Breeding For Rust Resistance in Daylilies: Part 4

Breeding For Rust Resistance in Daylilies: Part 4

In this fourth installment we will look at the importance of identifying recurrently resistant cultivars, the mindset required to make a good resistance breeder and the importance of exposure to pathogens in a resistance breeding program.

The Importance of Identifying Recurrently Resistant Cultivars

It is important when discussing rust resistance to first realize that there is no rhyme or reason to where resistance occurs. If one takes a wider view, of course, we could say that any cultivar that shows resistance likely had ancestral species (perhaps having evolved some level of resistance in the wild through interaction with the rust pathogen over an unknown amount of time) that carried some gene(s) for resistance and that from that point forward, each generation was passing those genes along until that cultivar encountered rust infection and revealed to us that it shows resistance. The same would probably be true of susceptibility. Yet, in the short term view, there is no way to know which cultivar will be resistant, even if we know it has an ancestor that is known to be resistant.

No specific phenotype traits seem to be linked with resistance. There is no way to know resistance or susceptibility based on foliage type, flower color, bud counts, branching, region of origin or even ancestry. Now, there does seem to be some tendency for certain colors to show more individuals with resistance than others (there are a lot of red cultivars that show resistance for instance), but there is no color where all the individuals are or are not resistant. There are resistant cultivars of every color and susceptible cultivars of every color.

Foliage type is also no indicator of resistance. There is no linkage between foliage type and resistance/susceptibility. Now, it is true that dormants will always have their leaves die off every year and that kills any rust mycelium living in those leaves, while an evergreen or semi-evergreen can have the mycelium survive even in fairly cold conditions, but that has nothing to do with actual resistance or susceptibility to rust.

As well, the entire notion of “hard dormant”, “dormant”, “semi-dormant”, “semi-evergreen”, “hardy evergreen” and “tender evergreen” is highly subjective. There may be some absolutes, but these categories commonly used in the hobby are more of a spectrum than a set of absolutes. For instance, I have purchased cultivars that are “hard dormants” where they were bred in locations north of me, only to have them display as “semi-evergreen” or even “evergreen” in my garden. 

As well, there are many examples of cultivars that are “evergreen” and “semi-evergreen” in southern climates that behave as “semi-evergreen”, semi-dormant”, “dormant” or even “hard dormant” when grown in northern climates. While the original species may in some instances be truly evergreen or dormant in all climates, even in some of those (fulva “evergreens” come to mind as do citrina clones) their foliage performance will depend a great deal on where they are grown. Since our modern cultivars are an amalgamation of many species, we can expect the genes of foliage type to be very mixed with great variability seen throughout the many climate ranges.

I would mention from personal experience that I have seen just as much susceptibility from northern dormants and semi-evergreens as I have from southern evergreens (or southern semi-evergreens or southern bred dormants, for that matter). In fact, I would say that susceptibility is much more normal from all locations than is resistance. In the first years of assembling a breeding colony of modern daylily cultivars, I bought a great many daylilies from breeders in the far north because I visually prefer the dormant foliage (all foliage types live well for me, but dormant looks cleaner and neater to me) and because I felt bad for all the northern collectors and growers who couldn’t get good dormants with nice, modern faces and I wanted to work on combining northern hard dormants with southern pretty faces in order to make daylilies that would be more useful for northern growers, but as soon as rust appeared, the vast majority of those hardy northern dormants rusted up and only got worse through the entire season. 

While their leaves died back that winter and the rust was gone, many of the plants were so weakened by the heavy rust of the previous year due to also having low tolerance that they performed very poorly with lowered fan number, bud counts, and branching the next year. As well, a great many of their seedlings were also highly susceptible, like having produced like in those cases. Most of these were removed from any breeding consideration with a few being used in salvage projects, while others that I retained are now here merely to use to ensure heavy rust to inoculate my seedlings when rust does appear.

With that said though, there have been a few northern bred daylilies that have shown excellent resistance to rust and I am using those heavily, so there is no way to say that “all southern daylilies are susceptible or resistant” or that “all northern daylilies are susceptible or resistant”. I would point out though that those cultivars from northern breeding that have shown good resistance often descend from other cultivars known to show resistance, even though no active selection for resistance had been applied due to lack of rust exposure. Further, I would also point out that I have obtained a great many daylilies from southern breeders that do show good to high resistance and are evergreen or semi-evergreen, but I have sought them out as there are many southern breeders who are making some selection for resistance in their breeding work, even if just through the very passive act of culling out the worst of their rust magnets, both in their breeders and in their seedlings. 

Both examples from north and south indicate that passive selection and removing rust magnets, rather than actively selecting for highly resistant or immune seedlings, can still be of benefit in a breeding program and that just the act of using a known resistant cultivar can increase the odds of producing rust resistance, even in situations where no selection pressures from rust infection are possible. So let us now consider the importance of identifying recurrently resistant cultivars.

In the years since rust has appeared in the US, there have been some small number of cultivars observed to not develop rust over a wide area of the country and for a long period of time. When this is the case, it is likely that these plants are strongly resistant and there is a chance that they may show resistance to more than one strain of rust. Nothing of that sort is proven, but the long term resistance seen in some cultivars, growing in many different locations, is encouraging and is perhaps the best point to begin in a quest for materials to use in breeding more resistant plants. Identifying such cultivars is a priority as while still anecdotal, numerous reports build a strong case toward the reality of their resistance. 

It doesn’t matter how old or simple the cultivar, though many may be neither old nor simple. All that matters is that those showing long-term resistance in many locations give the breeder a jumping-off point for potentially increasing the levels of resistance in their own seedling population. A further point is that when such cultivars also have many offspring that show recurrent resistance in many locations, we can then say that such cultivars and their family lines may represent a high level of breeding value for resistance.

Such cultivars, where they have been noted, need to be given more than a cursory glance. Even for a cultivar that is regularly immune to rust, one or more reports of some rust are to be expected. It is to be expected that even the most rust resistant may exhibit some level of rust sporation if there are other environmental stressors at work to lower their defenses. So we shouldn’t be too freaked out when the “proven resistant” cultivar shows up with rust in a given garden or a given year. Such a situation may also suggest that the individual has encountered a different strain of rust, and it is now confirmed that there are multiple strains of rust in the US.

Once large enough numbers of tests have been carried out in a wide range of conditions, an idea of overall resistance/susceptibility evaluation may be possible. In either case, I feel it is important to begin to identify truly, genetically based resistant individuals (as in ‘can reproduce the trait reliably’) and test them in both field and lab conditions to obtain further proof of their resistance, but most of all, I feel it is important for breeders to be using these cultivars and beginning to develop their own lines utilizing such individuals as part of their project base. 

The more anecdotal evidence that can be gathered, the better an insight we can gain into the rust resistance and potential breeding value of a given cultivar for rust resistance and this could then also build a foundation that professional researchers could use to establish proven resistance under proper controlled conditions. As professional researchers do more research, giving us a greater understanding of both rust strains and resistance factors, the greater our ability to breed for resistance will become, but at the same time, we mustn’t undervalue the observations of the hobbyist breeders. Both the anecdotal information of hobbyists and the empirical information of researchers are important in the pursuit of knowledge.

Without identifying those cultivars that actually show resistance, there is little clear direction for materials acquisition and breeding schemes to be formed. If there is genetic resistance to rust, then it will be heritable, and so determining those individuals that show the trait and can reproduce the trait is the first key in actively breeding for rust resistance. My experience and the information I have gathered from many sources (all anecdotes of course) have proven to my satisfaction that rust resistance has a genetic and heritable basis. This has not been ‘proven’ yet in any way by professional researchers that has been made available to the public at large, so only time will tell if we will have that ‘proof’, but I personally feel confident in proceeding with the certainty that there is a genetic basis to resistance. You, of course, will have to decide for yourself, but it seems that simply integrating one or two plants known to show high resistance in many areas surely couldn’t present a hugely undue burden to most breeders, unless their space was extremely limited.

There are many cultivars that have been stated to show resistance. Some of those reports are single-garden reports for one or more years. Other reports come from university studies. Some reports are derived from surveys and others come from scouring the Internet and daylily message boards as well as from personal communications. While none of this translates into “proof”, there are some trends that to me seem suggestive of actual resistance amongst some cultivars. There are a few cultivars that seem to show resistance in both field reports and university trials. I believe they may represent a jumping-off point for further testing and for hobbyist breeding projects. The scores for resistance do not always agree within all the tests, but there is an overall trend toward resistance in all evaluations of a handful of cultivars across several lists, and often with anecdotal reports lending credence, as well.

A final point I want to make in this vein is that I feel it is very important to work to identify newer and more modern types of daylilies that show good levels of rust resistance in order to have modern resistant material to work with and to cross to older resistant material to bring more modern traits in the resultant lines produced. Getting back to fully modern phenotypes will go much faster if there are very modern resistant cultivars to use from the beginning of such a selection project. Identifying even a tiny number of such cultivars would be very helpful and I am glad to say that some do exist. We will look at the issue of resistant cultivars in more detail in a later posting in this series.

Some Thoughts and Observations on Who Makes A Good Resistance Breeder

We could easily say that the best person to breed for resistance to rust lives in the deep south where rust is present the majority of the year, and that the person least suited lives in the far north where rust (is so far, nearly) never seen. However, that is not really what I mean by the title of this section. Instead, I want to focus in this section on what I feel is required, personality-wise, to actually pursue a program of selection toward disease resistance.

Beyond the observation and physical work required, my experience suggests that a certain mindset is required to pursue resistance breeding in an active fashion. That mindset requires focus and dedication, but perhaps the most important attribute required is detachment and/or a low threshold of sentimentality, and a large dose of practicality seems to help as well.

It seems to be ‘Murphy’s Law’ that your favorite individuals, the rarest individuals and the most expensive individuals, seem to show a higher frequency of low tolerance/low resistance to any given pathogen, regardless of the organism you are working with. For someone who is not very detached or is very sentimental, excluding such an individual from breeding may be emotionally very difficult if not impossible. Further, culling out such an individual may be unthinkable. With daylilies, culling out highly susceptible individuals may not really be necessary but excluding them from breeding, in many cases, may be. Exceptions would include such an individual being part of a ‘salvage project’ or in instances where northern breeders who simply can’t engage in a fully active resistance-breeding program and/or cannot fully or consistently test all their breeders and seedlings for resistance are pursuing a more passive system of increasing resistance.

With resistance breeding, maintaining highly susceptible individuals is important to keep a high level of rust in the garden for inoculation of each generation of seedlings, but it is also then important not to spray such susceptible individuals, or to at the very least not spray them in the fall. Even this may be more than some individuals can bear. As well, when a special individual seedling or cultivar is shown to have high susceptibility, it may be used in what I call a ‘salvage project’, which seeks to bring in the desired flower phenotype traits but recombined with resistance in a future generation (often not the first generation either). We will discuss this type of project later.

Beyond sentimental or very advanced cultivars, the culling of seedlings is a necessity. It is clear to anyone who has ever raised even a few daylilies from seed that you simply can’t keep them all. Anyone who has pursued daylily breeding with any seriousness will be used to culling seedlings, but they may not be used to culling seedlings with the most wonderful faces based upon plant traits. Rust is most certainly a ‘plant trait’ in that it does not make those beautiful faces any less beautiful, generally. It only disfigures the plant. So it may be very emotionally distressing to cull the most beautiful faces because a plant is rust prone and it may be equally distressing to not use plants in breeding that have exceptional flower traits, or that represent a heavy investment of cash, but after all, you can only really deal with so many salvage projects before you become overrun by the huge numbers of seedlings such work will generate. 

Resistance breeding requires a certain detachment to these things and an ability to drop sentimentality and cull where necessary and ‘salvage projects’ should remain special cases. Not everyone can do this and one must do some soul-searching before they begin to pursue such a program. The question one needs to ask oneself is, “Can I really cull and if so, can I cull or not breed from expensive, stunning, rare or advanced individuals?” If the answer is no, then perhaps this type of breeding is best left to those who can answer yes to that question, or a more passive program is most suitable for you.  

There is no shame in not being able to pursue such a program. In fact, I would suspect most breeders of any given organism wouldn’t find it easy. After all, who really wants to be immersed in disease and its unattractive manifestations when they can just ignore it or hide it with treatments and focus on beautiful flowers and/or other phenotypes? I want to stress that you need to be sure you can handle a resistance breeding program emotionally before you start one. Not everyone is cut out for it and that is fine. I have absolutely no judgment of anyone who isn’t. From my own experiences, I can attest that at times, such a program can be harrowing and emotionally draining, but in the end, is very worthwhile.

What I would suggest for those who do not feel up to undertaking such a project is that they simply attempt to keep informed about highly resistant individuals/lineages and incorporate those into their own breeding programs where possible. While this will not ensure resistance in all the offspring from such individuals, it does increase the likelihood that some resistance factors are entering those gene pools. This is the essence of a passive project, and while it isn’t resistance breeding, it is better than doing nothing and simply ignoring the problem. At the very least, you are educating yourself about what others are finding rust resistant, and you may even be able to send your seedlings south for further evaluation in very rusty settings, as well. If one or more of your own seedlings then turn out to show high resistance, all the better, and using such resistant seedlings of your own creation should not only be palatable, but joyful.

Finally, I want to stress that I do not advocate that all people should be following one goal or one system of breeding. We need diversity and it is the diversity of directions in the daylily world that has brought us the stunning and fast advances we see today. We need people who are just focused on the flower, creating advances that can then be incorporated into hardier or more practical lines, and we need people working on lines with more focus on plant traits including disease resistance. My point here is simply to instill an awareness of resistance and known resistant cultivars so that all breeders have more tools at hand regardless of their chosen direction.

Exposure Is a Necessity

Here is a point we need to look at for a moment. When breeding for resistance, it is important to have highly susceptible plants around to be certain your plants (cultivars, breeders and seedlings) are being exposed to sufficient levels of rust spores to ensure their inoculation. While a gardener who is not breeding may want to remove the most susceptible plants to ensure that they have low sporation, both to cut down on inoculation and to keep a more aesthetically pleasing garden, the breeder will want to do just the opposite.

I can’t stress how important this is, as some cultivars will not easily contract rust, even though they develop high spore levels when they do contract it. In some instances, a given plant may not develop rust unless it is given heavy exposure to spores, while others seem to literally suck rust out of the air and be dripping spores seemingly overnight. In order to identify those that are difficult to infect, but have high susceptibility when infected, they must be exposed to high spore levels. Thus for an effective resistance breeding program, there must be highly susceptible individuals maintained solely to ensure inoculation.

One of the great advantages in this for the breeder is that it then requires that some of the most susceptible be maintained. These can be those cultivars or seedlings with other good qualities that can be part of a salvage project, they can be sentimental favorites that you want to keep in spite of their high susceptibility or they can the expensive latest-and-greatest you just spent a fortune on only to find out they are highly susceptible rust magnets (and then maybe they can be a salvage project plant also).

This is probably the greatest difference between the breeder and the collector or average gardener in terms of how they manage the plants they grow. The collector and average gardener wants to reduce sporation, while the active resistance breeder wants to increase sporation to identify those that show the highest resistance under the highest level of sporation. However, for the person working with a more passive project, they may choose to not go to either extreme, not seeking to reduce or increase spore levels. As they are not actively seeking those with the most resistance under extreme spore conditions, but rather are just looking to remove those that are most susceptible under their garden conditions.

In the next post, we will begin to look at actual breeding methods, schemes and systems that can be used in the resistance breeding program...