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