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Characterization of 74 Roses for Blackspot Resistance and Ploidy The goal of the Earth-Kind® rose trialing program is to identify and promote regionally-adapted plants that perform well in low-maintenance landscapes. Roses are trialed for several years and locations throughout a region before they earn the Earth-Kind® designation. Based on the data, roses are then recommended that are highly likely to grow and bloom successfully with only basic plant care. What sets Earth-Kind apart from standard anecdotal recommendations and marketing-driven promotions is that there is a strong body of independently-generated data to back up the recommendations. So far the only region in the country to have a fullfledged list of Earth-Kind® roses is the South Central US. This is the only region where roses have gone through the whole program. Roses are being trialed in the Earth-Kind® manner in other regions of the US. The tests involve designated spacing, nutrient management, and irrigation parameters and consistent evaluation of traits in order to find roses that merit Earth-Kind® designation for other regions. Unfortunately, some have downplayed the contribution of the Earth-Kind® program, saying many of those roses do not do well in their region. The regional basis of Earth-Kind® has not always been well-articulated, and many have mistakenly misunderstood that Earth-Kind® roses should do well in every climate. It is difficult to force garden writers to emphasize the regionality of the program in their articles. The 74 roses we characterized for blackspot resistance and ploidy (all are listed in Table 1 in the appendix) are designated Earth-Kind in the South Central US, are part of the Earth-Kind® Brigade roses being trialed in the central US, or are part of the North Central US Earth-Kind® trials��"or they are being used as controls, or they are other roses being considered for future Earth-Kind trials or are key parents of healthy roses. In the South Central US, blackspot is one of the most important determiners of plant performance in the low-maintenance landscape. Plants with significant blackspot infection become weakened and compromised. Blackspot is also a key issue impacting plant performance in other areas of the country where summers are wet and humid. With the great resource of the international race array at our fingertips at the University of Minnesota, our goal was to characterize blackspot resistance of these roses with defined races. Specific race/rose combinations would allow us to better determine the type(s) of resistances each rose had (vertical and horizontal) and understand how well the laboratory tests paralleled relative horizontal resistance levels we observed in the field. We obtained and grew own-root plant of these roses in the greenhouse. They were placed in a walk-in cooler in the fall and brought to the greenhouse in January where they all uniformly flushed out with vigorous growth. New, fully expanded leaves were harvested for the pathogen inoculations in the laboratory. The goal was to have leaves at a common stage of development and to try to avoid differences in cuticle hardness due to leaf age, etc. Younger leaves are actually more susceptible to blackspot than older leaves. This may sound odd, but we see blackspot on older leaves of plants in our gardens first because these leaves are lower on the plant where humidity and avail- able water are typically greater. It takes longer for water from dew and rain to evaporate and the fungal spores needs several hours with water on the tissue to enter the plant. Harvested leaves from the greenhouse were placed on moistened paper towels in clear, sealable salad boxes and each box was sprayed with a common volume of solution containing spores of a particular race. We used three races (races 3, 8, and 9 of the international race array) which were collected from the eastern United States. We had different leaf harvesting and inoculation dates over time. We had at least four boxes of each rose by race combination with two leaves in each box by the end of the study. Of the 74 roses, two roses (‘Chorale’ and ‘Pariser Charme’) were susceptible controls��"roses we knew were susceptible to all three races and were expected to produce blackspot to confirm that our inoculum and techniques were effective. These control roses were chosen solely because of their susceptibility to blackspot and not for being contenders in the Earth-Kind program. At the end of about 14 days we rated the samples. The threshold for a positive disease reaction was the presence of visible lesions with fruiting structures of the fungus penetrating the surface of the leaves. In some rare situations we had small discolored lesions without fungal fruiting structures and spore production. Roses without visible lesions or fruiting fungal structures were classified as having vertical resistance to the particular race. When we had fruiting structures, we measured the three largest lesion diameters across different leaflets in each box. Lesion diameter after this common incubation period became what we used to rank and compare roses for horizontal resistance. Additionally, since we had own-root plants, I could perform root tip squashes and chromosome counts. Ploidy determination provides additional information to help support rose breeders in their efforts toward developing resistant roses. Two tables of data are presented from our experiment documenting vertical resistance (Table 1) and relative horizontal resistance (Table 2) to the three races of blackspot. Permission has been granted to reprint them from HortScience. The full reference is listed at the end (Zlesak et al., 2010). Ploidy Data Surprisingly, the ploidy level most commonly represented was triploid. There may be some very good reasons for triploidy to be so common as it is generally associated with traits people typically value in roses for the landscape. As ploidy level increases, typically plant parts are larger and thicker, with less branching and slower growth rates. Triploidy is a nice balance between the benefits of diploid and tetraploid levels and also has the added benefit of generally lower fertility for natural deadheading and quicker reflowering. Of course these traits associated with higher and lower ploidy need to be thought of in the context of comparisons made between plants of similar genetic background (2x and 4x polyanthas, etc.), and there is variation between individuals at any given ploidy level and genetic background. For instance, a diploid rugosa would likely have thicker leaves than a tetraploid hybrid tea. I have clearly seen trends in these growth characteristics associated with ploidy as I compare my diploid and induced tetraploid polyanthas and species roses and also tetraploid roses and the diploid plants I was able to recover out of them from regeneration of sex cells. As I continue to count even our classic roses, it is surprising how common triploids are (e.g. ‘Tropicana’ and ‘John F. Kennedy’). As breeders we may want to aim for triploid cultivars, especially for landscape use where we want relatively substantial blooms on well-branched, vigorous plants that generally selfclean and rebloom relatively fast. Practical Interpretation and Use of the Disease Information to Breed for Blackspot-Resistant Roses Nine of the roses in Table 1 displayed vertical resistance to all three races (Brite EyesTM, ‘Grouse’, Home Run®, Knock Out®, Pink Knock Out®, PaprikaTM, Peachy CreamTM, Rainbow Knock Out®, and Yellow SubmarineTM). This does not mean that no identified blackspot races can infect these roses. For instance, in the lab I’m culturing an isolate of D. rosae I isolated from Home Run®, and a more recent race of blackspot in the international race array that was isolated from Knock Out®. Fortunately, these two roses appear to have good underlying horizontal resistance when infected with blackspot. For most of the others, the underlying horizontal resistance is not clear. As we compare and contrast the roses in the Earth-Kind® pipeline, it is interesting to see the trend for percentage of cultivars containing at least one race-specific resistance gene. Those that have been in the Earth-Kind® program the longest and have won designation in the South have the least at 18%, those the next longest in the program (Earth-Kind® Brigade) have 37%, those in the Northern Earth-Kind® Rose Trials have 50%, and the last group (primarily newer roses first entering the program) have the greatest at 64%. This trend showcases how valuable horizontal resistance is for long-term durability and performance in the landscape. The trend for newer roses typically having a greater chance at having race-specific genes is likely a natural outcome of breeders interested in disease resistance becoming excited about the roses they have not seen any blackspot on. Roses enter the Earth-Kind® program based on positive anecdotal reputations for strong performance in the lowmaintenance landscape. During the trials, races that can infect each of the roses will emerge. The roses with strong horizontal resistance plus desirable ornamental traits are the ones that earn Earth-Kind® designation. One strategy for breeding blackspot-resistant roses, although risky, is to combine as many different race-specific resistance alleles as possible in a single rose. One can strategically cross roses with different vertical resistance factors in order to find the seedlings that accomplish the goal. Stacked resistance alleles will make it more challenging for a pathogen to have all the right keys in its keychain to enter and cause blackspot. For instance, one can cross ‘Barn Dance’ (vertical resistance to races 3 and 9) and ‘Prairie Harvest’ (vertical resistance to race 8) to try to obtain some seedlings with resistance to all three races. Although this strategy could be pursued, it is nearly inevitable that some form of D. rosae will eventually emerge that has all the right keys to overcome all those alleles and cause disease. Such a race can eventually move through commerce and become widespread. Since horizontal resistance has better durability, the data in table 2 are especially helpful. Notice that the relative position of most roses across races is quite similar, even though some of the specific isolates of the races used are more aggressive than others and generally can lead to larger lesions. There can be a range of how aggressive individuals of a particular race are (remember a race includes all the individuals that just share the same “keychain” and they may have differences in their background genetic makeup). For instance, the isolate of race 3 used produced larger lesions than the specific isolates of races 8 and 9. ‘The Fairy’ and LenaTM ranked first and second consistently for smallest lesions across all races! Other roses like ‘Alba Meidiland’ and ‘New Dawn’ consistently rank relatively high for horizontal resistance as well. This data highlights that there is good predictability for relative horizontal resistance based on lesion diameter. We have blackspot resistance field data for the roses that are designated Earth-Kind® in the South Central US. As we correlate the mean lesion size in the lab with the percentage of defoliation due to blackspot in the field, r=0.62. R values range from -1 to +1. A score of 0 means there is no predictive association. The stronger the negative number, the stronger the trend: the more one value increases, the more the other one decreases. The stronger the positive number, the stronger the trend that as one value increases so does the other. So in this case there is a pretty strong trend that as lesion size goes up for a rose in the lab, the more defoliation we see for that rose in the field. Since different isolates of different races can be more aggressive and lead to different lesion expansion rates, it is very promising to see such a strong correlation in the midst of that variability. As more blackspot resistance data are collected from other ongoing Earth-Kind® trials involving some of these 70+ roses, it will be interesting to revisit the correlation between the laboratory and field data. Clearly, horizontal resistance is the critical resistance type for us to aim for. We can use parents with relatively strong horizontal resistance to generate new seedlings that increase resistance and hopefully also combine desirable ornamental qualities. In order to assess horizontal resistance, we need to strip away vertical resistance so we can observe disease on all the seedlings and select for increased levels of horizontal resistance. Vance Whitaker and Stan Hokanson (2009) document the horizontal resistance of progenies that differed based on horizontal resistance of parents. They used both diploid and tetraploid rose populations and found the same trends. Their data demonstrated that parents with greater horizontal resistance tend to produce offspring with greater horizontal resistance��"an expected outcome if the genetics controlling the trait are primarily additive. Figure 2 illustrates the distribution of resistance in a seedling population one would typically find. For instance, if one crosses parents that differ in horizontal resistance, seedlings typically have a level of horizontal resistance between the two parents. The distribution or spread of the range of resistance among the seedlings can be very narrow with most if not all seedlings falling between the resistance levels of two parents. Sometimes the distribution is more spread out with a small percentage of seedlings that are more susceptible than the more susceptible parent and more resistant than the more resistant parent. When we cross a rose with high horizontal resistance with one with low resistance (maybe to bring in a different color or flower form from the more susceptible parent) it is clear why we typically end up with lots of seedlings with resistance levels not even close to that of our more resistant parent. In order to combine all the valuable traits in a single plant, one has to be patient and in many cases be willing to raise multiple generations before enough of the desired resistance genes are combined along with other desirable features in a single plant. Unveiling Horizontal Resistance in Our Breeding Programs One method to increase different races in our gardens in order to better strip away vertical resistance and view horizontal resistance is to introduce diseased leaves from different places. One challenge to this strategy is that in order to comply with APHIS (Animal and Plant Heath Inspection Service) regulations, we need to get permits to take pathogens like D. rosae across state lines, and then we must keep them contained and not let them loose in the environment. The international race array therefore is not a resource that can be released in outdoor spaces. If we keep our blackspotted leaves within state lines, our consciences will be clear as we work with increasing blackspot diversity in our gardens. On the surface, these APHIS regulations seem like a good approach for helping to manage disease-causing organisms. However, when I see roses at my local box store trucked up to Minnesota from other states with blackspot, I have to stop and scratch my head. One valuable strategy to try to obtain a reflection of race diversity in our own gardens is to plant indicator cultivars. For instance, one can plant ‘George Vancouver’ (resistant to race 8), ‘Candy Oh! Vivid Red’ (resistant to race 9), and Love and PeaceTM (resistant to race 3; described in Whitaker et al., 2010). If all three roses are infected with blackspot, that suggests there are likely to be a lot of keys and key combinations present in your fungi populations and you likely have good race diversity. Blackspot can take time to spread from one side of your garden to another. One can also try to take a mixture of blackspotted leaves from established beds and scatter them through a new bed to establish a culture of different blackspot races. An alternative would be to steep the leaves in cool distilled water for up to a day, strain out leaves and spray the solution over your new bed right before you’d expect several hours of dew on the leaves. Besides encouraging race diversity in our gardens, it is important to have environmental conditions that support blackspot and are as consistent as possible through our gardens. We want to be able to attribute the differences we observe in disease severity to differences in resistance in the roses. We want to find that delicate balance of environmental conditions which are conducive to the development of blackspot, but not so conducive that in short order our beds are completely defoliated and it is hard to detect the subtle differences in resistance between seedlings. An Example of Putting Theory into Practice My experience using Yellow SubmarineTM as a parent provides an interesting case study. So far, I have not seen blackspot on Yellow SubmarineTM in my region. Nor have I heard of blackspot infecting this rose in any other region (it does get cercospora though, a disease sometimes confused with blackspot). Yellow SubmarineTM has developed a very good reputation for blackspot resistance and logically so. I raised a population of RHA member Bill Radler has faithfully and strategically selected for higher and higher horizontal resistance over generations. He has done so by encouraging great race diversity in his testing beds to strip away the vertical resistance factors during the trialing phase. Bill was able to develop Knock Out® and is continuing to breed many other roses with health comparable that of to Knock Out®. As his roses are planted across the nation, most gardens have less race diversity than his yard. Hopefully the vertical resistance factors will be effective against the local races that do not have the right set of “keys” to get in. Even when vertical resistance fails in some regions, strong horizontal resistance kicks in to preserve the utility of such a rose in the landscape. Bill is a pioneer demonstrating the potential of what is possible in roses. As we use the tools before us to continue to make advances in breeding healthy roses, it is exciting to imagine what will be possible. b References Bolton, A.T. and Svejda F.J. 1979. “A new race of Diplocarpon rosae capable of causing severe black spot in Rosa rugosa hybrids.” Can- Plant-Dis-Surv 59(2): 38-40. Whitaker, V.M, Debener T, Roberts A.V., and Hokanson, S.C. 2010. “A standard set of host differentials and unified nomenclature for an international collection of Diplocarpon rosae races.” Plant Pathology 59:745��"752. Whitaker, V.M. and S.C. Hokanson. 2009. “Partial resistance to black spot disease in diploid and tetraploid roses: General combining ability and implications for breeding and selection.” Euphytica 169:421��"429. Whitaker, V.M., Zuzek K., and Hokanson S.C. 2007. “Resistance of twelve rose genotypes to fourteen isolates of Diplocarpon rosae (rose blackspot) collected from eastern North America.” Plant Breeding 126:83��"88. Zlesak, D.C., Whitaker, V.M., George, S., and Hokanson S.C. 2010. “Evaluation of Roses from the Earth-Kind Trials: Blackspot (Diplocarpon rosae Wolf) Resistance and Ploidy.” HortScience 45:1779��" 1787. Figure 1 (printed in part 1, fall 2012 issue). ‘Hansa’ routinely experiences significant defoliation from black spot in the upper Midwest. Figure 2. When crossing a parent with low horizontal resistance (Parent A) and high horizontal resistance (Parent B) most of the offspring typically have intermediate horizontal resistance.

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