Q: I have a black fungus on the stems of my dahlia plants. What is it and what can I do to treat/prevent this problem?
Sounds like sooty mold that grows on honey dew given off by aphids. Take a sample to your local garden center for an accurate Id. Get rid of the aphids that produce the honey dew that the sooty mold grows on. Sadly the black stain will not go away this season. Best Nancy