Wheat Disease Update – May 22, 2025

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.

Wheat Disease Update – May 8, 2025

Meriem Aoun, Small Grains Pathologist

Department of Entomology & Plant Pathology Oklahoma State University

According to the Fusarium Head Blight (FHB) risk tool (wheatscab.psu.edu), there is currently a high risk of FHB across most of Oklahoma, with the exception of the Panhandle (Figure 1). This elevated risk is largely due to recent wet weather and high relative humidity conditions that are favorable for FHB infection during the wheat flowering stage. In eastern Oklahoma, where wheat is often planted following corn, the risk is particularly high due to the presence of Fusarium inoculum in the fields. In contrast, in western Oklahoma, where corn production is not common, we lack information on the presence of Fusarium inoculum, making it difficult to accurately assess the FHB risk in those areas.

On May 7, a wheat grower in Kingfisher reported a very low incidence of FHB in his field (Figure 2). For wheat fields that have already passed the flowering stage, fungicides cannot be applied. For fields that have not yet reached flowering, the most effective window for fungicide application is at early flowering. Applications made earlier, such as at the heading stage, do not provide effective control of FHB. It is essential to carefully follow all fungicide label instructions to ensure proper use.

Fungicide products that offer good protection against FHB include Prosaro, Prosaro Pro, Proline, Sphaerex and Miravis Ace. While some growers have expressed concerns about the high cost of these treatments, an alternative is Tebuconazole, which is more cost-effective and provides fair efficacy in managing FHB. Wheat fields intended for seed production should be prioritized for fungicide application to preserve seed quality and germination rates, and to help reduce costs related to seed cleaning.

Wheat Disease Update – May 7, 2025

Meriem Aoun, Small Grains Pathologist

Department of Entomology & Plant Pathology, Oklahoma State University

Wheat streak mosaic (WSM) is a recurrent disease in the Oklahoma panhandle region however, this season WSM has been confirmed in several wheat fields in Blaine, Kay, Caddo, Garfield, Texas, Harper, Alfalfa, Kingfisher, and Payne Counties.

Rainfall during the latter half of April and early May has created favorable conditions for the development of several leaf spotting diseases, including tan spot (Figure 1), septoria tritici blotch (STB, Figure 2), spot blotch (Figure 3), and septoria nodorum blotch (SNB) in multiple fields across the state. SNB was observed on May 2 in Miami (Ottawa County) as both leaf blotch and glume blotch (Figure 4). Josh Bushong (OSU Area Agronomist Specialist) also reported SNB in Grandfield (Tillman County) on April 29. Symptoms of SNB and STB may appear similar, as both can occur on the same plant simultaneously. However, STB can be distinguished by the higher abundance of fungal structures known as pycnidia, which are easier to detect without magnification and appear as black specks on the lesions.

The incidence and severity of stripe rust (Figure 5) and leaf rust (Figure 6) have remained low across Oklahoma through late April and early May. Stripe rust has been observed in Stillwater (Payne County), Walters (Cotton County), and Lahoma (Garfield County). Leaf rust has been observed in Payne, Blaine, Tillman, and Garfield Counties.

Currently, the overall risk of stripe rust remains low. However, leaf rust pressure is expected to increase later in May, as the crop progresses into the later stages of development. Many wheat fields in Oklahoma have now moved beyond the flowering stage, which limits the use of fungicides. As a result, resistant varieties remain the most effective line of defense against diseases at this point in the season.

Dryland root rots were also observed in some wheat fields in Blaine and Tillman Counties. Infected plants were stunted, white, with discolored roots and lower stems. Peeling leaf sheaths in the lower stem internodes showed brown and sometimes pinkish discolorations on infected plants. Pink discoloration indicates Fusarium infection. Infected plants with root rot died prematurely and produced white heads that were either sterile or filled with shriveled grain (Figure 7).

Josh Bushong has reported cases of loose smut (Figure 8) in wheat fields in Woods and Alfalfa Counties. If loose smut is observed in a field, grain harvested from that field should not be used for seed in the next growing season. However, if using such grain for seed is unavoidable, it is critical to apply a seed treatment at a high rate.

According to the Fusarium Head Blight (FHB) risk tool (wheatscab.psu.edu), the current weather conditions indicate a high risk of FHB across most of Oklahoma, with the exception of the Panhandle (Figure 9). This elevated risk is primarily due to wet weather and high relative humidity that has prevailed since mid-April, which favors FHB infection during the flowering stage.

During a recent field tour in Miami (Ottawa County) on May 5, we learned that some growers have already applied, or are in the process of applying, fungicides to manage FHB. In this region, the rotation of wheat with corn (a known host of the FHB pathogen) further increases disease risk. Timely fungicide application is critical. The most effective window is at early flowering. Applications made earlier (e.g., at heading) are not effective against FHB. However, some studies showed that applying fungicide within 5–7 days after the start of flowering can still provide suppression of FHB.

In contrast, for many regions in western Oklahoma, where corn is rarely grown and with no past FHB outbreaks, the actual field-level risk should be low. This is due to the lack of local inoculum, despite favorable weather. Because FHB fungicide treatments are costly (~ $30/acre), it is important that growers consider both current weather risk and individual field history before deciding to apply fungicides.