Do disease-resistant wheat varieties pay a price in yield?
For example, the experiments that demonstrated Lr34’s effect in spring wheat found that with no leaf rust infection, the line that carried the gene yielded 6 percent less than the one that did not. But with leaf rust infection, the disease hurt the yield of the line with Lr34 by just 15 percent, compared with 43 to 84 percent losses in the line without Lr34. At K-State in the 1990s, my colleagues and I found that a leaf-rust gene transferred from Ae. tauschii provided a 42 percent yield advantage under heavy leaf rust, while it had no yield-depressing effect when leaf rust was barred by fungicide.
You never know at planting time which diseases will be the biggest threats over the coming season, but for diseases common in your area, the possibility that a gene may have a modest negative effect on yield in the absence of disease is probably less important than the risk of taking a much bigger hit to yield, test weight, and quality that comes with sowing a susceptible variety.
A final note: Wheat breeding programs around the Great Plains, the country, and the world expend extraordinary efforts on building genetic resistance into the varieties they release. As a result, could diseases have had another, less obvious effect on wheat’s productivity? That could be the case if resistance breeding has occupied much of the time, effort, and funding that wheat breeders could otherwise have spent on increasing yield.
If that’s true, as some have suggested, maybe breeders should focus their efforts solely on yield and quality, develop susceptible varieties, and let the grower use fungicides to deal with diseases. But the negatives of spraying, such as heavy expense, environmental hazard, and difficulties of timing and effectiveness, easily outweigh the relatively small negative yield effects of some resistance genes — effects that can be detected only in a disease-free environment anyway. And in Kansas, you can never assume you’ll have a disease-free environment.
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