Paul Price, Padgett, Guy B.
Originally published April 28, 2015
Over the past two weeks there have been multiple reports from producers and consultants throughout Louisiana of wheat scab, also known as Fusarium head blight (FHB). Reported incidences have ranged from 10 to 20 percent. The disease is mainly caused by the fungus, Fusarium graminearum, which also causes ear, stalk, and root rots in corn.
Symptoms of the disease will first appear 10 to 14 days after flowering as bleached heads which will be noticeable from the turn row (Photo 1). This symptom is often mistaken with the appearance of maturing wheat. Upon closer inspection, affected wheat heads will usually have infected kernels showing the characteristic bleached appearance with pinkish/salmon/orangish coloration along the glumes (Photo 2). This coloration is millions of microscopic spores (reproductive structures) of the fungal pathogen. There are usually healthy kernels along with the diseased kernels on the same head (Photo 3). In extreme cases, however, the entire head may be infected. At harvest, affected seed will be shriveled, off color, and much lighter than healthy kernels and are referred to as “tombstones” (Photo 4).
Since 1996, outbreaks of FHB have been as variable as the weather. Outbreaks have been reported in the Great Plains, Central U. S., Mid-South, and Southeast with reported losses of up to 20% and up to 80% in isolated fields. Conditions favoring development are wet, warm weather during flowering. The fungus may infect wheat from flowering to harvest with the most devastating infections occurring during flowering. This infection timing creates hurdles for managing the disease.
The pathogen oversummers corn, wheat, small grain residue, and other grasses. With that in mind, there are some cultural practices that may aid in management: crop rotation, tillage, mowing/shredding, or staggered planting/varietal maturity. At harvest, combine fan speed may be increased to remove infected seed, which is lighter than healthy seed. Additionally, seed cleaning equipment may help remove affected seed but may not be cost effective. These cultural practices alone will not completely manage FHB. An integrated approach is required to lessen the impact of FHB.
Triazole fungicides may be somewhat effective on FHB. Some of the earlier research showed that tebuconazole (Folicur and generics) may reduce incidence and severity of FHB. Later research shows that Prosaro (prothioconazole + tebuconazole), Proline (prothioconazole), and Caramba (metconazole) may be efficacious on FHB. THESE APPLICATIONS WERE MADE UNDER IDEAL CONDITIONS WITH IDEAL TIMINGS AND THE MAXIMUM CONTROL WAS AROUND 50%. AVERAGE CONTROL WAS ABOUT 40%.
Timing is critical. Essentially we have a 5 day window during flowering to make an effective application for FHB. The biggest problem is that ideal conditions (wet weather) for FHB infection are not ideal for making fungicide applications. Head coverage is also critical. Sprayers should be calibrated to deliver maximum water volume (minimum 15 GPA by ground, 5 GPA by air) and optimal droplet size (300 to 350 microns). For ground sprayers, nozzles angled at 30° to the horizontal will maximize head coverage. Some research has shown that dual nozzles angled in opposite directions will also increase head coverage.
The vast majority of fields in Louisiana are currently past the application window. Fungicide applications at this point would likely by off label and ineffective.
It is common to see 2-3 years of epidemics of FHB followed by years with little to no disease. Judging by the amount of calls and observations at this point, FHB has been more prevalent this year compared to previous years. If we have similar weather conditions next year during flowering, expect to encounter FHB again in 2016. An online (www.wheatscab.psu.edu) risk assessment tool that is based on temperature and relative humidity is available online, which has regional commentary that will help you to determine your risk at a given location next year.
For more information, please see the following resources:
Photo 1. A view of a field heavily-infected with FHB.
Photo 2. Closer view of a head infected with FHB. Note the salmon-colored fungal growth near the center.
Photo 3. Wheat heads with FHB-affected and healthy kernels.
Photo 4. Diseased kernels (left) vs. relatively healthy kernels (right).