Guess the Pest Week #23: Answer is Bacterial Blight on Soybean

Nancy Gregory, Plant Diagnostician; ngregory@udel.edu

The picture shows bacterial blight on soybean, caused by Pseudomonas. Bacteria need a wound or opening to enter leaves, which often happens during rain and wind events, mechanical injury, or insect feeding. Bacteria in a film of water can also be splashed around and enter through natural openings such as stomates and hydathodes on the edges of leaves. The picture shows leaf spots that started in hydathodes on edges of leaves. Insects can carry bacteria and the bacteria can also persist on or in some seed. Bacterial diseases are favored by high temperatures and high humidity. Use of resistant cultivars and pathogen-free seed is a part of management. Diagnostic tests are recommended if unusual foliar symptoms are observed. Spots on leaves caused by bacterial pathogens often have a water-soaked appearance and sometimes have a yellow edge or halo. Microscopic examination may reveal the presence of bacteria. Bacterial blight caused by Pseudomonas is favored by cool and wet conditions and appears primarily on young foliage. Resistance is available in some cultivars. Bacterial pustule caused by Xanthomonas is usually seed-borne and is not commonly seen in the Mid-Atlantic States. Small brown raised spots of bacterial pustule may be confused with soybean brown spot (Septoria) or mistaken for soybean rust.

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