Recent wet weather has brought on an increased risk of Fusarium head blight (scab) and leaf spotting diseases in parts of the state. Here we walk through current disease risks in Kansas and fungicide application reminders.
The weather is currently favorable for scab development in southeast Kansas, and risk is elevated moving into central Kansas according to wheatscab.psu.edu (Figure 1). This model considers weather conditions favorable for scab development over a 14-day window. It will be important to carefully watch risk in fields that are approaching the critical application window for a fungicide application (flowering- Feekes 10.5.1). The highest risk will be in fields of scab-susceptible varieties that are planted back into corn residue. Continued wet weather around flowering will increase our risk for scab in the state. For a reminder about the scab ratings for individual varieties, please refer to the Kansas Wheat Variety Guide: https://bookstore.ksre.ksu.edu/pubs/mf991.pdf.
Figure 1. Fusarium head blight (scab) risk for May 1, 2025. Red areas indicate regions with the most favorable weather for scab over the last two weeks, and yellow indicates less favorable weather. This model is calibrated for susceptible varieties of winter wheat. This interactive map can be accessed at www.wheatscab.psu.edu.
Scab infection occurs at flowering, but symptoms are not visible for 14-21 days after infection (Figure 2). Because of this, we cannot scout for scab the way we would stripe rust or other foliar diseases. Fungicide decisions need to be made according to the weather-based risk and the field’s yield potential. Not only can scab lower yield and test weights, it also produces a mycotoxin (vomitoxin, DON) and can produce grain that is “scabby” which can sometimes lead to discounts.
Figure 2. Fusarium head blight (scab) infection often begins with bleaching of infected spikelets and will progress throughout the head. When humidity is high, orange fungal structures are visible on the outside of the spikelet. The grain from infected heads may appear lightweight, white, or pink. Photos by: K-State Research and Extension.
Reminders for scab fungicide applications
Fungicide products - Fungicides such as Miravis Ace, Prosaro, Sphearex, Prosaro Pro, and Folicur (and generic equivalents) are known to suppress scab (head blight). Specific fungicide performance for scab and other diseases can be found here: https://bookstore.ksre.ksu.edu/pubs/EP130.pdf. Other fungicides are not labeled or not recommended for scab control, particularly products containing strobilurin (FRAC group 11 – azoxystrobin, pyraclostrobin, etc.). As a reminder, all products that control scab will also control stripe rust and other foliar fungal diseases.
Timing - Fungicides are most effective against scab when applied at early flowering (Feekes 10.5.1), but can provide protection even when applied later in the flowering window. It is important to pay attention to pre-harvest intervals at this point of the season and follow guidelines provided on product labels. The products listed above either have a 30-day pre-harvest interval (cannot be applied within 30 days of harvest) or cannot be applied after Feekes 10.5.4 (end of flowering, watery ripe growth stage).
It is important to remember that early flag-leaf fungicide applications will have little to no effect on scab control.
Rainfastness - With the current wet weather, we are getting many questions about fungicide rainfastness. Rainfast time is defined as the period of time that needs to pass between the application of a fungicide and a rainfall event where the fungicide will not lose efficacy. This information is often not included on the product label or is ambiguous. Rainfast time will be variable with temperature and canopy moisture, but most products recommended for wheat in Kansas will be rainfast within two hours, and likely within one hour under most conditions. Rainfastness is improved when a product is applied with a non-ionic surfactant.
Residual life - The residual life of the fungicide application is influenced by the product used, environment, and rate of application. In general, products belonging to the triazole and strobilurin classes of fungicide will run out of gas (you may start to see symptoms) after 21 days (about 3 weeks). Small differences in residual life among products typically do not result in large differences in grain yield. Some newer products are promoted as having much longer residual lives, but we don’t have university research that supports those claims.
Seed treatments - Seed treatments do not have any influence on disease development during the growing season, as the fungicides in these seed treatments wear off within 30-45 days. Seed treatments can improve the emergence of seed from infected fields as this pathogen can cause seedling diseases when infected seed is planted back.
Stripe rust update and outlook
We received our first observations of low levels of stripe rust in Kansas this week from both Ford and Labette Counties (Figure 3). To date, confirmed reports in Kansas have been limited to single fields in each of these counties. We are continuing to scout and monitor the situation and will update the regional map with new observations: https://wheat.agpestmonitor.org/stripe-rust/.
Overall, the risk of widespread losses in Kansas remains low. Although we have been experiencing favorable weather for stripe rust, disease levels have remained low in states to our south (see update from Oklahoma: https://spotlight.okstate.edu/wheat-pathology/2025/04/28/wheat-disease-update-april-28-2025/), limiting the number of spores available to infect our crop during critical growth stages.
For more information on the stripe rust outlook, see our Agronomy eUpdate article from last week (https://eupdate.agronomy.ksu.edu/article/should-you-spray-stripe-rust-and-other-wheat-diseases-to-watch-for-in-kansas-638-4). Scouting efforts should continue over the next few weeks as we move through critical growth stages across the state. Scouting efforts will be particularly critical in the Northwest portion of the state, where the wheat growth stages are much less advanced. As a reminder, the probability of a positive return on a fungicide application greatly diminishes when disease pressure is absent.
If you detect stripe rust, please contact me at andersenk@ksu.edu) so we can verify and update regional maps.
Figure 3. Counties where wheat stripe rust has been confirmed as of May 1, 2025. Real-time updates can be monitored at https://wheat.agpestmonitor.org/stripe-rust/.
Leaf spot diseases are starting to take hold
Although stripe rust levels remain low, wet weather has brought on higher-than-usual leaf spot disease pressure in parts of Kansas (Figure 4). As a reminder, tan spot and similar leaf diseases (Septoria tritici blotch and Stagonospora nodorum blotch) survive in wheat crop residue left over from past seasons. These pathogens develop spores in residue that are splashed or blown up to lower leaves. When weather conditions are right, these diseases can move into the upper canopy and cause yield losses. Periods of cool, wet weather after flag leaf emergence can favor disease progression. Fields with leaf spots moving into the upper canopy may benefit from a fungicide application. More information on fungicide efficacy for these leaf spotting diseases can be found here: https://bookstore.ksre.ksu.edu/pubs/EP130.pdf.
Figure 4. Symptoms of tan spot (Top two photos) are starting to show up in Kansas wheat fields. Symptoms of this disease include brown spotting surrounded by yellow halos. Brown spots may coalesce when disease pressure is high. Septoria tritici blotch (bottom photo) is a similar leaf disease that may have dark fruiting bodies in the center of lesions. Photos by Kelsey Andersen Onofre, K-State Research and Extension.
Kelsey Andersen Onofre, Extension Wheat Pathologist
andersenk@ksu.edu