The following is a quote from page 13 of the link below (actual page 13 as printed on the bottom of each page, not the PDF page):
“A diploid R. rugosa hybrid was resistant to all four pathotypes, but as a chromosomally doubled tetraploid it was susceptible.”
I can’t say I’m surprised. Although this is not the same situation to make the comparison, this is known to happen often when crossing two completely disease free species. I think we’ll discover that chromosome doubling doesn’t take us where we hoped it would. We are discovering that many of the most famous and useful breeders are Triploids, and so this smashes the mythology that Triploids are poor plants to work with, have sterility, etc. Its just not true.
Here is an excerpt I copied reading ISHS program:
“Most species rose are diploid and desease resistant whereas garden roses are commonly polyploid and more succeptible. Resistance to blackspot desease may be affected by ploidy”.
From a publication of East London University.
However german Thomas Debener when testing species for best desease resistance (he his certainly a specialist) did find tetrapoids such as R. bella and caudata along with diploids from the little infos i have actually.
I asked him full text.
So… maybe we should be looking at the tetraploid species as sources of disease resistance that might hold up better in the modern tetraploid gene pool???
Also… another thought/question… would the disease resistance of an amphidiploid (for instance Rosa kordesii – doubled wichuraiana X rugosa) still be fine, because their is only a diploid genome from each parental species. There aren’t four copies of each chromosome like in an autotetraploid – doubled Rosa multiflora.
I was at these meetings and listened to Andy’s talk (his student who was going to talk was ill and couldn’t come). Andy presented more information than in the abstract (abstracts are written well in advance of a meeting and sometimes eventual talks can focus or emphasize slightly different aspects of the topic). I summarized Andy’s talk and information he presented in the last RHA newsletter. He found variable differences between the diploid (or triploid) and induced tetraploid (or hexaploid) pairs of plants for their resistance responses. For some pathotypes of D. rosae the 2x/4x roses were both susceptible (s), both resistant (r), or s as 2x and r as 4x or visa versa. There is a lot of variability. Perhaps in general there may be a little more loss of resistance as one goes up in ploidy, but that is far from being documented very thoroughly. If anything we can say altering ploidy level and its effect on resistance is somewhat unpredictable.
I think another interesting finding he presented was that in a cross of ‘Scabrosa’ (resistant to all 4 pathotypes) and a R. rugosa alba genotype (susceptible to all four), all 40+ seedlings were susceptible for all pathotypes. Thomas Debener found a dominant blackspot resistant gene (Rdr1) derived from probably R. multiflora, but whatever effective resistance there is in ‘Scabrosa’ is not simply dominant. This also points to unpredictability even at one ploidy level in trying to breed for resistance.
Thomas Debener is looking for monogenic dominant resistance.
It is quite like he was looking for a gene that could be incorporated to some actual var through GMO technics.
You are to remember that the owner of a GMO plant is owner also of all plants derived from this GMO plant; particularly seedlings and seedlings of seedlings: all and every plant incorporating the manipulated gene.
A fantastic challenge as the owner does not allow amateurs or competitors to play with these plants.
Imagine major plant breeders releasing only the GM versions of the new vars…
Monogenic resistance are quite risquy as a single mutation may overcome it. When I was a young horticulturist the very first bremia resistant lettuce were bred with all known three strains resistance.
I trialed some and personaly found the 7 and 8th strains. There are actually more than 30 known strains. Funghi mutate often. The harmfull vegetative phase is haploid that allow instant selection of the recessive mutants that find an open field!
These gene to strain resistance is called vertical resistance as opposed to horizontal resistance that is reduced succeptibility to all this funghi strains and eventually reduced succeptibility to other funghi also. Horizontal resistances are often polygenic, are not easily overcome and even if much more difficult to inbred are actually prefered.
Results about rugosa are very interesting: from a recent phone talk Thomas Debener’s team tests are loboratory ones when field tests are not at all concurent. Did you think i.e. rugosa “Alba” were succeptible to all blackspot strains? He did find everything among rugosas from complete blackspot resistance to succeptibility to all known strains. It is obvious that rugosa field resistance is quite different from his findings. That this field resistance is not found in most rugosa hybrids points also to polygenic resistance.
Pathogen with host plant connections are obviously quite complex with other organisms interferences.
Hi Pierre, That’s a great point about the correlation of field and detached leaf assays (sometimes seems quite variable). For my potato work I used them with late blight to a limited degree to compare resistance data with my field results. Vance Whitaker, a friend who works on rose blackspot here at the U of MN, chose to innoculate whole plants for his study because of this issue. He wants to not only test for major or verticle resistance, but also get an idea of partial or horizontal resistance. I think in general for major gene resistance detached leaf assays are good enough, but definately for partial resistance it probably is not informative enough. Some resistance factors may be produced in other parts of the plant and transported. If all one has is a detached leaf in a super moist environment, it may be difficult or nearly impossible to reliably detect true small differences in partial resistance.
Andy and his student didn’t test allknown strains of blackspot (4 is very limited actually), but at least with those 4 those are his results. Diplocarpon rosae is very diverse as we look at the work done by the Europeans and others.
That’s a good point Pierre about GMO’s. A transgenic rose will probably be protected for use in breeding under autility patent. Like you also mentioned Rdr1 is just one major gene and is likely to break down anyway. The idea of Debener seems to eventually be to pyrimid multiple, different genes into one rose to make it less likely a single pathotype can break down all genes. Unfortunately, cloned blackspot resistance genes besides Rdr1 are not available yet. I think I might have heard a little bit about a Rdr2 he found, but I might have been dreaming. Multigenic, partial resistance seems to have the greatest potential for durability. However, much more work needs to be done to characterize factors for partial resistance to this disease, the heritability of such factors, and how to best use recurrent selection and other means to increase levels and combinations of these factors in rose populations from which potential cultivars can be selected.
Maybe unlocking some of the strong, yet not impervious, resistance mechanisms of roses like Knockout, Seafoam, and The Fairy will help.
I dont understand why R. rugosa alba is more susceptible than Scabrosa? Also, Seafoam blackspots badly here. The Fairy never has as far as Ive seen over the years but it will mildew (which is obviously another story).
Species are comprised of many individuals and some have genes for resistance, some don’t, and some have more than others. Some of us humans are more susceptible so certain diseases than others. One specific genotype of R. rugosa alba that Andy had and tested with four specific isolates of blackpot resulted in that rose being infected by all four tested isolates. ‘Scabrosa’ is one specific selection or cultivar of R. rugosa and it happened to be resistant to all four tested isolates of blackspot. Perhaps if four more are tested ‘Scabrosa’ would be susceptible to those four and R. rugosa alba selection would be resistant.
That’s a good point about ‘Seafoam’. It does get blackspot for me too and eventually leaves can get a lot of it, however, it does generally have very high horizontal resistance compared to most roses. That’s partly why it won the Earthkind award. Blackspot development happens relatively slowly on it, which is one aspect of horizontal resistance. Unlike other roses that once infected, even just a little, drop their leaves, ‘Seafoam’ generally tends to keep its leaves significantly longer before dropping them. This may make ‘Seafoam’ look bad because we can eventually see a lot of blackspot on it, but unlike many other roses it at least retained its leaves.
One key for rose breeders/researchers in general is to experiment more and understand the boundaries or differences between how ‘vertical’ and ‘horizonal’ resistances are expressed in roses, how genes act and are involved in different aspects of resistance, and then to more effectively breed for desired resistance forms and levels.
Ahhh! Thank you much. Now that actually makes sense.
As I said in another thread:
Seedling succeptibility or resistance are quite unreliable.
Without spraying: an healthy seedling may be quite desease prone after a few monthes to two years.
This happens often for new released vars such as Tropicana that was outstanding for health when released and desease ridden a few years later.
Same for new in my garden var.
On the contrary there are examples of individuals as well as full progenies quite desease ridden at first that after this initial episode behave differently some revealing even quite immune.
An horizontal resistance component is a better aptitude to host cooperating microorganisms. These are equivalents to what we have on our skin and the sterile environment raised human kids are lacking.
And a while is needed to find the right cooperative microorganisms or the more virulent/adapted desease strain for a seedling or a new in my garden var.
I asked about this when phoning to Thomas Debener. His reply is “This is an interesting field of research, most rose leaf surface microorganisms are bacterias; in our team we do not have competence about this.”
My belief is that there is a biological desease control oportunity to help roses fighting deseases selecting not harmfull very competitive bacterias.