Fusarium head blight (FHB) is also commonly called Fusarium head scab, wheat scab, and head scab. In Ohio, this disease is caused primarily by the fungus Fusarium graminearum, the same pathogen that causes stalk and ear rot of corn and seed rot and damping-off of soybean. On wheat, typical scab symptoms include premature bleaching of the wheat head (Fig. 1B) and shriveled, scabby kernels. The disease causes economic losses by reducing grain yield, test weight and grain quality. More importantly, F. graminearum produces mycotoxins, most notably vomitoxin (deoxynivalenol or DON), during colonization of the wheat heads. Vomitoxin may accumulate to high concentrations in harvested grain and may cause vomiting and feed refusal if contaminated grain is fed to livestock. Grain with vomitoxin exceeding 1 ppm and 5 ppm (parts per million) is unfit for human and livestock consumption, respectively. Consequently, wheat with vomitoxin levels above 2 ppm may be priced down or rejection at grain elevators.
The combination of fungicide and host resistance is the most effective management strategies for head scab. Although most of the widely grown wheat varieties are susceptible to scab, several moderately resistant varieties are available. Fungicide applications must be made at flowering (anthesis) when the crop is most susceptible to infection or at the very latest 4 to 6 days after flowering.
Improper timing of fungicide treatment will reduce efficacy. In addition, applications made when the risk of scab is low (conditions are cool and dry during flowering) are not cost effective. Therefore, a scab forecasting system was developed to help guide fungicide application based on the risk or potential for scab infection and disease development.
|Fig. 1. Wheat head A, at flowering (arrows indicate anthers) and B, With typical head scab symptoms (bleaching).|
The Fusarium Head Blight Risk Assessment Tool (www.wheatscab.psu.edu) helps growers assess the risk of scab in their field. For a scab epidemic to occur, three conditions must be satisfied: the variety must be susceptible and at the flowering growth stage, inoculum must be availability, and weather conditions must favor spore production and infection. Severe scab develops when these three conditions occur simultaneously. Think of these conditions as pieces of the “scab puzzle.” If all three pieces are not present at the same time, the puzzle will not be complete and severe scab is less likely to occur. Wheat is most susceptible to head scab during flowering (also known as anthesis or Feekes growth stage 10.5.1) and early grain-fill. Flowering usually begins three to five days after full head emergence (Feekes 10.5) and can be identified by the presence of exposed anthers on the wheat heads (Fig. 1A).
The scab forecasting tool helps to predict the risk of scab based on weather conditions leading up to flowering and variety susceptibility—the three pieces of the scab puzzle. For example, if it rains well before the crop reaches flowering, the risk will be low because the wheat will not yet be at the susceptible growth stage. The risk will also be low even if it rains during flowering, but a moderately resistant cultivar is planted away from a field with wheat or corn stubble and conditions are cool. Warm, wet, rainy weather during the days leading up to flowering enhance spore production and dispersal to the heads, infection, scab development, and vomitoxin contamination.
Website instructions: Go to www.wheatscab.psu.edu
State and Wheat Class Selection: Select your state from the dropdown menu. This will zoom in to the state map. Select between spring wheat (spring planted) and winter wheat (fall planted). This selection will activate different components of the system and customize the prediction for your crop. In Ohio, click on the “Winter” button.
Variety Susceptibility Class and Flowering Date Selection: To determine the flowering date, examine several heads at multiple locations across the field. Planting date and variety will affect crop development, so each field may have a different flowering date. It is therefore important to select the correct susceptibility class and flowering date for each field and variety. The models will make specific predictions based on your selections. Predictions will likely vary from one variety to another. There are also 24, 48 and 72 hour forecast buttons to estimate risk prior to flowering.
Know Your Risk To Manage Scab: Once the selections are made, the models will estimate the risk of a scab epidemic with greater than 10% field severity (percent head areas with scab symptoms). This risk will be indicated as different colors on the map. Although the color pattern will be applied to the entire map, the predicted risk will only be valid for the specific flowering date and variety susceptibility class you selected. Red indicates high risk for scab exceeding 10% severity, yellow indicates moderate risk, while green indicates low risk. High scab risk is associated with high risk for grain contamination with vomitoxin, but the tool was not developed to predict vomitoxin. Even if scab risk is low, vomitoxin may still exceed critical thresholds if wet, humid weather occurs between flowering and harvest. Contact your local extension educator and state specialist for further assistance with interpreting the tool and to discuss your management options.