Periwinkles are a member if the vinca family. (There are several other types of "vinca" don't think the ground covers are susceptible...) They are highly susceptible to stem rot. However, I have understood that stem rot is actually a virus, not a fungus which is why no fungicides work on resolving the problem. A tip, periwinkles are very drought tolerant. They cannot tolerate wet feet. Only water them as often as the soil dries out. If you water them everyday, you will always have problems with stem rot. If you have a bed with poor drainage,, forget about Periwinkles. Learned this one the hard way after loosing 40+ flats. I'm with turfdawg. Pentas are the way to go as a sub.
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