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Bacterial Leaf Blight Developing in Winter Wheat

By Emmanuel Byamukama
Recent winter wheat scouting found bacterial leaf blight developing in some fields. Bacterial leaf blight is caused by the bacteria, Pseudomonas syringae pv. syringae. The disease develops under frequent rains between 59 – 77° F. P. syringae pv. syringae is a weak pathogen and disease progress will be curtailed once wheat is exposed to periods of low humidity and warm temperatures (above 77° F).
Bacterial leaf blight symptoms can be confused with tan spot or chemical injury. The photos below illustrate the key differences among these conditions.
The best way to distinguish bacterial leaf blight from tan spot is to look for water soaking along the lesion margins (Figure 1) .
Figure 1. Bacterial leaf blight symptoms in winter wheat. Notice the water soaking on the margins of the lesions.
Tan spot has a small, dark-brown center surrounded by a yellow halo (Figure 2) .
Figure 2. Tan spot symptoms on winter wheat. Notice the dark brown center and yellow margin surrounding the lesion.
Chemical injury symptoms do not have water soaking or yellow margins, and they tend be uniform on plants in a large area in a field (Figure 3).
Figure 3. Symptoms caused by chemical injury on wheat plants. Lesions have no water soaking or yellow margins.
As with other bacterial diseases, bacterial leaf blight cannot be managed through application of fungicides. Bacterial leaf blight is a minor disease except where a planted cultivar is highly susceptible.
Scouting and proper disease diagnosis is important for effective disease management in wheat or any other crop. Growers should not rush to apply a fungicide when symptoms are caused by non-fungal pathogens.