Stripe rust incidence in 2016 wheat plots
Stripe rust was the most significant disease on wheat for the 2016 growing season. The disease got an early start this year and was found at low levels in central and southeast Kansas by the middle of April. Dry conditions slowed the development of stripe rust early, but when the rains came, the disease spread rapidly.
The effects of stripe rust were evident in many of the variety demonstration plots we visited this year. In some cases, the severity of stripe rust was approaching 100% on susceptible varieties (Figure 1). This level of disease was common throughout the state, but many growers were aware of the threat and responded with fungicide applications. The visual evidence of fungicide application was striking at many locations. In some situations, it appeared the fungicide was applied a little too late and the leaves were already damaged by stripe rust. The fungicide still killed the fungus and prevented further spread, but the leaves had tan, dead areas that were damaged by the disease Figure 2.
Figure 1. Stripe rust damaged the leaves of many susceptible varieties this year. This image shows a susceptible variety that was severely damaged by disease during the early stages of grain development. Photo by Erick DeWolf, K-State Research and Extension.
Figure 2. Partial control of stripe rust with a fungicide. The fungicide killed the fungus, but the damaged cells on the leaves continued to die, resulting in tan, dead areas on the leaves. Photo by Erick DeWolf, K-State Research and Extension.
The benefits of genetic resistance were also evident this year (Figure 3). The stripe rust population this year was very similar to last year and most varieties with moderate or high levels of genetic resistance made it through the season with only minor damage to the leaves. Stripe rust developed much slower on varieties with intermediate levels of stripe rust resistance. Many growers reported that these varieties had enough disease to warrant a fungicide application, but the slower disease development provided some extra time to respond to the emerging threat of stripe rust and protect the crop with a fungicide treatment.
Figure 3. Comparison of wheat varieties with different levels of genetic resistance to stripe rust. The susceptible variety (right) has symptoms of severe stripe rust evident on the flag leaves. The disease is so severe that the leaves appear yellow even from a distance. Photo by Erick DeWolf, K-State Research and Extension.
Varieties with moderate or high levels of genetic resistance include some well-established varieties such as WB-Cedar and T158. Other relatively new varieties such as Gallagher, Oakley CL, SY Monument, TAM 114, WB4458, and WB-Grainfield also appeared to be holding up well this year. It was also encouraging to see recent releases with good levels of stripe rust resistance, including Larry, LCS Chrome, SY Grit, Tatanka, WB4721, and Zenda. Selecting varieties with genetic resistance to stripe rust could greatly reduce the risk of severe stripe rust in the future and reduce the need for extra costs associated with fungicide application.
Leaf rust, barley yellow dwarf, and tan spot were also present in some of the demonstration plots this year. Thankfully it appears the incidence of these diseases was low or occurred late enough in the growing season that they are not likely to cause severe yield losses this year. Fusarium head blight (head scab) was detected again this year. Most demonstration plots had less than 3% incidence of head scab and we remain cautiously optimistic that we may have escaped widespread significant losses from this disease. The full impact of these diseases will become clearer as we move further into harvest.
Erick DeWolf, Extension Plant Pathologist