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Frogeye Leaf Spot on Soybean (09/10/20)

We are beginning to find Frogeye Leaf Spot in North Dakota. Yield loss to Frogeye will not occur this late in the growing season.

We are beginning to find Frogeye Leaf Spot in North Dakota. Yield loss to Frogeye will not occur this late in the growing season. 

To the best of our knowledge, Frogeye leaf spot has not been confirmed before in North Dakota. We have observed the disease in Sargent and Richland Counties last week, and are conducting a survey this week to better understand the prevalence of the disease. The disease occurs widely through the soybean-corn belt and has been reported from South Dakota and Minnesota.

Symptoms of Frogeye leaf spot begin as small dark spots, and enlarge to ⅛ - ¼ inch, circular to irregular leaf lesions (Figure 1). Lesions centers are commonly gray-brown with a darker reddish-purple border. Visible gray fungal growth may be visible on the underside of the lesions (Figures 2 and 3). Other diseases and chemical injury can mimic Frogeye symptoms, and an excellent publication produced by a multi-state group of pathologists examines common look-alikes [Link https://crop-protection-network.s3.amazonaws.com/publications/cpn-1017-frogeye-leaf-spot.pdf].

The warm and humid growing season this year was very favorable for Frogeye leaf spot development. Most likely, the pathogen (Cercospora sojina) came from adjacent states where the disease is more common. The pathogen can survive in crop residue for two years, but it is somewhat unclear how well it will survive in North Dakota’s winter. We hope to have a better understanding of how widespread Frogeye leaf spot is in North Dakota by next week, and we thank the North Dakota Soybean Council for providing funding to conduct a survey. 

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Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

Berlin Nelson

Soybean Pathologist

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