Tracking Fungal Disease Development with Online Tools

May 18, 2017

By Emmanuel Byamukama

Assistant Professor & SDSU Extension Plant Pathologist

Winter Wheat: Disease Update

The winter wheat growth stage as of the week of May 15 ranges between jointing and flag leaf emergence. The flag leaf emergence growth stage often coincides with fungal disease development, mainly due the microclimate created by canopy closure. Flag leaf and the leaf below flag leaf should be protected from fungal diseases because these leaves contribute the most to grain yield. The two common diseases being observed in wheat fields currently are tan spot (Figure 1) and powdery mildew.

Small Grains Disease Forecasting System

Certain weather conditions are required for fungal diseases to develop. To decide whether a fungicide is needed, a weather tracking system that indicates the likelihood for fungal pathogen development can be helpful. The Small Grains Disease Forecasting System can guide producers on the risk for fungal leaf spots to develop based on weather conditions. The system requires producers to follow three steps in order to determine the need to apply a fungicide. The overall objective of this forecasting system is to help the grower protect the top two leaves, which contribute the most to grain yield and to avoid unnecessary fungicide application if not needed.

Establish the presence of disease in the lower leaves at late jointing growth stage. Scout for leaf spots on the second leaf below flag leaf (F-2 leaf on the main tiller) at 5 stops for at least 10 leaves at each stop. If half of the leaves have fungal leaf spots, proceed to the next step. Otherwise, scout again every 3 days.
Consult the model table, select the nearest weather station, the growth stage, and count the number of “Yes” in the table (Figure 2). If the table has 6-8 consecutive “Yes” infection periods, proceed to step 3. Otherwise repeat step one after 3 days.
Check the weather forecast. If rainy, humid weather is in the forecast in the next 3-5 days, consider applying a fungicide. Fungicides need at least two hours before raining to avoid its washout. View a list of fungicides that are effective against leaf spot diseases for more information.

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