Wheat Disease Update – May 22, 2025

Categories: Disease updates

Meriem Aoun, Small Grains Pathologist

Department of Entomology & Plant Pathology, Oklahoma State University

This season, wheat streak mosaic (WSM, Figure 1) has been reported in several wheat fields in Blaine, Kay, Caddo, Garfield, Texas, Harper, Beaver, Alfalfa, Kingfisher, Washita, Woods, Kiowa, Custer, and Payne Counties (Figure 2). Controlling volunteer wheat and grassy weeds at least two weeks before planting remains the most effective management against WSM. About 2/3 of the infected samples with WSMV were also infected with Triticum mosaic virus (TriMV) based on diagnosis performed at the OSU Plant Disease and Insect Diagnostic Lab.

This season, Septoria nodorum blotch (SNB), also known as Stagonospora leaf and glume blotch (Figure 3), and Septoria tritici blotch (STB) have been the most prevalent fungal foliar diseases across multiple locations in Oklahoma, with the exception of the panhandle region. The widespread incidence and severity of these leaf spotting diseases were largely driven by the abundant rainfall in April and May. Septoria nodorum blotch and Septoria tritici blotch were observed on both the lower and upper canopies of infected plants. SNB symptoms on the leaves can be confused with those of STB, especially because both diseases can affect the same plant simultaneously. Pycnidia (small black round structures on the lesions) produced by the STB fungus are more abundant and easier to see without magnification. In addition, STB is usually more prevalent under cool temperatures (50-68°F; 10-20°C), whereas SNB is more prevalent under warmer temperatures (68-81°F; 20-27°C).

During wheat field tours in the Oklahoma Panhandle earlier this week (May 19–20), a high incidence of dryland root rots (particularly Fusarium crown and root rots) was observed, especially in Hooker, OK (Texas County, Figures 4 and 5). These diseases are favored by drought conditions and have been widespread across Oklahoma over the past three growing seasons (2022–2024). Infected plants were stunted, white, with discolored roots and lower stems. Peeling leaf sheaths in the lower stem internodes showed brown and pinkish discolorations on infected plants. Pink discoloration indicates infections by Fusarium species. Infected plants with root rot died prematurely and produced white heads that were either sterile or filled with shriveled grain. Dryland root rot symptoms can be confused with those of freeze damage. However, freeze damage only affects the heads, whereas root rots are usually associated with the whole plant turning white with brown or pinkish discoloration in the lower stem and roots.

Although genetic resistance to root rots is complex, some varieties in the variety trial at Hooker, OK, appeared to show better tolerance than others. Management practices such as fungicide seed treatments and delayed planting can help reduce the impact of dryland root rots. While fungicide seed treatments may not directly control root rots in the spring, they generally support better stand establishment and can contribute to improved yield.

Leaf rust incidence and severity have increased in some locations in Oklahoma. For example, on May 12, Dr. Brett Carver reported high incidence and severity of leaf rust at Chickasha (Grady County). High levels of leaf rust were also observed in one of the fungicide trials conducted in Stillwater, OK. This season, leaf rust has been reported in multiple counties, including Payne, Blaine, Washita, Tillman, Caddo, Jackson, Cotton, Garfield, and Grady. However, since the disease appeared late in the crop’s development, it should not cause significant yield losses.

During the second and third weeks of May, low levels of Fusarium head blight (FHB), also known as head scab (Figure 6), were observed in variety trials at Stillwater (Payne County), Kildare (Kay County), Kingfisher (Kingfisher County), and Lahoma (Garfield County). According to the FHB Risk Tool (wheatscab.psu.edu), the risk for FHB was high across most of Oklahoma (excluding the Panhandle) during the first week of May. This elevated risk was primarily driven by wet weather and high relative humidity during the flowering stage, which created favorable conditions for infection. However, to date, there have been no significant FHB outbreaks in commercial wheat fields in Oklahoma.