Do disease-resistant wheat varieties pay a price in yield?
Over the years, such controlled experiments in wheat and other crop species, all of them designed to answer the question, “Does the plant pay a price for resistance?” have provided us with a very clear, definitive answer: “Well, it depends.” A survey of these comparative studies, published in the 1990s, found that in exactly 44 out of 88 cases, covering a wide range of species and genes, resistant lines were less productive than susceptible ones in the absence of the relevant disease, insect, or herbicide. In the other 44 cases, there was no difference or, rarely, the resistant line was more productive. The results of studies done since that time have continued to vary in whether they find a yield effect and if so, how big that effect is. It depends on the specific resistance gene(s) involved as well as environmental conditions.
In wheat, as in other crops, some resistance genes reduce yield while others do not. Many of wheat’s genes for resistance have been transferred from related species. In the process, long stretches of DNA extending to either side of the resistance gene come along for the ride. Once in a wheat variety, some of those hitchhiking genes may hurt grain yield even if the resistance gene itself is benign.
This has happened in the past with a chromosome segment from Aegilops umbellulata carrying the Lr9 gene for leaf-rust resistance; it depressed yield by 5 to 14 percent. The Lr47 gene from Ae. speltoides was associated with a 4 percent yield reduction, but in some of the environments and genetic backgrounds examined, there was no effect. There are other interspecific genes such as Fhb1 for fusarium head blight that appear to have brought no yield-reducing hitchhikers with them, and the 1RS rye chromosome arm carrying genes for leaf, stem, and stripe rust resistance has actually had a positive effect on productivity, even when the resistance genes are superfluous or ineffective.
Sometimes the resistance gene itself appears to have a direct impact. The Lr34 gene, which confers adult-plant resistance to leaf rust, originated within common wheat, but its yield-depressing effect is well known. A much-studied powdery mildew gene in barley, Mlo, also reduces yield directly. But whether a yield reduction is caused directly by a resistance gene or indirectly by its bad neighbors, it is crucial to remember that these negative yield impacts have all been measured when there is no disease present. To a wheat grower, such an effect may be less important than the impact of the disease when it does strike.
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