Possible Xylella fastidiosa in a Cup Plant (Silphium perfoliatum) at Troy Historic Village, Oakland
This plant is in an ABC garden in a mainly sunny location at the Troy Historic Village, Oakland County, MI. It has always performed well; very tall and prolific flower producer. The plant is probably 8=9 years old. This year the plant appears stunted. Aphids have been an issue and are controlled by hosing off the leaves and 2 weeks ago I released lady bugs acquired from English Gardens. In the past 7 days the plant has taken "a turn for the worse". It is now wilting, with leaf scorch, curling leaves,dieback and really looks terrible. I googled "Cup Plant" and found a collection of photos. One looked exactly like this plant. The condition was called "Phony Peach" caused by Xylella fastidiosa. The article said it is a devastating disease as it invades the plant's water conducting system. Darn, this is one of our "S" plants. What ever is going on is moving quickly.I hope I am wrong! HELP. If nothing can be done it probably is not wise to place another cup plant in that location. We also have a very large lavender plant in the garden, but on the opposite side of a path so not growing in the same bed. The article said lavender is susceptible too. Thank you, Karol Carter
Oakland County Michigan
It sounds like you have done considerable research.
Cup plant is visited by leaf hoppers and they can transmit many diseases including Xylella fastidiosa. However we can’t positively identify bacterial diseases from pictures. And there isn’t much on diseases of cup plant in my research-based references. This is probably because cup plant isn’t commercially important like grapes or other crop plants.
This also looks like powdery mildew. Are the plants watered from overhead? Wet leaves would encourage mildew. Watering and hosing off should be done so that leaves dry as quickly as possible, sometimes hard to do in humid weather.
If you would like to confirm your diagnosis start by submitting an email request to MSU Plant Diagnostic lab. During Covid19 they request you email them first at firstname.lastname@example.org including your pictures. If the disease is continuing to progress it is probably best to remove it, isolate it so you can send the lab samples if needed.
If they need samples they will contact you.
Even if the disease is confirmed, you can start new plants from seed or bring in a new plant start. The disease doesn’t reside in the soil.