No. 18 Article 4/July 27, 2007

Similar Symptoms on Soybean: Phyllosticta Leaf Spot Vs. Frogeye Leaf Spot

Phyllosticta leaf spot was found recently in a soybean field in McLean County. Although this disease is not new to Illinois, it is somewhat new to me. The particular leaf sample was collected assuming frogeye leaf spot, but closer examination indicated the problem to be Phyllosticta leaf spot. The diseases are similar in that both are caused by fungi (Cercospora sojina for frogeye leaf spot; Phyllosticta sojicola for Phyllosticta leaf spot). The diseases also cause similar symptoms on leaves: dull gray to tan lesions with dark brown to purplish borders. Magnification reveals the differences between the two diseases. Phyllosticta leaf spots may contain dark "specks" (known as pycnidia) in the center. Frogeye leaf spots will not contain pycnidia in the center; rather, a gray to steel blue "fuzziness" may be seen in the center of frogeye leaf spots if they are sporulating.

Phyllosticta leaf spot on soybean (magnified). Note dark "specks" (pycnidia) in the center of the spot. Photo courtesy of Dianne Pedersen.

Frogeye leaf spot on soybean (magnified). Photo courtesy of Dianne Pedersen.

Why is this distinction important? Some soybean varieties may contain the Rcs3 gene, which provides resistance to all known races of the frogeye leaf spot fungus present in Illinois. If the types of spots described above are being observed on varieties with the Rcs3 gene, it would be important to know if they are being caused by Phyllosticta or by a new race of the frogeye leaf spot fungus. In addition, several fungicides are available that list control of frogeye leaf spot on the label. None of the fungicides available for soybean list control of Phyllosticta leaf spot on the label.--Carl A. Bradley

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