Mysterious Silphium dieback summer 2014

Have other prairie restorationists observed this?

At Black Earth Rettenmund Prairie a major dieback of Silphium species is occurring. Strikingly, at Rettenmund this has only occurred in the areas that were not burned in the spring of 2014.

This die-back was most dramatically revealed with compass plant (Silphium laciniatum), one of the signature species of the prairie, but rosin weed (Silphium integrifolium) is exhibiting the same phenomenon. In other sites (also unburned) in southern Wisconsin, the two other Silphium species, cup plant and prairie dock (not present at Rettenmund), are also suffering from this die-back. 

 As often happens in restoration ecology, this dieback creeped  up on me. The leaves of compass plants grew normally in late spring/early summer. Most plants had a lush complement of typical leaves. But in mid-June, no flower stalks developed at the time they normally would appear. Since the compass plants in the areas that had been burned were flowering normally, I knew something was wrong. But what?

By mid August all the compass plant leaves had turned brown. A survey of compass plants at Rettenmund indicated 100% dieback. 

Because of its structure, the situation with rosin weed was somewhat different. All the rosin plants grew normally but at the time they would be flowering they began to turn brown. The flowers on any plants that had already started developing aborted. Again, 100% effect.

Since we did not have the other Silphiums at Rettenmund, I visited a few other sites in Dane County. Both cup plant and prairie dock also showed the same phenomenon. 

The end result? Unless the site was burned, there may be no Silphium seeds to collect!

I am trying to document the extent of this dieback and would welcome reports from others who have experienced it.

One more thing. All of the other species at Rettenmund are growing and flowering normally except flowering spurge (Euphorbia corollata), which has also shown 100% dieback.

Let me emphasize that sites that had been burned this spring are not exhibiting this dieback.