Thursday, August 25, 2011

This is frogeye leaf spot of soybean. In order for frogeye to do much yield damage, it has to be active early. The frogeye is only able to infect leaves that are expanding or are scenescing. In other words, fully expanded leaves are basically immune to new infections and will only become susceptible once again when the leaves start to turn (pretty late in the game). The leaves that are showing symptoms now were infected when they were still expanding and you are just now seeing the results (infections probably 2-3 weeks old). Fungicides can only be used to prevent infections, but have no ability to control infections that have already taken place. Small lesions on leaves will continue to mature whether you apply a fungicide or not.

In reality, frogeye can look pretty rough, but it must be pretty severe before yield is hurt much. Disease in the lower canopy is of no consequence. It must be severe in the upper 1/3 of the canopy to do significant yield damage. In order to have prevented the lesions you are now seeing, fungicides would have had to have been applied about 2-3 weeks ago, before infection. Then it is questionable if later applications after you begin to see symptoms, are of much value because by the time older leaves become susceptible again, the crop is likely to be made.

Bottom line: it is either too late to do anything, or nothing needs to be done. The only outlier is if you have a late planted field that still has a long way to go (like currently at R3), then treatment may be justified since the plants will still need to put on new growth. The fungus can also infect pods and reduce seed quality, so if you are thinking about saving seed, it may be good to apply a fungicide to protect seed quality. Both strobilurins and triazoles give good control of frogeye.

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