Over the past couple of weeks, we have had several reports and soybean samples of top dieback.

The last time we had a widespread outbreak of this disease in Iowa was from 1997-1999. This season ²⁰⁰⁷, the disease has been found in eastern and central Iowa.

Differences in susceptibility among varieties have been noticed. In addition, affected plants are often defined by rows — although patches of top dieback, not unlike those you would expect to see with soybean cyst nematode damage, have also been reported.

Top dieback symptoms are usually seen between late July and mid-August when most soybean plants are in the reproductive phase.

Initial symptoms are characterized by yellowing and death of leaves in the upper portion of the plant canopy, followed by discoloration of internodes. Plants die prematurely when the disease is severe. An Ohio study showed that fungal species belonging to the genera Phomopsis and Diaporthe, a group of plant pathogens that cause various diseases in soybean, are associated with top dieback.

No systematic study has been carried out on this disease, and as a result, we do not know when this disease strikes or what factors contribute to its appearance. It has been speculated that potassium deficiency may be associated with top dieback symptoms because they somewhat resemble those of potassium deficiency. Several years ago, it was observed in southern Iowa that the disease ceased after an application of potassium. Soybean cyst nematodes are also frequently found in these fields. In collaboration with Greg Tylka, who specializes in plant-parasitic nematodes, we will be collecting soil samples from affected and non-affected areas of a field showing top dieback symptoms to investigate if there may be a relationship between the 2 pathogens.

If top dieback is severe, consider changing soybean varieties for the next growing season. Do not save seeds from fields with severe top dieback because the pathogens causing it can be seedborne. If necessary, soil and plant tissue analyses could be conducted to ascertain the involvement of potassium deficiency in symptomatic fields.