Plant disease resistance is a term used to describe the reduction of pathogen growth on plants to minimize damage from the pathogen and potential yield loss. Qualitative resistance comes from a major gene or genes and provides a clear benefit when it matches the pathogen race present. Quantitative resistance can come from many genes and is often considered a baseline of protection from major losses. Disease can still occur with resistance but resistance traits should mean the plant has lower disease severity and yield loss than plants that are susceptible.
So how can we lose a resistance trait? For example, how does a clubroot-resistant (CR) variety work one year and not the next?
Resistance breakdown occurs because of:
1. Natural selection. One clubroot gall produces millions to billions of resting spores and that population of spores can contain multiple Plasmodiophora brassicae pathotypes. However, one or two pathotypes tend to be dominant across a field, with other pathotypes present only at low levels. First generation CR varieties are resistant to the most common pathotypes across the Prairies. Natural selection can occur with repeated use of the same type of resistance, which will keep the common pathotypes from increasing but opens the door for rapid increases of some less common pathotypes that were present in the field at low levels. Over time, these less common pathotypes could become the new dominant pathotypes in the population. This is the “selection pressure” that makes natural selection work on clubroot. Through repeated use of the same resistance sources (especially in short rotations), you select for the pathotypes that can overcome the resistance used.
2. Mutation. This refers to a permanent change in the DNA (nucleotide sequence) of an organism, which can be passed on to its progeny. Mutations can occur randomly, and while most are not advantageous (and are often harmful), occasionally a random mutation confers some advantage to the organism in question. For example, a mutation in a P. brassicae could result in a brand new pathotype developing which can cause infection in a resistant canola variety. Mutations are one way that diversity can be introduced into a pathogen population, but mutations like this are rare and slow.
3. Recombination. This refers to the exchange of genes during sexual reproduction (chromosomal crossover) that leads to the offspring having different gene combinations from the parents. This can also add diversity to the pathogen population and result in the emergence of new strains. While recombination is possible with other diseases, this is not believed to occur with P. brassicae.
Other common terms used in a discussion on pathogens and resistance:
Race/strain/pathotype. These terms are often used interchangeably in common language and in general refer to subgroups of a pathogen that can be distinguished based on virulence, symptom expression or host range, but not by morphological features. Usually “race” is the most precise of these definitions, and often specifically implies the presence or absence of specific avirulence genes in the pathogen that interact with matching resistance genes in the host (so they’re often based on the clearly defined genetics of the interaction). “Pathotype” is a somewhat looser term, since these are often distinguished based on their virulence (ability to cause disease) and/or the type of symptoms they cause, while the molecular or genetic basis of the interaction may not be known. Finally, “strain” is the loosest of the terms, implying groups that are different in some way (e.g., virulence or some other parameter) but which are not always defined by any agreed upon system (for example: we recognized that there were multiple strains of P. brassicae, but needed to develop the CCD to classify them into pathotypes).
Isolate. This is a subpopulation of a pathogen or other microogranism that is separated from its parent population (for example, by collecting a single-spore or single-gall) and maintained under controlled conditions.
Genotype and phenotype. The genotype is the set of genes responsible for particular trait – the genetics present. Phenotype is the physical expression of a trait – what it looks like in the field this year.
Pathogen. This refers to the agent that causes a disease. Sometimes use of the terms “pathogen” and “disease” are confused. The pathogen causes the disease. The disease is the result of the pathogen, host and conducive environment coming together. For example, P. brassicae is the pathogen that causes clubroot. So that means that P. brassicae is what infects canola. Clubroot is the name of the disease that results from infection.
–Thanks to Steve Strelkov at the University of Alberta for help with these terms.