Seedlings and blackspot

Reading old post on the forum I see that their is general differences to when people will begin to cull for powdery mildew. Some cull as soon as it shows up others cull later in hope that maybe the seedlings will grow out of it.

I was wondering what peoples thoughts are on blackspot. If they cull as soon as they see it or wait?

Lastly I was wondering at what point do you consider the blackspot to be at an unexceptable level? I no longer live in blackspot country but when I did some roses I saw always got black spot but never got it bad and lived through it and other completly lost all of their leaves so at what point do you consider a seedling no longer worthy?

I have a few new seedings that grew under lights over the winter. After I moved them outside this spring 3 seedlings developed major powdery mildew. I was tempted to toss them but decided to wait a bit. 2 are PM free and the 1 remaining is much improved. I think I won’t be so quick to think about tossing PM seedlings and wait to see how they do. Interestingly, the parents are Leverkusen and Lichtk

I think it depends on one’s tolerance level and the number of seedlings produced.

At this point I toss anything that shows PM as seedling, unless it’s a really unique cross.

Yes, they can outgrow PM somewhat, but the propensity will always be there.

I find it’s best to concentrate on the best of the best when possible, especially now that chemical cures are finally falling out of fashion.

I an very interested in this topic as I live in one of the worst blackspot areas of the country. I would like to rephrase the question slightly – Has anyone had experience with a seedling that showed some blackspot susceptibiity initially, but later seemed to develope resistance to it? Is this even possible?

Also, for the first time ever, my Mermaid that I like to brag is bullet proof had a lot of blackspot this spring, but it shed all those leaves and is now covered in bright glossy green foliage without the use of any fungicide. This is the first time that I have ever had a rose appear to self cure. And it is not that the weather has changed – the weather favors blackspot here year round and it is becoming more of a problem on my other bushes, but Mermaid now appears to be blackspot free – and i don’t even bother to remove the affected leaves or rake them up. I have heard of resistant roses breaking down under heavy blackspot pressure or losing resistance to certain strains, but is it possible for a rose to mount an immune response after initially becoming infected?

Hope to hear from some wiser and more experienced experts, which would include almost anyone else on these forums.

Many thanks, Bob in New Orleans

One of the questions I was wondering how big do the seedlings have to be to show some resistance, or is their any build up in resistance as the seedlings grow.

Acquired resistane is a great question. Definately more work needs to be done in roses to understand what genes are turned on after infection and their products and how they may ward off continued attack and future infections.

One other reason for variable blackspot incidence of course is different races present in the garden and perhaps changes of races in a garden. One might infect a particular rose, but another may not. Depending on what is predominating and where it is in the garden will influence what roses get infected.

Widespread testing to get an idea of a rose’s suceptibility to different forms of blackspot is important before a rose is widely introduced as “resistant”. This can be done in the laboratory as well with the different characterized races preserved and individually used to challenge roses. I was doing such work this past winter with the EarthKind roses. The purpose is to document the resistance for each of the trial cultivars (Northern Collection and mid America collection) and compare it with building field data. Perhaps this screen will be a useful means by which to help decide which cultivars to enter the trials in the future and increase efficiency. Those that limit infection to the races and/or have widespread race specific resistance are more likely to hold up in the landscape. There may be additional races we don’t have in the lab, but it definately is more than just guessing and limited experience with whatever was at a single or limited group of sites. I think lab screens hold promise for larger nurseries/breeding firms that can support such trials.



David Zlesak:

Thanks for informative and almost instantaneous response. One of the reasons I asked, especially in relation to seedlings, is that I know that in humans the immune system is not fully developed at birth, but I do not know if there is a botanical analogy to this phenomenom, i.e., do seedlings have the potential to develop improved immune systems as they mature. Climate conditions here are rarely favorable to mildew and when it occurs the weather changes quickly enough to cure it, but blackspot is relentless and eternal. Accordingly, I have been following your EarthKind info avidly, and have recommended to my daughter that she stick to resistant varieties – but the pictures in rose catalogues often look like the apple must have looked to Adam. I also find Pual Barden’s critiques on his web site regarding resistance very helpful for a beginner like me.

Bob in New Orleans

Robert I have the opposite situation than you. I rarely see black spot here but powdery mildew starts early and never disappears here. I asked the beginning post because I am interested in pest and diseases.

So David what is the procedure for testing in the lab.

Another question how come certain varieties that are in the beginning very resistant over time loss resistance and yet certain species continue to show resistance. Is it because our cultivars only have a few resistant genes and species tend to have horizontal resistance.

If it is horizontal resistance how do you breed for it. I know how to breed a resistant gene into a breeding line but I do not know what it takes to breed horizontal resistance. I have read up on the subject but it seems a little fuzzy to me and it seems like you could be breeding for horizontal resistance and mistakely breed vertical resistance instead.

On a side note Baby Love is suppose to be resistant to powdery mildew right? Because last year I saw it on it at the end of the season which does not worry me. But what worries me is that I have it on it right now (not a large amount but enough to notice).

That’s a great question and point Adam. For horizontal resistance it tends to be quantitative and seedlings tend to vary in expression. Although there is some exceptions and variations, the following tends to be a good way to think of it. There are a lot of minor genes that contribute to the trait of resistance with horizontal resistance and the goal would be to accumulate alleles at these genes that contribute to resistance. As you intermate roses with good horizontal resistance one should find a whole array of expression among the offspring. Hopefully there will be some with greater resistance than either parent and can be selected for future breeding and over time more and more of the positive alleles can be accumualated.

For race specific resistance which tends to be a single gene single allele like you mentioned is easier to select for. If a breeder selects resistant offspring that has race specific resistance, they may be surprised later. A race may overcome it and then they find there is little underlying horizontal resistance. It would be nice to combine both. One can cross parents and by using the race(s)that overcome any race specific resistance they may have then they can look at and select those with good underlying horizontal resistance. Later in the garden that particular race isn’t present, but if it comes, there should be good underlying horizontal resistance. Perhaps if multiple race specific resistance alleles can be pyrimided it can provide extra protection as well. This all relies on characterization of different blackspot races and resistance types. Some single gene resistances are race specific, but do allow some minor infection and can make the whole process confusing and hard to work with and understand, especially for us without the benefit of characterized races and ability to perform controlled innoculations with each.




It is hard for me to imagine mildew as a serious problem having had no experience with anything but inconsequential transitory bouts with it. How bad can it get if it persists?

Bob in New Orleans

Following is an excerpt of my words on another forum a while ago.

"As our human skin and internal cavities are colonized by a lot of globally beneficial microorganisms (MO), thousands of different ones ; there are a similar MO population on plants. These MO, competing for space and food, are eventually antagonist or predatory. They have different competitive habilities. As Baker’s first concurrent is another Baker; there is a strongest competition among same species individuals or strains.

Funghal plant deseases mutate quite easily. When plant pathologists examine a funghus they generaly find a mix of strains ranging from high virulance low competitive strains (HVLC) to others that are higher competitive lower virulance (LVHC). For pathogens high virulance means damaging, eventually letal, when low virulance often has no damaging effects.

Linkage of low virulance and high competitivity is reliable enough for inoculating such LVHC strains being a fully successfull strategy. It is with many human/animal vaccines. As to plants this strategy is applied to i.e. some Elm, platanus and castanea deseases. It is also applied to some annual horticultural plants viruses.

Evidently when spraying we whip out the not damaging LVHC MO and open the door to damaging pathogen HVLC funghi.

When first introducing a new var we often find it quite desease resistant and after a few monthes to up to two years this initial resistance vanish. High virulance is most readily selected and expressed on single clone populations. That is why a top selling var that was initially quite desease resistant is no more when widely grown."

Reversely a succeptible var may “find” low virulence strains able of helping it to resist the higher virulence ones. It is something I observe frequently in my never sprayed rose field.

I.e. some six years ago a 75m long row of young foliolosa crossed seedlings was without exceptions heavily rust ridden, soil was red under the plants. They did not rust for the following two years. I still have many plants from this lot. Never saw rust on them since.

I do actually have young seedlings a few generations further this line that have heavy rust something rarely seen here in spring. It is temporary only, probably as foliage is close to soil actually.

Banksiae sp. seedlings are usually PM ridden when in the seedling tray. When adult these plants are free of PM.

Actually there are two roxburghii OP seedlings lots in my rose field some 40m distant; in one all true to species (80%) plants are hit by PM when the other is totally sound.

A friend of mine reported recently that after reading the forementioned words he ceased spraying for BS an old var he cherish and that it is much healthier since.

As I am breeding for desease resistance in a climate that favour at times all and every rose desease I never select early. All my seedlings are field planted and observed for three years. Culling is nature made.

I consider succeptibility to BS as well as to other deseases acceptable when it does not hinder building up and decorative performance in my conditions that are pretty rough: seedlings are at 15cm on the row so that foliage is always intermingled and there are many seedlings quite succeptible to each desease.

Here Knock Out if not spotless qualifies, Baby Love that defoliates does not.

Some plants like lilac get it pretty bad here but it doesn’t seem to affect them that much I suppose that is due to some kind of co evolution or something else.

On roses here it depends alot on the rose (the PM that effects lilacs is slightly different then that which effects roses). Some I never see any Pm on like woodsii (though I found an interesting gall on it this morning round red gall with spikes). Other like Baby love get a little bit of it but nothing bad. Then their are those that get alot of it. Most floribundas I see get alot of it. Some of these it just seems like the foliage is sort of grayish white but it doesn’t kill them. Others it is as if you painted the leaves and the stems with fake snow. The young stems will curl up and the leaves curl dry up and either drop off or just hang their and look bad. It also will grow on flower petals. Mostly on modern dark reds from what it seems. I think it may have to do with Charlotte Armstrong being in alot of their parentage but I am not sure. A rose in this catagory even if you manage to stop the mildew is almost certainly a goner.

Another note of interest is if you want to see the spores that spread the disease around they are on the underside of the leaves. Some species of Powdery Mildew like that found on lilac make it easy because the spores on the bottom are a different color than the mycelim that grows on the top. These spores are spread by the wind to the upper leaf surface sprout and the mycelim will grow through the leaf surface until it reaches the bottom where it can begin to flower per say.

You can stop PM from spreading by spraying backing soda on the leaves. Other than that I get rid of anything that is too bad and I have been weeding out any seedlings that show it at all (I have weeded out whole seedling tray so far).

Living at one time in Portland and knowing vaguely what blackspot is like (I was not really into roses at that time but I did grow a few) and dealing with extreme PM. I would probably take the PM over blackspot anyday. It seems PM is easer to control useing natural means but I would say they are both as deadly to those cultivars that are not resistant.

Others who have alot of powdery mildew do you also find that it runs in conjunction with out breaks of spider mites. I find the best weather for powdery mildew is also the best weather for spider mites. And from a distance both of these things look similar. until you look at the leaves.

I hope I answered your question robert. I been learning alot of this as I go especially on roses which I only have had a real big interest on the last three or four years. Plus I have a bad habit of rambaling and thinking of a million ideas at once which sometimes even manages to get myself confused…

David is their anyway to tell if a variety has horizontal resistance or vertical without testing? Or is it just by takeing in the evidence like say Baby Love, Knockout and Home run where we know that the resistance fails misrably at certain areas and yet almost fool proof at other (making it vertical). While on the flip side The Fairy getting blackspot in many areas but it seems to be able to live with it (so I am assuming it would have some horizontal resistance).

And assumeing my assumption was correct then crossing one of the first group with The fairy could give both horizontal and vertical resistance if your lucky? In most cases I would assume you would then have to cross the F1 together in hopes of regaining most of the horizontal resistance? But that still leaves a problem without a way to test around the vertical resistance because you still won’t know which ones have the horizontal resistance unless you lived in an area where the races are present or you could test for it.


Great contribution. Many thanks. My gardener took it upon herself this week to start spraying my roses unbidden. I will ask her to stop and see how it goes as all of the few plants that I have are supposed to be resistant and the blackspot has not gotten to be too bad yet. Part of the problem is that the sprinkler system could be better aimed so that it doesn’t spray the leaves in the evening, but I would rather have some blackspot than stand out in New Olreans heat supervising the watering myself when I come home from work.

Bob in New Orleans

Sorry about all the questions I have been thinking alot on the issue lately.

Another question I have is I was reading on breeding for horizontal resistance and it seems one of the methods is to gather varieties that have no resistance at all and are not so closely related into a population and then though random breeding and inbreeding over time select out the ones that seem to be the most resistant. After doing this 15 generations or so if you do it right it sounds like you can build up horizontal resistance. To me this sounds like something ou could do well with a seed crop and not somethingthat may work outwell with a crop like roses that are propagated by clone. I would love to know yor thoughts on this?

Lastly it seems that the main thing that stands in the way of good horizontal resistance in roses is the economics of the whole thing? If it takes generations to breed good horizontal resistance and you can lose it in a single generation of crossing. It seems to me that it is more economic to work with several vertical resistance at once and try to layer them together especially if you are breeding at a company like weeks or jackson and perkins and just try to stay a little bit ahead of the pathogen.

Adam, I think the reason Knock Out and the like look like they do is just for the reasons you have stated.

They are chosen primarily for disease resistance instead of floral and growth characteristics.

Interbreeding those already selected for disease resistance seems to be the most efficient method of retaining these characteristics.

As time goes on I’ll bet flowering characteristics will gain importance, as long as it can be combined with ease of maintenance.

I think it is not only important for us to try to use these few disease resistant plants but we also need to develop as many new source of resistance as possable. I would not be surprise if Knockout, Baby Love and Home runs resistance runs out. They are all of the same line and I think they all have the same gene creating their resistance.


You will soon know if your watering timing and mode are bearble for roses in your climate.

Here I would never advise wet leaves at night.


Breeding for desease resistance is not restricted to seed crops. It is not so often done for ornamentals, but few are grown as collections of plants first bred for flower qualities when most species grow widely scatered among other plants.

And yes breeding for desease resistance has not been done. Only lately searching for dominant resistance gene to introgress. A strategy that is bound to fail in my opinion.

When I was a lettuce grower and young horticulturist, trialing the then new Bremia resistent vars (to make it short Bremia is lettuce PM), unfortunately I discovered race 3 and 4 that were sampled by Dutch breeders. Actually there are at least 24 well known Bremia races. And a lot of race specific resistance genes. None universal and every one superseded over time. Accumulating them is when done as it was over a few decades for Butterhead greenhouse vars leads to ineluctable impoverishment of diversity as it is allmost impraticable to outcross to not fully “accumulated” vars. Too large progenies with too much testing: too costly.

Horizontal resistance then was considered. It is rather easily encountered in cross breeding populations that evolved wild or cultivated densely grouped heterogenous for long periods and many generations. Conditions that were met for many annual or biennial agricultural plants. As David explained horizontal resistance is allmost impossible to backcross. It is easier to introgress the desirable features in the population with horizontal resistance.

Conditions favoring horizontal resistance are rarely met for vegetatively propagated perennials with at least one strong exception: the potato that is still grown in the original countries by the uneducated uncivilized as a mixture of clones.

About roses infos on populations density are not overabundant but most grow quite scatered among other plants and are protected by distance more than resistance.

There are a few species that are able to grow in dense populations.

The northern or alpine ones are not the most interesting as funghi spores do not overwinter well.

More interesting are tropical Asia species with a long annual hot wet season from which stem the Teas, Chinas, Giganteas, Noisettes and many Ramblers… southerners know that they are not immune. Just highly tolerant another word for horizontal resistance.

Or seaside ones with very long growth season in wet environment like rugosa.

Or ground hugging sp like wichuraiana and sempervirens.

You make some excellent points Pierre.

I can add that in my relatively short experience in dealing with tropical species, it is as you suggest. There are those resistant to disease such as Powdery Mildew, but not totally immune.

Very few are clean all the time.

I suspect if one were to observe very closely, perhaps even to a microscopic level, disease symptoms will be observed even on those thought to be immune.