NEVER fertilize an otherwise stressed plant ... it just adds more stress ...

w/o a picture.. its really hard to help you ...

sacrifice of lower leaves.. is usually assoc'd with lack of deep proper water ...

but i have no clue how a plant looks worse after watering.. unless there is some root rot going on ...

how about digging one up and see what going on in the root zone ...

no comment on pH levels.. w/o a soil test.. but if you have a wide ranging garden.. i dont know why one type of plant would have a problem ... sounds fishy to me ...???

and get us a pic ... check out the link

ken

ps: were they planted at the proper planting time in spring.. or are they rather recent plantings????

Here is a link that might be useful: link

Thanks Ken. These were planted Mid-May, and are at the forefront of the perennial bed. Three that I had in a no-drainage hole pot started to look the same way after we had a lot of rain. I moved them to another pot, and they are doing great now. That's why I thought maybe the clay is keeping them too damp.

As soon as the camera charges up I'll post a picture!

From what I've read, vincas don't like humidity nor cool wet weather. Had it rained a lot lately or has the humidity been high? Perhaps the pot is able to dry out better than the ground. I've avoided growing them because our New England weather can be unpredictably cool and wet, even during the summer, not to mention days of high humidity. They are breeding new more humidity tolerant strains of them, though.

Here are the photos of the distressed Vincas. It has been extremely hot and humid here, and it almost looks like black spot on the leaves of one. My neighbor has roses with a black spot issue just on the other side of the fence.

Thanks in advance for your help!

Here is a link that might be useful: Photobucket

Correct that Catharanthus (commonly called Vinca) doesn't like cool temperatures, but I've never known them to shy from humidity...as long as they are growing in a well drained site. They are at their best in hot, sunny, even arid locations and have earned their reputation as a successful parking lot plant.

However, these plants are quite susceptible to a string of soil borne pathogens, including several fungal disorders and nematodes. Soils that stay consistently wet for even a short period of time can trigger the fungal relationship.
These pathogens all exist naturally in the soil and won't cause a problem unless you have that combination of disease-perfect environment-susceptible plant species.

I think of Cathranthus as happiest in a xeric setting, planted with others of the same ilk. Though commonly grown AS an annual, it is actually a perennial sub-shrub, one that gives us a very long lasting bloom period as long as the conditions are right. Hot and dry.

Probably too much water - vincas don't do well in overly moist conditions, prone to rot.