Do disease-resistant wheat varieties pay a price in yield?
For wheat growers, it’s a truism: Plant varieties that are resistant to prevalent diseases. But what if the wheat plant has to pay a price for resistance, possibly reducing its yield? Is the resistance worth it?
It’s not easy to detect any possible negative effects of resistance genes on yield. Any two wheat varieties you can imagine will very likely differ in their geographical adaptation, yield potential, and reaction to the range of Central Plains diseases. If, for example, you happen to compare a specific stripe rust-resistant variety with a susceptible variety, the resistant one might yield less than the susceptible one in a year without stripe rust. Depending on which ones you’ve picked, it could also yield more. However, that does not mean that resistance causes lower yield. The two varieties differ not only in that stripe-rust gene, but also carry contrasting genes throughout their genomes. Any of those genetic differences could contribute to a difference in yield potential.
Having said that, there is a legitimate question as to whether resistance genes themselves may have a yield effect. The theoretical basis for this comes from plant species in nature. Plant populations are highly variable when it comes to disease reaction, with some plants having more resistance genes and some fewer, and it’s a different situation for each disease. Therefore, ecologists and evolutionists have long reasoned that resistance to pathogens must come at a cost to a plant’s fitness; otherwise, natural selection over many generations would have driven populations toward accumulating every available resistance gene, and plants would be uniformly resistant. That’s not the case. So in crop species, if the goal of artificial selection — the plant breeder's full-time job — is to accumulate resistance genes in crop populations, will genetic yield potential suffer?
Answering that question requires field experiments to estimate the impact of resistance on yield. To be valid, such experiments must compare lines of wheat that are almost identical in their genetic makeup, except for the resistance gene(s) being tested. Either that or the experiment must compare two groups of lines from the same population, one with and one without the gene, so the genetic backgrounds would cancel out. Then these lines must be yield-tested in a replicated, randomized experiment in which the disease in question is totally absent or, better, in which the plots are chemically protected from disease.