Prevent Scabby Heads – Assessing Fusarium Head Blight In Wheat

DR. HEATHER MARIE KELLY

JACKSON, TENN.
   Most wheat in Tennessee has started or will start to bloom in the next week or two which is the critical timing to apply a triazole fungicide for prevention of Fusarium Head Blight (FHB or Head Scab).
   While wheat is susceptible to infection from head emergence until harvest, infections initiated at and soon after anthesis have the greatest destructive potential. FHB/Head Scab can not only reduce yields but the fungus can also produce mycotoxins, fungal chemicals that are harmful to animals if ingested. The major toxin associated with Scab is deoxynivalenol (called DON or vomitoxin). Wheat will usually docked at DON levels of 2 parts per million (ppm) and loads can be completely rejected at 5 ppm.
   FHB/Head Scab infection is favored by extended periods of high moisture or relative humidity (>90%) and moderately warm temperatures (between 59 to 86°F). These conditions present before, during, and after bloom favor spore production, flower infection, and colonization of developing grains. The Fusarium Head Blight Prediction Center (www.wheatscab.psu.edu) uses models to estimate Head Scab risk using weather variables observed seven days prior to flowering. A note on the forecast website – it is best viewed with the following web browsers: Chrome, Firefox, and Safari (Internet Explorer has some compatibility issues).
   While evaluation of these models have shown them to be correct 75 percent of the time, it is still recommended to evaluate your local weather and wheat field as it approaches bloom to better guide fungicide application decisions. While Tennessee is shaded for low risk, over the next 72 hours on the prediction site, more point specific risk estimates using the ‘Query’ option has West Tennessee between 10 and 25 percent risk (out of 100 percent) and the rest of the state at 10 percent or less. Where you see green on the map indicates low risk. Yellow represents medium risk (between 30 and 50 percent) and red indicates high risk (above 50 percent). Other factors that can increase Scab risk include wheat planted after corn or wheat and susceptibility of the wheat variety.
   I believe the later planted/later blooming wheat will be at higher risk for Scab as well as some wheat going into bloom now, but individual fields and local forecasts of rain should be used to better guide if and when a fungicide application needs to be applied.
   Only fungicide products that are solo triazoles (FRAC or fungicide group 3 on the label) should be applied for prevention/management of Head Scab. Products that contain a strobilurin can increase the mycotoxin levels in FHB infected wheat. The best products currently available for reducing Scab symptoms and DON levels are Caramba and Prosaro. Although these fungicides, like all fungicides, are not going to provide 100 percent control, research trials have shown that FHB symptoms and DON levels can be reduced by approximately 40-60 percent relative to a non-treated check. Multiple factors contribute to this level of control which include fungicide efficacy, application coverage, timing in respect to wheat growth stage (all the heads in a field will not be flowering at the same time), and disease risk (i.e. weather before, during, and after application). The bottom line is that fungicides can reduce disease and DON levels, but pairing fungicide with a resistant variety will provide the best protection and both the FHB forecast model, along with your local forecast and information about your own field will better guide fungicide application decisions. ∆
DR. HEATHER MARIE KELLY: Extension Plant Pathologist, University of Tennessee