Wheat Disease Update – May 7, 2025

Categories: Disease updates

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.