Breeding for Rust Resistance in Daylilies: Part 12
So now we come to the final post in this series. In the previous eleven posts we have looked at a wide range of subjects relating to rust and selecting for rust resistance in daylily breeding programs. It is my hope that these posts have been of benefit to you and that some few of you who have read them may now make use of some of this information and apply some of what I have offered herein to your own breeding programs.
I would like to make a short review of what I consider to be the most important points made throughout this series.
1. Rust is now an inescapable reality on the North American continent. We have it, so there is no going back to the old pre-rust days. Even for those in climates cold enough to kill all foliage and suppress rust each winter, rust may still enter your areas through shipped daylilies or through the use of greenhouses. For those of us in warmer climates, even when an exceptional winter suppresses rust for one year, rust is still a reality and no single harsh winter is going to permanently eliminate rust. Rust is a reality we all face and the further south you go, the more persistent that reality is and the more extreme the impact.
2. There is resistance to rust in the daylily population. While resistance levels will vary from cultivar to cultivar, and may even vary from garden to garden or year to year, depending on the environmental factors at work and/or the strain of rust present, there is some consistent resistance amongst some cultivars and that resistance appears to have a genetic, and thus heritable, basis.
3. As rust resistance appears to have a heritable basis, active breeding and selection for rust resistance can be practiced. Simple Mendelian segregation ratios should not be expected, and we should be realistic and look for individual resistant seedlings, rather than being focused on a more idealistic goal of developing "resistant lines". While there is some circumstantial evidence that some family lines do exist that show higher than average percentages of resistant individuals, a more realistic and basic breeding goal is to simply select resistant individuals. Equally important is the reduction of the use of very susceptible individuals in breeding programs, except perhaps in 'salvage programs'. Finally, of equally great importance is the reduction of the number of susceptible individuals that are introduced as cultivars and put on the market.
4. It is important to remember that the term 'resistant' it not synonymous with 'immune'. They are different things. Further, 'susceptibility' and 'resistance' are broad terms that do not simply define one particular phenotypic expression, but represent a bell curve of phenotype variations with the two terms representing the extreme ends of the spectrum with many variations in between. Further, tolerance means the ability to tolerate the presence of rust without severe repercussions and lack of tolerance means those that suffer severe repercussions due to the presence of rust that can range from suppression of vigor to death in some specific environmental situations.
5. There is known to be more than one strain of rust. Rust will continue to mutate, but it is highly likely that one cultivar or another may show resistance to any given rust mutation. A daylily resistant to one strain of rust is valuable for breeding even if it shows lack of resistance to another strain of rust. The ultimate goal would be a layered or pyramidal resistance that combines multiple forms of genetic resistance into one individual.
6. You need to make observations in your own garden when rust appears and develop your own observational system and approach to breeding and management based on your own experiences. The information I have imparted to you in this series is my experience, my basic approach or system and the gathered anecdotal and peer-reviewed information that I have been able to find. This information is presented to give you a jumping off point, tools to help you develop your own approach, for use in developing your own program based on your needs, interest, and available space and time. The information I have presented here is not set in stone.
7. Any cultivar listed as resistant, immune, susceptible, etc, is merely a guide and may not apply to all situations or all strains of rust. You need to find what is best in your situation and work from there. Further, a cultivar that shows excellent resistance in one year, but susceptibility in another, may have encountered different environmental conditions or a different strain of rust. Don't be too hasty in eliminating a cultivar from your program due to variability between its response in one year or another. A better view is gained of resistance if the cultivar is observed for many years in one particular setting. Any list of resistant cultivars is merely suggestions and is presented as a guideline and no cultivar therein will be resistant permanently in all settings or all years.
8. There is considerable difference between the approach of an average gardener, a collector, a seller and a hybridizer. Each will have different needs and may also have different approaches. The average gardener may choose to eliminate susceptible cultivars and may want to focus on only maintaining resistant cultivars in order to not have to spray and to keep a neat and attractive garden setting. Collectors and sellers may choose to maintain susceptible cultivars and may also spray, with sellers in most states being required to spray, at least for part of the year. Hybridizers who have any interest in increasing resistance to rust in their program will need to maintain susceptible cultivars, but are advised to not breed from such any more than is absolutely necessary (salvage projects). While hybridizers who are also sellers may be required to spray, those who can forgo spraying are in the best situation to observe actual resistance levels and make selections toward resistance.
9. Your location will determine how you will approach your resistance breeding project. Those in the most rust prone areas, where rust is present in spring and fall, have the very best environment for rust resistance selection. They can engage in an active resistance breeding program. Those in less rust prone areas or in the far north (and who do not use a greenhouse) are less able to actively select for resistance and are usually only able to participate in a passive program.
10. An active resistance selection program requires rust (at least in the fall and better if in fall and spring) yearly and requires no spraying so that all plants are equally exposed to rust and can reveal their resistance levels. A passive program involves the use of known resistant cultivars as breeders, rust present in the garden on occasional years, no spraying when rust does appear and active observation and notation of the performance of the resistance levels of the cultivars and seedlings in the garden with the use of the most resistant in the breeding program. Passive programs may also send seedlings into more rusty environments for testing and the gathering and recording of anecdotal evidence from those growing the plants from that program is also important. Finally, in a passive program, it is very important to keep up with information on resistant cultivars and advances in research as well as exchanging information with other breeders.
11. Greenhouses are an excellent way to keep rust active in a northern program and can allow a northern breeder to maintain rust for an active resistance breeding program.
12. Sprays will suppress the formation of rust spores, but do not kill rust. Some sellers are required by law to spray. This is unfortunate, but unavoidable where that is the case. As spraying does not kill rust, but only suppresses spore formation, any plant can still carry rust even if sprayed to control rust. As sprays are expensive and environmentally questionable, I do not advocate spraying for anyone who is not legally required to spray. I would recommend anyone who does use sprays or is considering using sprays to look into some of the side effects on the environment and beneficial insects of fungicides. Recent research suggests that many fungicides may play a role in bee colony collapse syndrome among other problems. This makes spraying even more questionable to me, especially since spraying does not kill rust, but only suppress spore formation. You will have to decide where you stand for yourself.
13. Keep researching. New information appears. Keep on top of it. This is especially important for those engaged in more passive programs.
What I have presented here in this series is a starting point. I have presented you with tools to use in developing a resistance breeding program, passive or active, of your own. However, this is not the final word, nor is it comprehensive. I have shown you my thoughts, feeling and observations - shared my methodologies and understanding. Now you need to develop your own.
I hope that even a few who have read this series of posts takes this information seriously and begins to apply even a small amount of it in their own programs, but I can't force you to do that. All I can do is lay out the basic points and then you have to pick up the ball and run with it. Most of you probably won't. It is too easy to spray, or ignore it, or feel that your cold winters insulate you from concern, but if even one for every thousand readers makes some effort, that is more effort than was being made before.
I believe that in time, this subject will become unavoidable for all of us, but I also hope I am wrong. Time will tell. I know that selection produces results. I also believe that those who do not ignore rust and who apply even some of the information I have presented in this series will be ahead of the curve and will be better prepared if we do see a rustier and rustier future. I leave it to you now to do what you will.