OSU Wheat Pathology

Wheat Disease Update – 2 April 2024

Meriem Aoun, Small Grains Pathologist

Department of Entomology & Plant Pathology

Oklahoma State University

Stripe rust continues to increase in Southwestern Oklahoma (Jackson County and Tillman County). On March 31, Dr. Brett Carver, the wheat breeder at Oklahoma State University (OSU) noted high stripe rust pressure at the OSU research station in Tipton (Tillman County, OK). Dr. Carver stated that he has not witnessed this severe and early stripe rust infection in Oklahoma since the early 2000s. The wheat crop at Tipton has not yet reached the flag leaf stage, thus many breeding lines from the 2024 Southern Regional Performance Nursery (SRPN) have shown high susceptibility to stripe rust (Figure 1). This susceptibility is attributed to sparse seedling resistance sources, also known as all-stage resistance, in U.S. Great Plains hard winter wheat. Breeding lines carrying all-stage resistance genes such as Y5 and Yr15 showed high levels of resistance (Figure 2). Many of the OSU varieties have stripe rust adult plant resistance which is usually evident around flag leaf stage. In Tipton and prior to flag leaf stage, the OSU variety Smith’s Gold showed the highest level of protection against stripe rust. On March 31, I also observed initial stripe rust infection at the OSU Agronomy Research station and the OSU Entomology and Plant Pathology farm in Stillwater (Payne County). Based on weather forecast for the coming days (cool and moist weather), stripe rust is most likely going to spread further across Oklahoma.

Given these reports and observations, it is advisable for producers to watch their fields closely and prepare for applying a fungicide, especially if growing a susceptible variety. It is important to apply fungicides to a susceptible variety sufficiently early to prevent widespread infection. For more information on fungicides and their use, see OSU CR-7668, which can be found at: https://extension.okstate.edu/fact-sheets/foliar-fungicides-and-wheat-production-in-oklahoma-march-2016.html. The most updated information about the efficacy of fungicides can be found at https://cropprotectionnetwork.s3.amazonaws.com/cpn3002_fungicideefficacywheatdisease2023-1684787198.pdf

**Figure 1**. Severe stripe rust infection (yellowing of the leaves) on susceptible breeding lines from the 2024 Southern Regional Performance Nursery (SRPN) of hard winter wheat at the OSU research station in Tipton, OK (Photo: Dr. Brett Carver; March 31, 2024).

**Figure 2**. **The plot on the left** is for a resistant breeding line carrying all-stage stripe rust resistance gene (s). **The plot on the right** (showing yellow leaves) is for a susceptible breeding line (Photo: Dr. Brett Carver; Tipton, OK; March 31, 2024).

**Figure 3**. Stripe rust infection on a susceptible hard red winter wheat variety (Stillwater, OK; March 31, 2024).

On March 29, moderate to high barley yellow dwarf (BYD) incidence and severity were observed at the OSU Agronomy Research station and Entomology & Plant Pathology farm in Stillwater. The symptoms appeared as yellow discolorations on the leaves as shown in Figure 4. BYD virus is transmitted from plant to plant by cereal aphids such as bird-cherry oat aphids which were also found on infected plants (Figure 5).

**Figure 4**. Barley yellow dwarf symptoms on hard red winter wheat (Stillwater, OK; March 29, 2024).

**Figure 5**. Bird-cherry oat aphids on wheat leaves (Stillwater, OK; March 29, 2024).

On March 31, I also observed a few hotspots of powdery mildew (Figure 6), Septoria tritici blotch (Figure 7), primarily on lower wheat leaves at the Entomology & Plant Pathology farm in Stillwater.