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Frogeye Leaf Spot on Soybean (08/12/21)

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We are seeing some limited Frogeye Leaf Spot in North Dakota. Given the low severity and advanced growth stages, yield loss is not a concern. Further, without frequent rains, an increase in disease is not likely.

Symptoms of Frogeye leaf spot begin as small dark spots, and enlarge to 1/8-1/4 inch, circular to irregular leaf lesions. 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 1). 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.

 

While yield loss is not a concern this year, it is important take note if you find frogeye leaf spot. The disease was first identified in North Dakota late in the 2020 growing season. With support from the North Dakota Soybean Council, we conducted a survey in the first week of September 2020. Frogeye leaf spot was common throughout the SE counties of the state. Additionally, of the over 300 pathogen isolates we collected, 23% were already resistant to FRAC 11 (QoI, strobilurin) fungicides.

 

Figure 1. Upper and underside of Frogeye leaf spot lesion.png

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops