Wheat variety selection and the emerging vulnerability to stem rust in Kansas
In recent years, diseases like stripe rust and wheat streak mosaic have drawn the attention of many wheat growers in Kansas. This attention is justifiable given that recent outbreaks of stripe rust and wheat streak mosaic have cost growers more than $200 million in the last 3 years alone. Unfavorable weather patterns such as drought or freeze injury, and heat stress during grain fill, have also further damaged the crop in recent years and have contributed to declines in overall productivity and loss of potential income for growers.
Wheat growers in Kansas are making some adjustments to reduce their risk of disease and other hazards influencing their wheat production. One way that growers are responding is by selecting varieties that better able to tolerate drought and mature slightly later than other varieties, thus reducing the risk that the crop will be damaged by a late spring freeze. Some producers are also moving toward varieties that have better genetic resistance to wheat streak mosaic and stripe rust. While these are important adjustments that can reduce the risk of yield loss to the current problems, a closer look at these varieties suggests that we may be unknowingly exposing ourselves to other wheat production threats simply because we are making some potentially incorrect assumptions. Let me explain…
In a sense, the population of wheat varieties available to growers goes through a selection process every year. That sounds very academic, but here is the pattern. We are naturally drawn to select wheat varieties that yield well in our area. This performance is a product of how well a variety handles the weather conditions and diseases that dominated this year or during the most recent 2-3 years of testing. On the surface, this makes perfect sense, but let us look at some potential pitfalls of this approach. This strategy works well as long as production conditions remain similar to the years of testing, or if the varieties have characteristics that allow them to be successful over a broad range of conditions (weather, disease, soil types, etc.) that occur within a region. Problems can emerge; however, if we select varieties that perform well in the short term (1-3yrs) but are not well adapted for the full range of conditions that may occur. When the variety encounters conditions that expose a previously unrecognized vulnerability, its productivity can drop dramatically.
This pattern repeats its self with respect to many characteristics, but let us consider an example related to disease susceptibility. Specifically, let us look at the trend toward growing wheat varieties in Kansas that are highly susceptible to stem rust. Now you might be thinking to yourself, “Do they really mean stem rust? When was the last time we saw an outbreak of stem rust in Kansas or the Great Plains?” These are valid questions. It has been a long time since we had serious problems with stem rust in Kansas (it was 1986, in case you are wondering). Interestingly, there is a strong correlation between the decades without an outbreak of stem rust and the wide spread use of wheat varieties with genetic resistance to the disease. It turns out that the last time we were planting many acres to stem rust susceptible varieties was… (Yep, you guessed it) 1986. The concern here is that it has been so long since we have experienced major problems with stem rust, that we are assuming that this disease is no longer a threat. As a result, we are allowing this base of genetic resistance to erode through our variety selection decisions.
Figure 1. Stem rust is able to attack leaves, stems, and heads of the wheat. When severe, stem rust causes yield losses exceeding 40%. To make matters worse, the stem damage caused by stem rust also predisposes plants to lodging and complicates harvest operations. Photo by Erick DeWolf, K-State Research and Extension.
Recent data on the wheat varieties grown in Kansas indicates that the percentage of acres planted to stem rust susceptible varieties has increased dramatically in the past decade (Figure 2). In 2018, stem rust susceptible varieties were grown on almost 35% of the acres in western Kansas, as compared to near 0% of the acres in 2009. The trend is also present to a lesser extent in central Kansas, where the acres planted to stem rust susceptible varieties is approximately 10%. There are multiple varieties responsible for this trend including (in alphabetical order) Avery, Byrd, Brawl CL Plus, LCS Mint, LCS Pistol, T158, and TAM 114 (Figure 2, inset). Given the recent release of high yielding varieties that are also susceptible to stem rust (Langin and Lonerider, for example), this trend is likely to continue.
Figure 2. Trends in number of varieties widely marketed and planted in Kansas that are susceptible to stem rust. Figure by Erick DeWolf, K-State Research and Extension.
A similar trend toward stem rust susceptibility is occurring throughout the southern and central Great Plains. Recent variety surveys indicate that more than 40% of the wheat varieties planted in Eastern Colorado are highly susceptible to stem rust. The distribution of susceptible varieties extends into Oklahoma, Nebraska and Texas with more than 9% of the acres planted to susceptible varieties within some important wheat producing regions (Figure 3). The presence of stem rust susceptible acres in the southern US is critical because the disease is more likely to survive the winter in these regions. An outbreak of stem rust in Texas dramatically increases the risk of disease problems in Kansas, Colorado and Nebraska.
Figure 3. Percentage of acres planted to stem rust susceptible varieties is the central and southern Great Plains. Map by Erick DeWolf, K-State Research and Extension.
Returning to the patterns of variety selection and underlying assumptions, we would ask, “Are we unknowingly creating a vulnerability to stem rust through our variety selection?” The current variety surveys indicate that we are assuming that conditions will remain similar to recent years and that stem rust will remain a minor concern. At the same time, our variety choices are eroding the base of genetic resistance that help to ensure that stem rust remains a minor problem.
Disease epidemics are influenced by many factors including weather and regional use of genetic resistance. If an outbreak of stem rust did occur within the next few years, many growers would likely use fungicides to slow the spread of disease and reduce yield losses. This costly and incomplete solution is complicated by the need to manage multiple diseases each potentially requiring different application timing and methods. Fortunately, there is an alternative. We can take corrective action by simply selecting varieties that are well adapted for the wheat production in Kansas and have stem rust resistance (Table 1). We have good options available, so let’s use them. Hopefully we can address this vulnerability before we have serious problems in our wheat.
Table 1. Wheat varieties with a good yield record and resistance to stem rust.
Hard Red Wheat
Hard White Wheat
DoubleStop CL Plus
Varieties listed in alphabetical order
For a more complete listing of see “Wheat Variety Disease and Insect Ratings 2018” (MF991) from K-State Research and Extension.
Erick DeWolf, Extension Plant Pathologist
Romulo Lollato, Wheat and Forages Specialist
Lucas Haag, Northwest Area Agronomist