Evaluation of roses from the Earth-Kind trials: Black Spot (Diplocarpon rosae Wolf) resistance and ploidy.
Hortscience 45(12):1779
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I’m really excited with the resources that are in place with characterized, preserved races of black spot from international sources to help in better understanding resistance genes in roses. When Vance started his MS, he had to start from scratch with North American races as the races that has been previously published on were lost due to not being preserved well.
The manuscript I spearheaded took the three first races Vance characterized from North America and screened roses in the Earth-Kind trials. We are interested in roses with good resistance that can hold up in the landscape and being able to use tools like these characterized races in the laboratory to better characterize rose resistance and see if it is predictive of field performance would be very valuable. We learned that black spot resistance as documented by lesion size after set periods of incubation was very predictive of field performance. There are two basic forms of resistance- Vertical (sometimes called Race-specific) and horizontal (sometimes called field or non-race specific).
THe vertical resistance can be thought of as a series of locked doors in the rose. Each door is basically a single resistance gene/allele. The race needs to have on its keychain every virulence gene/allele in order to unlock all the doors and get in. Roses like Baby Love have a series of locked doors that so far most races do not have all the keys necessary to unlock. This is changing as the race that does is becoming more widespread. Vertical resistance as you can imagine is what is easy to select for as breeders. In our location with our races of black spot, it is nice to find roses that do not show any black spot. We get excited and select them which is totally understandable.
What is more important is the underlying horizontal resistance. After black spot gets in, this resistance governs how much damage it does. It influences the rate of growth and reproduction of black spot. It generally tends to be similarly effective across the different races. Since we cannot predict when an infective race can come in and take down vertical resistance, horizontal resistance is what is most important for long term landscape value and durability of the cultivar (at least in terms of bs resistance). Horizontal resistance can be thought of as quantitative with varying degrees.
So, as breeders, it is great to have roses with both vertical and strong horizontal resistance. Knock Out is a great example of that. When vertical resistance is stripped in some regions by a race with the right set of keys, there is strong horizontal resistance still acting that helps keep the plant from falling apart as fast as less resistant roses.
So, as breeders, it would be great if we had a wide mix of races in our garden that can strip away all the vertical resistances in our material so we can then select and build horizontal resistance (seems to be what Bill Radler was able to accomplish with Knock Out). Then, as our roses go out into the world, hopefully vertical resistance will keep them performing great in many places without bs and in other places bs would at least be limited.
Relying strongly on just vertical resistance is problematic. We don’t know the level of horizontal resistance in the rose until vertical resistance fails. My Baby Love fell apart hard from black spot when it did become infected.
In our research we found a trend that the newer introductions tended to have more vertical resistance genes and the older roses that have endured and have earned Earth-Kind designation in the South generally did not have much to any vertical resistance to these 3 races.
The international race collection was able to have some race(s) infect every rose trialed to date. This is a great resource to challenge upcoming landscape roses with in order to understand what the underlying horizontal resistance early on.
These races are also important to characterize more vertical resistance genes in rose. Perhaps if we stack enough of them into a single rose it would become more difficult for a race to have all the “keys” to overcome them all. Additionally, the races and controlled infection environments are helpful in understanding and working with characterization of horizontal resistance.
These resources that are coming in place set the stage for some exciting advances in black spot resistance. It is very exciting to have a part in this important research.
David