with Powdery Mildew so I stripped all his leaves off so hopefully it doesn't spread anymore through the garden.
I'll be taking TA out in the late fall...

    Bookmark   August 14, 2014 at 10:26AM
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strawchicago(zone 5a)

I have Thomas' parent, Bayes Blueberry, its root is straight down, like a singular long rope, no cluster whatsoever. My Bayes Blueberry did well in a tall raised-bed, but when I moved it to wet clay, it declined.

The straight-down stick root-system gets much bigger with gypsum in the planting hole. The cluster-root doesn't benefit much. When Blue Mist (multiflora) was in a pot, I didn't give it gypsum, but its root was a solid-2-gallon cluster. The straight-down stick root is more woody, thus more calcium is needed.

    Bookmark   August 14, 2014 at 11:54AM
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jim1961 Zone 6a Central Pa.

That wet clay soil I showed in my "Lincoln & Stuff" thread was the wettest part of our yard. (Upper End)
Thomas Affleck is on the opposite end of our property which has less clay and is not near as wet.
Our Mister Lincoln grew there from Sept 2010 until April of 2014 and did good. No blackspot or mildew and leaves stayed nice.
There was too much sun for Lincoln which caused his blooms to fall off too fast but that's about it...

Next year Earthsong and Prairie Harvest will be planted in that wettest section in the upper yard. ( But I took off a lot of mulch so it has dried up)
Our Double Knockout is growing and blooming very happily in that section.

Kordes Plum Perfect will have a area down toward the house which has less clay and is not as wet.
(Right now we have a young Double Knockout growing in that spot and its doing really well with blooming and has nice shiny leaves.)

Easy Does it will go in Thomas Affleck's spot...
Since Mister Lincoln previously did well in that spot I'm anxious to see what direction Easy Does it will take...

(Now I grew Easy Does it here before and it did NOT get Mildew) It only got BS but it was surrounded by 4 other severely BS-ed roses. This time Easy Does it will be in another location off by itself away from other roses.)

This post was edited by jim1961 on Thu, Aug 14, 14 at 15:10

    Bookmark   August 14, 2014 at 12:40PM
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