Guess the Pest! Week 23 Answer: Soybean Stem Canker (Diaporthe)

David Owens, Extension Entomologist,

Congratulations to Mark Wainwright and George Petitgout for correctly identifying last week’s soybean disease as stem canker. This one caught my eye and I first thought Sudden Death Syndrome, but having learned my lesson about disease diagnosing from just a photo, I sent a sample to Dr. Alyssa Koehler, our Extension Plant Pathologist.

This from Alyssa:
This week’s guess the pest was a tough one. There are many soybean diseases that produce foliar symptoms that often are lumped as Sudden Soybean Death (SDS). The leaf symptoms of this image were caused by one of the ‘look-alikes’, a different fungus called Diaporthe. The Diaporthe complex can affect soybeans at about every growth stage. The many species of Diaporthe cause diseases like stem canker, pod and stem blight, and later Phomopsis/Diaporthe seed decay. While it can be tricky to distinguish late season soybean diseases, I have prepared a table with some of the features to look for. One of the diagnostic signs for SDS are the blueish structures at the base (sporodochia). These can turn white once exposed to air, so it is best to look for them right as you are digging plants or if you have had plants incubating. Splitting stems is another way to sort out SDS and Brown Stem Rot (BSR). When splitting stems, SDS usually has brown outer vascular tissue (cortex), while BSR is brown in the center (pith) and the vascular tissue is usually still healthy looking. Diseases from Diaporthe spp. will tend to have healthy looking vascular and pith tissue, but you may see black zone lines.

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