Wednesday, December 25, 2013

Breeding for Rust Resistance in Daylilies: Part 7

Breeding for Rust Resistance in Daylilies: Part 7

In this installment we will look at techniques of rating rust resistance and susceptibility. I want to stress that this is not a comprehensive coverage of the subject, but is a look at the techniques I use and how those techniques compare to the methods of various other breeders. A comprehensive discussion of techniques and methods of rating rust resistance in daylilies is beyond the scope of a simple blog. This is a complex subject and I hope this small overview can spur further thought and discussion.

Rating Resistance/Susceptibility Levels

This is a complicated issue. The rating of resistance or susceptibility levels is subjective and there is no formal method for evaluation that has been devised which is universal for Hemerocallis. Each person I know doing a resistance selection program has a slightly different rating method, though they are all similar. As well, rust itself can be very variable. In some cultivars, rust is only on the undersides of the leaves. On others, it is on the underside and the upper side of the leaves. This can severally complicate the evaluation process. In time, I would love to see a formal set of working guidelines established for rust sporation evaluation. In the meantime, I will set out here some loose, informal guidelines based on how I and a handful of other breeders who I am in contact with, tend to evaluate for resistance levels.

Before we discuss actual rating scales and systems, I need to stress that the subjective aspect is that people can color their observations with emotion. A great example can be found in various message board posts dealing with rust. Take for instance that one person may have a cultivar, say ‘Joan Senior’, and they post that is it ‘resistant’, then a few posts later another person pops in and posts that “’Joan Senior’ isn’t resistant because it got rust in their garden". The problems here are many. First, while the one person saw no rust, that doesn’t mean it is necessarily “resistant” or “immune”, though it seemed to be in that situation. On the other hand, because the other person saw some rust doesn’t mean that it is ‘not resistant’ either. The later person doesn’t in any way indicate the level of rust seen with such a loose statement, and seems to be equating ‘resistant’ with ‘immune’ as we have discussed earlier when defining these terms.

My experience of the diploid version of ‘Joan Senior’ mirrors that of many other people who are actively selecting for rust and regularly evaluating plants – it shows high resistance, but it isn’t immune. In other words, it can get some rust, and often does in severe outbreak seasons, but it doesn’t get more than about 25-30% coverage in my experience and I haven't seen rust on the upper sides of its leaves. To me, that then is a resistant cultivar, but if I was deluded into thinking that any rust at all meant “no resistance” and I thought they had to have no rust to be “resistant”, I might have an emotional reaction and make a broad, subjective statement like, “It got rust here. It is not resistant.” Real resistance is a more nuanced thing than generalizations allow. Further, the misconception that the only resistant plants are those that never show rust creates an artificial expectation that is harmful to any actual realistic effort toward evaluating resistance levels and can likely lead to an emotional reaction that might color one’s ability to rate the subject.

The most important tool you can take into the garden with you during an active spore outbreak is detachment. More than anything else, detachment will help you to remain calm and not give in to emotionalism, which is very counterproductive to accurate observation. Scientists value detachment, and a few actually achieve it: those who do make great researchers. Likewise, those who can achieve some detachment when evaluating their own plants for rust will be able to achieve a more accurate and consistent rating that is closer to the reality of what the plants are actually displaying.

In my own rating style, I prefer to use a general 4 level range, which I label ‘A’ through ‘D’. I also allow for half steps (yes, I have studied music also) that are represented by a + or – symbol. The highest rating in my system is ‘A+’, which we could also call ‘apparently immune’ (I use ‘apparently’ to not imply false certainty). In this way I can actually chart about ten possibilities within the overall arc of the four main rating levels.

I know several people who use a numerical rating of 1 through 5 and a couple who use a rating of 1 through 10. Others use titles and their abbreviations such as ‘Rust resistant’ (RR) for the highest level through ‘Poor Resistance’ (PR) for the lowest with other nomenclature in-between. Others simply use phrases such as ‘excellent resistance’, ‘average resistance’, 'good resistance’, etc. The rating system at All Things Plants database uses a numerical scale of 1 through 5, but they each have many shadings between, such as 2.5, 1.3 or 3.8, etc. The university research studies I have seen tend toward a simple numerical rating system. All of these systems work and each are basically saying the same thing. Through observation and comparison, I can use most anyone’s scale and translate it to mine. Some are more detailed than mine, some less. I find a looser scale more effective, as it is very difficult to be very specific, especially in systems that are using the evaluations of many people with different situations and different ideas about what each level is.

I will now set out the parameters of my system and then you can use that as is or you can adapt it in any way you like. The best possible thing to do is to talk to many people, compare their systems and create a hybrid system that allows you to interface with many systems. That is what I did. I will also attempt to make some comparisons of other systems with mine so you can have an idea of how it interfaces with other’s nomenclature when you run across it.

The system I use is based on the first four letters of the alphabet – A, B, C, and D. In general terms, A is high resistance or low susceptibility, B is good resistance or moderate susceptibility, C is low resistance or high susceptibility and D is very low to no resistance or very high to total susceptibility. Immunity is something higher than any level of resistance, and is rated as A+. Any rust spores at all knock the A+ allegedly immune plant down to an A rated plant. An A plant is still exceptional, highly desirable and very usable in breeding. They may in fact be the most useful plants for breeding in many ways.

Now, let’s look at the numerical percentages I equate with each letter. A+ is no rust spores or 0% spore coverage on either side of any leaves on the plant, while A is 1% to 25% spore coverage on the underside of any number of leaves on the plant. B is 25% to 50% rust spore coverage on the underside of any number of leaves on the plant, while C is 50% to 75% spore coverage on the undersides of the leaves on the plant. D is 75% to 100% coverage of the underside of the leaves by rust spores.

To complicate things, rust can also appear on the upper side of the leaves of some cultivars while never appearing on the upper sides of leaves of other cultivars, regardless of how resistant or susceptible they are. To me, this seems to either be higher susceptibility or a further genetic factor that allows spore formation on the upper sides of the leaves. In either case, I don’t like it and when this occurs, that knocks the rating of the plant down by a half notch to a full notch, depending on how severe the spore coverage is on the upper side of the leaves.

For instance, if a plant shows 20% coverage on the underside of, let’s say, half its leaves, it is an A rated plant, but if that same plant also shows ten percent coverage on the upper sides of the leaves, then it becomes an A-. Further, if the same plant shows coverage as heavy or heavier (almost unseen in my experience) on the upper side of the leaves as on the underside of its leaves, it is then knocked down a full ranking to a B. Since I am not literally counting every spore and measuring the square millimeters of coverage versus uncovered areas of the leaf, this is subjective.  The key is to practice, spend a lot of time looking at infected leaves from various cultivars and compare infected leaves from various cultivars to each other to gain as impartial and subjective an eye as you can. Some people are going to be better at this than others, but I do not rule out the ability of the hobbyist to do this. After all, hobbyists often learn to observe a great deal of minutia such as petal width, teeth and edge details, shading of colors, etc. Either acting as if you can’t do it or getting emotional over rust being on a cultivar you have a given attachment to (positive or negative) will derail the process faster than anything else can and lead to (sometimes wildly) inaccurate perceptions.

I can’t stress enough how important it is in all aspects of rust to just take a deep breath and realize that the world won’t end over it. In most instances, taking a step back and realizing that the problem is manageable and that you aren’t about to be ‘wiped out’, ‘utterly ruined’ or ‘totally annihilated’ is a good first step. Then, rather than running at break-neck speed for the sprays or starting to rip things with a speck of rust out of your garden willy-nilly, you can calmly proceed to begin making observations, some careful notes and over the course of a few weeks, begin to get a feel for what cultivars are more or less effected and formulate a sensible plan from which to proceed. In that calmer mindset, you can then make the critical evaluations with an eye less colored by emotion or sensationalism.

Learning to properly evaluate the level of rust on any given cultivar is not easy, but it is also not extraordinarily difficult. It is not impossible! It can be done and I have faith that you can do it. There are some simple points to consider that will help. In some instances, the rust actually manifests in ways that can help you determine what you are seeing. It is not uncommon to see rust on fairly resistant cultivars clustered only on the tips of the undersides of the leaves, or in some instances, at the bases of the leaves. In those instances, you can fairly easily gauge how much of the underside of the leaf is covered just by measuring how long the leaf is, and then how much of the leaf is covered in rust, or you can then prepare an average from several leaves on that cultivar for even more accuracy overall. A little math is required, but it isn’t a quantum formula.

In other instances, the rust may be scattered over the entire surface of the underside of the leaves or on both sides of the leaves, much like a scattering of salt. The amount can vary widely. The easiest way to come to a conclusion of coverage in this instance is to attempt to judge how much leaf is showing through the salting of spores. This is more difficult to gauge than when the rust is restricted to a specific portion of the leaf, but it is not impossible to gauge. I hope in the future that we can come up with a series of photos showing leaves with different levels of rust to create a general visual guideline for those new to evaluating rust levels, but until that gets done, I hope these general ideas can help you to make some fairly accurate evaluation assessments in your own garden.

In comparing my system of A-B-C-D to other systems, I have found that the numeric system of 1-5 equated quite closely, with 1 = A+, 2 = A, 3 = B, 4 = C, 5 = D, generally speaking. If decimal points are used, then the various decimal points indicate shadings between the main letter/number scores, much as I use a plus or minus. When 1-10 is used, it seems that 1 = roughly A+, 2-4 is about A, 4-6 is about B, 6-8 is about C and 8-10 is about D, approximately. For those systems that use descriptions of resistance I find that Rust Resistant (RR) = A+, Highly Resistant (HR)= A-B+, Moderate Resistance = B-C and Poor Resistance (PR) = C- to D. In other description systems Excellent Rust Resistance seems to be similar to the upper end of my A range, Very Good Rust Resistance is in the A-B range, Good Rust Resistance seems to be about my B level, Average Rust Resistance is about C and Poor Rust Resistance is about the D level in my rating system.

Before we finish this section, we must consider some other eccentricities of rust that will have a bearing on evaluating resistance levels of various cultivars.
First, rust infestation can progress throughout the season, so what may look like an ‘A’ rating in August could well be a ‘C’ or ‘D’ rating by the end of October. It is thus important to start weekly or twice monthly evaluations from the first appearance of rust up through the first killing frost (in climates with enough cold to get killing frosts) when rust sporation will end for that season. There can be tremendous variation in the actual final resistance/susceptibility level and that which the subject starts out with. Some very susceptible cultivars will have heavy spore coverage from the first appearance of rust. Others only gradual increase to heavy spore levels. If I had to choose a plant that shows heavy spore coverage for breeding, I would prefer to use the individual that has slower progression of coverage to heavy levels.

Those that spore heavily almost as soon as rust appears may be the least suitable selection for breeding, as they show the most susceptibility and due to the heavy sporation from the beginning of spore appearance, these cultivars often exhibit a strong impact on vigor and subsequent performance, showing low tolerance to rust, while those that gradually move from light spore coverage to heavy spore coverage over a few months seem to have less impact on their vigor and performance due to the rust infection and would be examples of higher tolerance to rust. In the most tropical regions of the country, these susceptible plants with low tolerance to rust can even dwindle and die from the long-term impact of rust infestation. It is very important to regularly evaluate the levels of rust throughout your rust cycle, but it is also important, where highly susceptible cultivars are not culled out of the garden (as in a breeding program for resistance where some rusty individuals are required to inoculate the seedlings each year) to evaluate their performance the next season to see if their vigor and performance is heavily impacted by the heavy rust infestation of the previous year.

Another point that we often hear is that some will say, well, this or that cultivar was highly resistant or immune last year, but this year it is showing a lot more rust. I suspect that a given cultivar does not build up or loose its genetic resistance to the same strain of rust over time. I suspect in such instances there is either environmental difference between the years that have impacted the plant(s) in question, or you are seeing variable resistance from one year to the next because of the presence of different strains of rust in those years. Certainly, those who regularly spray cannot make a qualified statement on resistance levels at all, nor make observations about the levels of resistance in a given cultivar or clone from year to year. As the years wear on and we keep moving plants around throughout the country, we will see more and more strains of rust in our areas. Hopefully there will be some labs that will offer testing for strain identification in the future.

One final point I want to touch on is that any garden can have variable environmental factors, while different gardens will likely represent environmental differences as will different regions around the continent. Environment can influence the expression of resistance. One report from one year from one garden is highly unreliable. Multiple reports representing many gardens gives a much better chance of understanding actual resistance. When multiple years are also factored in from multiple gardens, that gives a much better picture of actual resistance. With that said though, even one garden can make for better evaluation by using multiple clumps of the same cultivar or seedling scattered throughout the garden. Both multiple clumps and random arrangement, even within one garden, gives a much better insight into the actual resistance of a given plant and helps to neutralize environmental effects. This is but one technique to help eliminate environmental effects and come to a better evaluation of actual resistance. There are also other techniques used by professional researchers to formulate statistical models that can help to eliminate environmental effects.

The evaluation of rust has not yet been standardized, and that is a shame, but we must move forward anyhow, learning to evaluate rust on our own, if necessary. Trust me, after a few seasons of closely observing rust outbreaks and spore coverage, you will develop a keen eye for the various manifestations of spore levels and you will begin to know the range of acceptable sporation (resistance/susceptibility) that you can live with and want to aim for in your own breeding work. Keeping some sort of record system is also very helpful, in order to compare resistance levels from one year to the next in order to gain an overall impression of resistance in any given cultivar and to perhaps also gain some insight into those years when anomalous results may indicate the presence of an alternate strain of rust. In closing I can’t stress how important it is to not freak-out the first time you get an outbreak and to proceed cautiously and carefully, observing with the most detached stance you can achieve. Make observations and evaluate your plants, It doesn’t matter if you did the evaluation ‘right’, just that you are making the attempt and at least learning the variations of resistance/susceptibility and tolerance in your own garden. That is the first step and all else follows from there. 

In the next installment we will look at many points that don't conveniently fall into one particular subject heading...

Merry Christmas!