Bacterial Pustule on Soybean (07/21/21)
I am following up last week’s soybean leaf disease article (bacterial blight, frogeye leaf spot and Septoria leaf spot) with another disease we are beginning to see, bacterial pustule.
Bacterial pustule is not an economic threat to soybeans in our region. Bacterial pustule is very rare in North Dakota. The disease is favored by periods of hot weather, so it is not be a surprise that we are beginning to see it. Leaf symptoms begin as very small light green spots with raised centers. Unlike most bacterial diseases on most crops, these symptoms do not have a water-soaked appearance. Lesions turn brown and small and tan, raised ‘blisters’ often develop in the lesion centers. Light green to yellow halos may develop around the lesion, but are less pronounced that those that are characteristic of bacterial blight. Lesions may coalesce into larger brown spots and infected areas often tear away from the leaf.
Importantly, and the main reason I am writing this article, is that bacterial pustule has been commonly confused with soybean rust in areas where both exist (very far southern US). To be clear, we do not have (and have never had), soybean rust in North Dakota. So, if you see some ‘pustules’ in your soybean fields this summer/fall, think bacterial pustule.
For more images and information on soybean diseases in North Dakota and Northern Minnesota, visit: https://www.ag.ndsu.edu/publications/crops/soybean-disease-diagnostic-series
Extension Plant Pathologist, Broad-leaf Crops