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
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
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
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
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
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