Monday, November 18, 2013
Breeding For Rust Resistance In Daylilies:
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.