MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Wheat has begun to flower in fields in the southern-most portions of the state bordering Indiana and Ohio.

Elsewhere, wheat ranges from second node in the north to the boot stage or heading. Most of the wheat is likely to flower this week. Once wheat begins to flower, it enters the critical stage for fusarium head blight infection. The Penn State model, a prediction model for risk of fusarium head blight epidemics has been field tested for the last three years in Michigan and has accurately predicted risk. So far, the predicted risk levels for fusarium head blight for Michigan at the Penn State Model site remain low for 2009. The web address is: http://www.wheatscab.psu.edu/riskTool_2009.html The Penn State winter wheat model uses only data from the week before flowering to estimate the risk of a scab epidemic with 10 percent or greater field severity. Risk is calculated based on the number of hours when both relative humidity is greater than 90 percent and temperatures are between 48°F and 86°F during the week before flowering. Humidity and temperatures in this range are favorable for the reproduction of the fungus (Fusarium graminearum) that causes head scab. The spores must germinate and infect the flowers for the pathogen to cause disease. Moisture, in the form of high humidity, rainfall, dew or ground fog has the greatest influence on infection. Generally, warm temperatures (75-86°F) and extended periods of wetness (overnight and morning) tend to be most favorable for infection. Cool temperatures tend to limit fungal growth, but long periods of wetness can compensate for cooler temperatures.

How to use the model

You will be asked to identify the flowering date for your fields. It is a good idea to start checking the model a week or two before flowering to look at the patterns of disease risk during this time period. So far, risk level has been low throughout the state; however, recent rainfall and temperatures in the optimum range for fusarium head blight may increase risk levels. Visit fields to determine the date when anthers are exposed on 15 percent of the heads. Wheat is most susceptible to scab during the flowering growth stage, and the model uses weather conditions observed during the seven days prior to the flowering date you select. Planting date and variety will affect flowering dates so each field may have a different flowering date. (The 24, 48 and 72 hour forecast buttons in the upper left corner of the tool can be used to help you estimate risk several days before the actual flowering date).

Using weather forecasts

Select the button labeled 24, 48 or 72 from the upper left corner of the tool window to access disease predictions that incorporate the weather forecasts. The disease risk predictions for individual weather station locations are only available while the system is in “forecast mode.” To reset the system to use only observed weather data, and access individual station information, select the button marked with a zero. Clicking on the zero button provides the risk levels for the assessment date chosen, using only observed weather for the seven days prior to that date. In the zero mode, an individual map location icon can be clicked on to get more detailed forecast information for that location. Clicking on the 24 button provides the risk level based on the previous six days observed plus one day forecast weather data. Clicking on the 48 button provides the risk level based on the previous five days observed plus two days forecast weather data. Clicking on the 72 button provides the risk level based on the previous four days observed plus three days forecast weather data.

Individual station information:

Clicking on a circle or square within the state will take you to a screen that provides risk information for that weather station location chosen. You will find graphs showing the current risk level (low, medium or high) and graphs showing the risk levels for the previous seven days and the temperature and precipitation for those dates.

Clicking on the “view model parameters” arrow will bring up a graph that shows the number of hours each day during the previous week when temperature was between 48°F and 86°F (yellow bars), hours with rainfall (purple bars) and number of hours when both the temperature and humidity were in those ranges (blue bars). The blue bars represent the number of hours each day when both the temperature was in the 48°F and 86°F range, and relative humidity was over 90 percent, conditions that are favorable for the development of fusarium head blight. Moving the mouse over any of the bars will display the number of hours it represents. You will also find relative humidity ratings for the week in a separate graph.

State commentary

You can also access Michigan-specific commentary related to weather conditions, fusarium head blight risk, and development of foliar diseases that may provide additional information needed to determine whether or not a fungicide treatment at flowering is advisable.