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Bacterial Blight's Bark Worse Than it's Bite

Bacterial Blight's Bark Worse Than it's Bite
Make positive identification to avoid spraying for nothing.

A disease that typically doesn't show up that much on Indiana soybeans is showing up in more fields and on more plants than normal this year. It's called bacterial blight, caused by a bacteria. The good news is that it likely won't affect yield.

That's the word from Kiersten Wise, Purdue University plant pathologist. The lesions caused by bacterial blight are angular with a yellow halo. As the disease progresses, tissue turns brown and falls out, giving leaves a tattered appearance.

Since this disease is caused by a bacteria, spraying a fungicide just for this disease will do no good, she notes. If you were going to spray a fungicide for other reasons anyway, that's one thing. Don't let someone talk you into spraying a fungicide because you see these symptoms, since fungicides will do no good in controlling it- the causal agent is a bacteria, not a fungus.

However, the key is to make sure your plants have bacterial blight and not some other disease. It can be hard to tell apart from brown spot, she says. Brown spot is caused by a fungus. However, finding one leaf with brown spot may still not justify applying a fungicide unless you intended to apply one anyway, specialists note.

Plants coming into the Morgan county fair for judging last week typically contained at least one or more lower leaves that displayed the symptoms of either bacterial blight, brown spot, or both. However, for the most part, the plants appeared to be otherwise healthy.

Wise suggests sending a sample to the Purdue Plant & Diagnostic Lab to make sure which disease you have. Whether you want to spray a fungicide for brown spot or not, it helps to know so you can select varieties with more resistance in the future. There is a difference in varieties when it comes to genetic resistance to both of these conditions.

Crop rotation will also help lessen the chances to see bacterial blight. Growing soybeans after soybeans allows inoculum to build up, Wise says. Tillage may also help where tillage is feasible without causing soil erosion.
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