Despite the late planting dates, we did not escape many of the late season diseases that attack soybeans.
Sclerotinia stem rot – we found this disease in our research plots today in a field that was planted on June 19. Sclerotinia is caused by a fungus that survives from season to season and over several years from sclerotia. The infections actually occurred during flowering when the canopy was closed, and cool nights can really enhance and favor this disease.
Sudden Death Syndrome – this disease is also beginning to develop and symptoms typically start just after soybean growth stage R5. Symptoms include irregular yellow spots, which turn brown or necrotic between the veins. Interestingly the veins are surrounded by green. The center of the stem or pith is bright white in this disease.
This is a fungal pathogen and infections most likely occurred shortly after planting. Even though we planted in mid-June, soil temperatures were still relatively cold this year and I won’t relive how much rain we had, but suffice it to say, this field received over 3” the weekend after we planted. These conditions greatly favor infections. Note in the picture the resistant cultivar.
Diaporthe stem canker (northern and southern) have both been problems, but I have not received any samples to date. On susceptible cultivars the plants will die early in patches. For Northern, there is a canker at the third node which girdles the plant. For Southern, there can be several reddish cankers on the stem and the internal tissue is a reddish brown.
Phytophthora stem canker – we are finding this way too often this year and in places that have not reported it very frequently. The plants will wilt first, turn yellow, and a chocolate brown canker will form from the bottom of the plant to almost mid-height. The key difference between this and Northern Diaporthe stem canker is the length of the canker and where it originates. If the canker begins below ground, it is Phytophthora.
Another look alike symptom on soybean leaves that you might be seeing is potash deficiency. We have had a number of calls late this summer asking what might be causing discoloration of soybean leaves – see the photos of a leaf and of the field edge.
What appears to be happening is #1 – it’s dry, or root systems are compromised is some other way (think excessive wetness and compaction) or #2 the symptoms tend to be at the edge of the field. Here the guess is that with larger high capacity fertilizer applicators, we may not be getting that full rate all the way to the edge of the field.
As a result, we run low on K there when we likely have adequate amounts across the rest of the field.