Consider threat of seedling diseases in soybeans

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With soybean planting almost complete in South Dakota, Connie Strunk, SDSU Extension plant pathology field specialist, reminds growers to scout their fields for seedling diseases as plants begin to emerge.

"Even though most soybean seed has been treated with a fungicide seed treatment, no seed treatment product is effective against all seedling diseases and under all conditions," Strunk said. "It is important to look at what protection the seed treatment offers and the performance it delivers."

Most seed treatments offer a limited time of protection against seed and seedling diseases. Depending on soil temperature and soil water content, the chemicals are effective for about two weeks after planting.

"Damping off can occur in two ways; before emergence, where the seedlings fail to emerge because of a fungal attack, or post-emergence, where the seedlings are killed by fungal attack after emergence," Strunk said.

If your soybean field has been affected by seedling blight or damping off, Strunk says it is helpful to identify what seedling disease caused the infection. Proper diagnosis will help with fungicide seed treatment selection for replanting purposes and even for next year's planning.

The two fungi most often associated with damping-off in South Dakota soybean fields are Pythium and Phytophthora. Strunk says it is difficult to distinguish between these two seedling blights without lab testing.

"Phytophtora typically has a brown discoloration extending from the soil line up the stem of the soybean plant. Pythium attacks soybean seedlings planted in cold, wet soil whereas Phytophthora attacks soybean seedlings in warm weather generally around 80F," she said.

She adds that Rhizoctonia is another seedling disease that attacks soybean seedlings in warm weather (80F) and is often seen in late-planted soybean fields.

"With Rhizoctonia you will often see a reddish-brown colored lesion on the hypocotyl and lower stem of the plant but this is not observed above the soil line," she said.


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