Breeding For Rust Resistance In Daylilies:
Part 2
In this installment, I want to focus on some basic definitions that we absolutely must have some understanding of before we can proceed further. I frequently see discussion on various forums that don't get anywhere and often because people are using the same words, but meaning different things. The greatest fallacy is that 'resistant/resistance' implies fully immunity to rust. This is not the case, as I discuss below, but the notion that this is the case seems to be one of the biggest stumbling blocks to understanding and communication.
Resistance, Immunity,
Susceptibility and Tolerance
Before we begin to talk about various strategies for
breeding for resistance, there are a few terms we need to define and discuss a
bit. As this sections header suggests, resistance and immunity are not the same
thing, and while resistance and susceptibility can encompass various points of
a spectrum, they are neither the same as tolerance. So what do I mean when I
use any of these terms.
First, let us consider immunity. This is the simplest of the
terms to define, as immunity means exactly what it implies. An individual that
shows immunity to a pathogen is not able to be infected by or show symptoms of
infection of a given pathogen. In terms of rust, this means that a cultivar
that is immune or that shows immunity does not display any symptoms of rust to
the naked eye. Under a microscope or in lab conditions, rust may be found to be
present, but such an individual will not show the visible effects of rust –
spores or active, visible rust.
Some individual plants may show immunity due to
environmental reasons, but others may be due to an actual gene or genes giving
full immunity to rust. This is likely rare. More rare would be complete
immunity to multiple strains of rust, so while cultivar may prove to be immune
to one strain of rust, it may not be immune to another, and the level of
susceptibility to any given strain could vary, even in the individual plant is
highly immune to one or two strains of rust. While full immunity to multiple
strains of rust is likely very rare, if it even exists, there is always the
chance that it could exist. Without gathering information on individual clones
over many locations and many years, the discovery of such a multi-immune plant
will be very difficult.
In many plants and animals, total immunity is often a
single, dominant gene expression and is frequently more easily circumvented by
new mutations of the pathogen than multigenic resistance. However, if a single
dominant gene that confers immunity (or high resistance) is combined with
multiple other genes giving resistance in various levels, that can create a
very strong level of immunity/resistance that can be both difficult for a
pathogen to breach and offer some resistance to multiple strains of a given
pathogen. Thus these single gene dominants should not be ignored and make an
important component of a resistance-breeding program.
Now let us consider resistance and susceptibility together,
as they go hand-in-hand and are actually interchangeable terms. First,
resistance is not immunity. I can’t stress this enough. The only time that
resistance is immunity is if we were to refer to ‘total resistance’ or
‘complete resistance’. However, more frequently, resistance is aimed more at
meaning the ability to resist a pathogen to some extent, variable by cultivar
or clone, though certainly in the hobby, the term resistance is often used to
mean immunity by some, but then will be used to infer variable levels of
resistance, but not immunity by others. This has caused a great deal of
confusion and when the term resistance is used to imply immunity or the meaning
is left vague, it can leave people both intimidated as to the difficulty of
achieving such a goal and disheartened when a “resistant” cultivar then shows
some rust in their garden. We need to be very clear about what we are actually
meaning when we use a given term.
When dealing with multigenic, quantitative traits, we
generally use a bell curve to express the array of phenotype expressions seen.
So the two extreme points of any bell curve could be strong resistance/poor
resistance with many shadings in between, or equally, the two extremes could be
called low susceptibility/high susceptibility and be equally accurate. Said
either way, they basically are saying the same thing. However, I have heard
those who don’t want to acknowledge the importance of highly resistant plants
to the actuality of there being genetic resistance that is selectable within
the Hemerocallis suggest that because the two terms are interchangeable, that
means that all daylilies get rust and even if you have one that is highly
resistant and you see a few spores on it, then you still have rust, so it is
really no better than the most susceptible rust magnet. This is complete
nonsense, of course and is usually being stated by persons selling or breeding
and selling daylilies. Any grower who has ever experienced rust knows that the
visual effect, as well as the effect on the health of the plants, is profoundly
different between a cultivar that gets a few rust spores and one that is such a
magnet as to become an orange rust-fountain. We will discuss this more a little
later in this section when we look at realistic goals for our breeding
projects. Suffice to say for now that ‘resistance’ and ‘susceptibility’ is
interchangeable terms that describe a wide range of expressions, but not
necessarily immunity.
Tolerance is a somewhat different term that means the
ability to be infected by a pathogen, express the pathogen, and yet show little
negative impact on health, vigor or performance. In short for our example, it
is the ability of a plant to have rust, show sporation and yet have little
negative impact on the performance and/or health of the given plant, even in
some instances when the rust expression is extreme (low resistance or high
susceptibility). This is also an important point in breeding consideration and
one that is often overlooked. There are plants that will contract rust and show
strong negative effects either the year that rust is present and/or in the next
growing season, even if the rust is eradicated by winter freezes or spraying.
These plants may show symptoms such as reduced scapes count and/or height, poor
bud count, poor branching, poor plant habits, reduced increase or fan-count, or
even withering of the overall plant size along with several of the other
expressions mentioned here. In extreme cases, some plants are so compromised by
rust that they never fully recover or perform at full capacity again, and in
areas where rust is seen in both spring and fall, such plants may become so
compromised that they die outright, even if their other cultural needs are
being met.
In our breeder selection and breeding consideration,
especially where we want to pursue a ‘salvage project’ with a very susceptible
plant, we may make better strides by using a highly susceptible plant that
shows high tolerance to the presence of rust, rather than one that is highly
compromised by rust infection. Then, at the very least, even if we do not make
great strides toward high resistance in the first generation, our f1 should at
least flourish and survive to make an F2 or BC1 (back cross first generation)
and perhaps then draw together the flower traits we want with higher
resistance. In short, tolerance to rust can’t hurt and likely will help, but
low tolerance to rust can be deadly and certainly creates more obstacles to
overcome.
