Hi, if it's the PERIWINKLE (vinca) it COULD be a fungus (Phytophthora). Then, there's stem canker which is from watering overhead/too much watering. I have a lot of periwinkle in pots alone and/or in pots with others. I first thought I had a "bad batch" from the vendor(s) but I couldn't remember who I got what batch from so I had to go back to the drawing board (Some vendors have stopped selling vincas because of the phytophthora, btw).
I began to look at the soil (too fluffy? too dense? poor drainage? crummy pot-mate? loneliness? were they chloritic? Then I realized, (and this is after a year of vinca problems), hey, wait a minute: I get up at 3:45 a.m. to go to work (yeah, I drive a long way), I don't have TIME to water in the morning, so when I get home (sometimes well after 5) I water. So, the plants that have the SERIOUS ISSUES, are the ones that have gotten "water cooties" on their leaves late in the day AND the ones with poor drainage.
Once, I did cut them back to death and had a good resurrection; however, that only lasted one season. This year I got the guts to pull one completely out and saw stem cankers. Sad.
HOWEVER, I will add this: The POTTED (not IN THE GROUND) periwinkles (vincas) I have are just creeping up on their 4th birthdays; I've seen some in the ground years and years old. So, age, I think, is a factor, too.
ULTIMATELY, I'm convinced it's the stem canker because of the watering and, sometimes overcrowding doesn't give them a chance to become dry. They do like dry and arid conditions....
As far as the vinca minor (the trailing one w/blue flowers
) goes, I have that on a slope and in the Southern California heat, they wilt, sure, but water'll perk them up. In fact, they only flower
when I water them regularly; not just when I "remember".