Wednesday, January 2, 2019

Rust Resistance A Few Words

Rust Resistance: 
A Few Words
(January 2, 2019)

In the five year rust resistance screening I performed from 2012 through 2016, I was able to observe a large number of cultivars and seedlings under late summer-fall rust. I made observations followed by selections. Selection aimed toward those most consistently rust resistant, showing both the greatest resistance to rust year-after-year combined with high breeding value for rust resistance in my garden.

My article for the American Hemerocallis Society, Daylily Journal: Spring 2018 issue titled "Breeding and Selecting for Daylily Rust Resistance: Anecdotes From One Gardener's Experience"details the five year rust screening program I did from summer 2012 through fall 2016. I outline the most important aspects of such a program, drawing from my own experience and from the scientific literature on breeding for rust resistance in other plant genera. To read that article, Click Here.

Throughout that five year cycle, there were a number of cultivars and seedlings that were present in 2012 and so went through all five years of my rust resistance screening. Both cultivars and seedlings (including some of my introductions) that consistently showed extremely high resistance (seeming-immunity) through all five years, and significant breeding value for resistant offspring, are of special interest for future testing and breeding for those interested in working toward greater rust resistance in their daylily lines.

I understand why people would be very concerned about rust resistance in climates where mild winters makes for endemic rust, and honestly, much of my work with rust resistance selection and breeding has focused more toward evergreen and semi-evergreen breeders, as I could find the most information on plants growing in areas of endemic rust. While I did find cultivars and family lines of plants that show various levels of dormancy with high rust resistance, most of the resistant plants turned out to be a touch tender for me, but I have persisted with these, as the people who need rust resistance the most will also need plants that can survive their milder-winter climates. 

Many of my introductions for rust resistance, especially amongst the four and five-year tested plants, will be semi-evergreens that have been selected for performance and hardiness here, but that are clearly of the growth pattern to do even better further south, where they can continue to be tested for resistance (hopefully gathering some of that data!) and where their genetic potential can be put to use by people who really need it in their programs and their gardens.

That isn't to say that I don't have cold-winter-climate-loving, hardy, fully deciduous, dormant, resting-bud-type plants with rust resistance. I do, and I felt it was important to impart as much resistance into this type of my base plants as possible, and so there are a handful of plants of this foliage behavior that also show extremely high resistance. Having rust resistance in northern hardy plants couldn't hurt them, after all! :-) But, for the most part, the rust resistance selection material skews toward less extremely dormant plants, and more toward semi-dormant, semi-evergreen and evergreen plants, as the bulk of need and interest will fall more toward those plants.

As I mentioned above, there were five years in total in my rust resistance screening program: 2012, 2013, 2014, 2015 and 2016. In each of these years, there were new seedlings, so that there were five waves of testing, with plants ranging from one-year tested, all the way up to the five-year tested. 

I feel most confident that any of the plants that were five-year tested and consistently showed extremely high resistance have good provenance as breeding material for rust resistance, even if in time they have a failure in the face of new rust strains. The presence of a new rust strain doesn't mean old strains are gone for good, and old strains, for which a tested plant likely still has resistance, may reappear. Many of the most highly resistant plants I tested have also shown extremely high resistance in other gardens, some for over a decade, and a few (like H. fulva 'Korean) going back to the beginning of rust in the United States nineteen years ago. 

Plants with such provenance deserve to be central to continued rust resistance breeding efforts, and in many cases, already are, as some of these resistant cultivars have been know to be resistant for well over a decade. The seedlings that I am most interested in seeing tested further from my own program, in rust-endemic gardens, are those that went through five or four years of screening. I would like to know if that was an artifact of the environment, or if there is consistency in other locations. Reports from some early testing in gardens with endemic rust suggest that the resistance is high in other gardens with some of these.

The seedlings that were tested between one and three years all have lesser provenance, in my opinion. They are certainly not useless or valueless, and will make up the bulk of fancy faces I take back over the base plants, but undoubtedly some of them will yet fail and show lower resistance to some strains of rust. That may not be a problem in all instances. I would expect only a small percentage would turn out to have the longterm resistance that I so value, but some few most likely will. I hope growers of my plants can help to uncover those in their gardens!

Tested Level
5-year (highest level of testing)
4-year (high level)
3-year (Moderate level)
2-year (Moderately Low level)
1-year (low level)

The length of time during which a given plant was tested is not the same as the level of resistance it showed.

Plant rust resistance levels: 
As presented in a chart from the above-mentioned AHS Daylily Journal article:

Daylily Rust Resistance Rating System
Lesion coverage
Very high
No visible susceptibility
1% - 25%
26% - 50%
51% - 75%
Very low
Very high

I consider both ratings, with a five-year tested A+ ("Extremely High Resistance") being much more noteworthy than a one-year tested A+ ("Very High Resistance"). A five-year tested B-level plant ("Moderate Resistance") may be much more noteworthy than a two-year tested A+ ("Very High Resistance"). To me, the long, consistent resistance is much more impressive than very high resistance that doesn't hold up year after year. 

Each year as I screened plants through the late summer and early fall, what I was paying the greatest attention to was those plants that displayed extremely high resistance, and who's seedlings did the same, with each new round of rust I introduced each spring. 

Each year, some things had to be lowered in rating. Each new round of rust I brought in might bring different strains, or some artifact of the environment could have skewed my results toward higher apparent resistance, but not in fact be stable or genetic. Just because something proved to need to be lowered in rating, didn't mean it always ruled it out for breeding. That all depended on how much of a decline in resistance actually occurred, how its seedlings behaved and if the plant and/or flower was something I thought was exceptional enough to keep with lower resistance. In most instances it wasn't, but in some it was.

So the plants that showed total resistance through all five years decreased in number each year, and in the end, those that had remained resistant throughout tended to be a handful of cultivars and species clones, and their offspring (from crosses with each other or to other things with less resistance). These are the core of my base plants. 

I am now beginning to release some of these rust-resistance-screened daylilies and will introduce more over the next few years. These plants should go out to continue being tested and having their seedlings tested, in turn, for rust resistance to help establish breeding-value levels in multiple locations. From my first introductions in 2016 to those of this year, there are some of these five-year and four-year rust resistance tested seedlings amongst the introductions, and it will be clearly spelled out in the description of that cultivar. I will continue to introduce some of the best plants and flowers from my entire rust screening program over the next few years. 

It is inevitable that most daylilies will fail to show resistance against some future mutation of daylily rust. In wheat, the longest-lasting rust gene know has begun to fail after fifty years of extended resistance. There are daylily clones and cultivar that have shown high resistance from the start, and are still showing it. There are multiple cultivars and family lines that show variable resistance, ranging from high to very high in most situations, for a decade or more. There are a few highly resistant species clones that show provenance of resistance back to the early 2000s when rust had first appeared in the US. Some forms of rust resistance in the US seems to indicate longterm resistance is present in the North American population of Hemerocallis. Future work needs to continue to identify both new mutations for resistance and new mutations of the rust fungus itself.

When rust resistance levels appear to fail, it is important to watch the actual level of change, as it frequently is not a huge variation from previous levels. I have never seen a cultivar or seedling that has previously been seemingly-immune, which then suddenly became highly susceptible the next year. The charts in Buck, et al 2013 would seem to suggest this as well. While total resistance (seeming-immunity) is nice, and would be ideal, plants with lesser levels of resistance can still be useful, both for breeding (if proven to carry recessive resistance genes) and for growing in gardens (lessened spore production). I personally prefer plants that range in the A+ to A to mid-B rating levels. I must stress though, if you want to select for resistance you need to maintain plenty of rusty plants to produce a high spore load in the garden, but in a flower garden the presence of highly resistant to moderately resistant plants can result in very light spore production and a good garden display even in rust-endemic areas.

Click to go to My Introductions (Scroll down for list of cultivar information page links)

Click to go to Rust Resistance and Tolerance Cultivar Listing (Scroll down for list with links)

Click to go to Other Hybridizer's Cultivars Information Pages (Scroll down for links - list rust resistance in many of the information pages)

Click to go to Rust Resistance Articles Main Page (where many articles on the subject can be found)

Click to go to Monday Night Lights 2018 Diploid Breeding Program Presentation (where much is discussed about rust resistance)