I attended the ISU Integrated Management Conference and one of the presentations was on soybean sudden death syndrome. Here's what I heard:
The fungus survives for several years on various crop residue. And the worst host is corn kernels.
Infection can occur soon after planting and is made more likely if the weather is cold and damp.
Soil compaction and SCN tend to make it more likely to occur.
Crop rotation doesn't help
Fungicides and herbicides don't help.
What can you do about it?
Delay planting may help
Deep till or break up compaction
Tile (but drainage won't help on a flood year)
Use SCN resistant seeds.
Some beans are more resistant to SDS
Probably every county in Iowa has SDS. The fungus can sit on the roots and not ever bother. It has to get inside the root hair. If it does and some event triggers a release of toxin, it acts like a herbicide on it's own host and attacks it.