How to select for Vertical and/or horizontal resistance

I read the latest RHA Newsletter a couple of times and have a lingering question about verticle and horizontal resistance. David Zlesak’s article explained the concept so a non-scientific mind like mine can understand (thank you, sir.) The question I have is how does a breeder select for horizontal resistance without a ton of DNA data for each potential breeder rose? David mentioned 11 races of black spot, but what if I only have two or three in my area? Kim’s article about inducing fungal problems brought up a few more related questions that I will save for another day.

I’m trying to find stock roses that are listed as disease resistant. After reading the RHA Newsletter I guessing that I may just be taking a shot in the dark as far as disease resistance. Are there ways to find out which roses display horizontal resistance to BS or other diseases like PM, and other fungal diseases? Am I over thinking this?


Unless one proceeds as Bill Radler did, deliberately exposing his seedling/plants to various strains of fungi, your resistance can only be local. One way to overcome that as members on here have done is to send small plants to rosarians throughout the country for testing and get their feedback or else make use of test gardens, if available.

I don’t think one can ever be 100% sure since strains of fungi are always evolving and growing conditions are extremely variable even in one’s back yard which can stress the plants and then weaken their immunity fighting abilities just like us.

If you go on HelpMeFindRoses and do an ADVANCE search, one of the criteria one can select for is disease resistant, highly disease resistant, etc.

Jim P

Although it doesn’t directly provide an answer to your question, I highly recommend the book “Return to Resistance” by Raoul A. Robinson. It’s all about horizontal vs. vertical resistance, especially in food crops. A really fun read; not too technical. It really helps to solidify the understanding of the two different types of resistance.

It is available for free online as a pdf if you google it.

Thanks Joe,

It is a good read. I’m reading it instead of working right now, lucky we’re not that busy.

Thanks Joe. I just downloaded the book.

Jim: I read someplace where either Radler or Buck ground up infected leaf samples and sprayed seedlings.

I’m trying now to get my breeding stock together. I’ve literally spent hundreds of hours researching available roses and then their disease resistance through HMF. Last night my grandson was here and my wife took him to the “playroom” for about an hour. When they came back to the livingroom I barely noticed because I was digging through the internet reading about roses and their resistance. I thinks she believes I have an obcession about this. She is very much correct. :slight_smile:

Hi Jeff!!

Great question. It is nice having a single race to do a controlled innoculation. For all those roses that display infection, we can compare the relative disease progression. Those that have disease that progresses slower have greater horizontal resistance. Those that have no disease likely have vertical resistance and you cannot determine the rose’s horizontal resistance with that race.

There are different isolates and races of black spot here and there around the country and then they move around. Good thing we have some understanding and tools we can use that address the challenges imposed by different races in different locations so we don’t have to throw our hands up in the air in surrender. The changing and variable race structure in different localities has a greater impact on the vertical resistance reactions we see than horizontal resistance. For horizontal resistance, if you try to increase your diversity of black spot in your garden so there is at least one race hopefully capable of breaking the vertical resistance in every rose, that is very powerful and practical. We should try to plant our roses with as uniform as possible conditions (spacing, air flow, etc.) and then select those roses that consistently have less disease. Sometimes we may have a race / isolate that may be a little more aggressive than another that can complicate determining relative horizontal resistance precisely. However, as we encourage race diversity in our gardens and watch our roses over years, those roses with the most consistent relative horizontal resistance should become clear.

The challenge I see in increasing horizontal resistance is that it takes time and commitment. Bill Radler put the time in building high levels of horizontal resistance. We need to be patient, committed, and not give up. It typically takes multiple generations to build horizontal resistance. Using parents in the mix that have moderate to low horizontal resistance, but great color, etc. typically dilutes much of the horizontal resistance we are trying to bring in from our more resistant parents. Using parents with lower horizontal resistance is fine, but we should realize we can expect to have to raise more generations and seriously impose selection for resistance each step of the way before we typically can really push the envelop of high horizontal resistance again along with bringing in additional ornamental or other traits.

There are some recent roses that have Knock Out or Double Knock Out as a direct parent with the other parent being something with low to moderate horizontal resistance. Some nurseries are trying to capitalize on these new hybrids saying they have the health of Knock Out and the ______ beauty of the other parent. Some RHA members growing some of these roses already are saying the roses have defoliated from black spot in their garden. This just highlights commercial nurseries as well as the rest of us should understand horizontal and vertical resistance and do the work evaluating for it before we let ourselves get too excited and in the end exaggerate claims.

As we consciously select for horizontal resistance along the way in our breeding programs and are truly committed to it, eventually a greater and greater ratio of our seedlings will have increased levels of horizontal resistance. As we continue our breeding program with our different objectives we should also experience more roses having the various colors, etc. we are interested in too from some of the often more susceptible parents we brought into the mix. Kordes is another example besides Bill of truly being committed to moving towards greater horizontal resistance. Many of their newer roses are definitely a step ahead of most of their competitors for resistance to black spot. They are pushing the envelop now with some of their newer hybrid tea roses that have greater health. Conard Pyle and others are pushing that direction too with ‘Francis Meilland’ and others that are a definite step forward.


Is there a certitude that Kordes roses desease resistance is horizontal. Among the latest some are amazing in year one here. Wedding Bells leaves now january 30 are as tough and shiny as ilex aquifolium ones.


Selecting for horizontal resistance you need desease presence from either susceptible, not immune vars or inoculation.

Selecting for a lower level of parasitism is advised.

No good source of resistance is needed but it does not harm to start from usefull level of horizontal resistance and a wide enough genetical base is an imperative.

The difficulty to apply this to roses is that there are so many variables the species are far from when most have ample horizontal resistance.

These variables including plant model and flower sophistication are linked to low genetical diversity and mostly from failling at vertical resistance mainstream vars.

Another is that building up resistance is better done by random crosses as gene complementation cannot be guessed.

Hi Pierre!!

Great points!!! There is variability among the Kordes roses for horizontal resistance and some definitely have some vertical genes too. Overall as a group there are a good number that have significant underlying horizontal resistance. They encourage race diversity in their fields and are trying to select for it consciously. They need to balance all the traits important for commercialization, so especially with the hybrid teas there are steps towards improvement, but not necessarily the level achieved in other rose classes yet.

Making crosses using diverse germplasm and then trying to accumulate multiple genetic factors (genes / alleles) that contribute to resistance is a good way to proceed. There definitely is a bottleneck imposed by breeding especially within some market classes of roses. If we become to limiting working with recrossing a limited germplasm pool we may become too limited in the kinds of genes and alleles we are trying to accumulate. I’m not sure what you mean by random Pierre. “Random” crosses and gene complementation may fit better for vertical resistance than horizontal. Horizontal has general combining ability being the most significant source of variation and can be handled as an additive and predictive trait to work with. There may be a unique minor gene contributed that can add to resistance from some of the more susceptible parents worth capturing, but one has to be patient in bringing it in.

For the idea of random and unpredictable results this is how I see that best coming to fruition. For vertical resistance, the genes can be thought of as locked doors. You can have two susceptible roses in a garden that each have a locked door and there are two races of black spot. One has the virulence allele (key) to unlock one door and get in and the other can get in and infect the other rose. Crossing those roses, one can get a hybrid with both resistance genes. The fungal races in the garden may not include one that has both keys on its “keychain” and therefore there is no black spot in the garden to infect the hybrid and suddenly there is that complementation and an unexpected resistant offspring. This happens and is nice, but someday somewhere there will be a race to overcome both those genes. That is why horizontal resistance is more durable and what we should aim for. It is harder to select for than vertical resistance for breeders, but in my opinion is a good direction for us to head. The best case is having strong horizontal resistance and then some vertical resistance genes too.

In part 2 of my article in the next issue, I talk about an example that fits complementation of vertical genes.

I’m excited for the future progress we will make in disease resistance in roses along with bringing in valued ornamental traits.

So it sounds like I need more races of the BS fungus. Anybody want to trade fungi? :wink:

If you’re like most of us you have gotten your roses from all over North America. I’ve gotten mine from Canada, California, Texas, Oregon, Washington State, West Virginia, Minnesota and Wisconsin. With maybe a few other places I have forgotten about. Among them I’m sure there must have been different races of BS from what was already here, so who knows how many different races I now have.

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I will be willing to trade with you. I do not get a lot of blackspot in my area but when I see it I will certainly send you some. I have been trying to collect samples from other areas and spread them in my garden. I figure home depot is a worse offender to the rosarians in my area than I will ever be. So far I have not noticed much of a difference but my climate only has a good black spot year every 3 to 4 years. That good black spot year would probably be called moderate to most of you.

Also does anyone know what the best way to introduce black spot. In fall before winter in crushed leaves or something else? What a weird question?

Bill Radler mostly used blackspot infected leaves from his own garden. He would collect them, dry them quickly inside, powder them in a blender, and sprinkle them on the wet leaves of the roses he wanted to inoculate. It seems to me it would be best to do it in the evening; mist down the leaves and then sprinkle the dust so it stays wet long enough to inoculate them. He told me that only once did he intentionally bring in a new strain of blackspot; it’s the strain he calls the Brite Eyes strain, because it infected his rose of that name. I believe that strain also hits Radler’s Winner’s Circle.

I’ve tried collecting dirt in the early spring from near the base of roses that routinely defoliate, sprinkling that dirt through a sieve onto the misted leaves of roses growing in the greenhouse, keeping them under a propagation dome to keep the leaves moist for 24-36 hours. It didn’t work.

I get lots of BS in my mid-atlantic state. I’ve had to pull Westerland, Autumn Sunset and Quadra, to name a few, because of complete defoliation from BS. I’d be happy to send infected leaves to anyone this season.

I sure did not have any problem getting the “herd inoculated” last year, but accidental timing had a lot to do with it. That and the fact that I live up wind, up canyon, and all the spores in the world beat a path through my roses. I did intentionally set several lightly infected young plants (yearling seedling and recent purchases) on the down hill side of the seedlings, did not attempt to control the disease (rust and blackspot mostly) and then the weather alternately turned very warm and humid, then warm dry days with cool moist nights, and I had more rust and blackspot than I knew how to handle. I did dump the worst, although the BS came roaring back late last fall when the weather again cooperated brilliantly, and many that did not react or show prior infection did so this time. There are only 6-8 that did not show any infection, but I have maybe 20-30 that did not show an appreciable amount of any infection. I again dumped some more, and now with them just starting to re-leaf for spring, am going to watch what happens again. For myself, I thought it was more than a little scary watching all that fungal outbreak, and I even have misgivings about trying a repeat for this year. Surely there was more than one strain of BS involved, or at least that is what I thought. I was very happy to read about horizontal resistance and understand it a little better, because I seem to have a few that demonstrate some degree of this. Most of the yearling seedlings showing the best resistance are coming from one particular parent, and this parent has not shown any infection. I have a hunch that one of the best times to introduce the fungal strains are before warm humid spells of weather, and then spray the foliage with water before sundown if the humidity is not cooperating enough.

When I exhibited, I honestly got depressed by blackspot in July, especially on ‘Bill Warriner,’ which I loved. The poor thing would defoliate from head to toe, and have beautiful coral pink blooms up top. It did this for 5 years before being replaced by Pink Abaundance, which while healthy-ish, was a really, really boring pink floribunda.

Now, I dont worry so much, because what I select only gets some blackspot, which I dont care about, and some often do not get any.

Which reminds me, someone shoud cross a modern floribunda to the Pernet types, because theyre really pretty, but just REALLY naked on the bottom of the plant. They have horrible internode spacing, lol.


Here it is in “Return To Resistance” Third edition revised by Raoul?A.?Robinson p328:


Cross-pollination is an essential feature of recurrent mass selection. It ensures that the final selections of each screening generation, which become the parents of the next screening generation, will share their genes to the maximum extent. Each final selection is cross-pollinated with all the others, either randomly, or with controlled crosses, depending on the crop in question.


Anybody want to trade fungi? :wink:[/quote]

Trade? Hey, I’d love to give you all of mine. You can keep yours. :wink:

While I applaud our goals for horizontal resistance, the idea of trading pathogens and introducing them to new areas kinda makes me cringe. (Is it just me?) I wouldn’t want a local pharmaceutical company introducing a disease into my neighborhood so they could test a new drug. (I know that metaphor is kinda overdoing it, but I think there are a lot of parallels in the concept

I do accept that in time, new pathogens will find their way into different gardens, and we should have plants that will withstand them. But (and I’m taking this on a far-reaching tangent) I tend to think we should be breeding for our climates and our regions, and not aspire to something that everyone everywhere will love. I suppose that’s bad business, but I wonder if targeting too big a market has partly been to blame for the downfall of the rose industry. The “All American Rose” does not perform well in all of America, and so folks are disenchanted when “the best” fails for them.

Your thoughts?

Pat Shanley and Gene Waring announced at The Great Rosarians of the World, the formation of an AARS replacement. I apologize I don’t have the proper name for it, but I’m sure it’s available on the ARS web site. They’ve created a committee with a number of gardens participating to select the best garden roses “by region” with the first winners announced for sale beginning in 2017. Disease resistance is the highest point rated criteria of the evaluation. They will also have an annual fragrance winner.

The committee consists of her, William Radler, Clair Martin, Paul Zimmerman, Jeff Wycoff and two or three others whose names escape me. Philip’s comment about the AARS winners failing in gardens reminded me of the announcement.

As I posted earlier if you’ve purchased roses from other parts of the country or Canada you’ve probably already introduced different races of BS to your garden. And even if you just buy locally, who knows where those roses originally came from, so you could have introduced different races that way as well. Because of that, I don’t think the intentional swapping of pathogens is necessary or wise. We all know how devastating Dutch Elm disease and Chestnut blight were to our forests and our city streets. So I’m in agreement with you there Philip.

I have been told that the ARS’s AARS replacement is called American Garden Rose Trials (AGRT). It is expected to accept roses for trial soon, maybe as early as this year unless something has come up to prevent that. From what I understand, it is patterned after the ADR trials in Germany and will try to use ADR scoring in the old AARS trial locations.

Another initiative, American Rose Trials for Sustainability (A.R.T.S.), has been launched by a group of people with a different orientation. If I understand correctly, A.R.T.S. is putting greater emphasis on regionality and has filed or will file for non-profit status.

I don’t know whether non-profit status is planned for AGRT (the ARS plan). Apparently it is not part of the current program (as far as that has developed). As you know, the AARS was a marketing scheme, a for-profit cooperative endeavor.

Perhaps at some time these two initiatives will find a common ground and combine their efforts.