I've lost a few plants due to all the rain & muggy air, poor plants dont get a chance to dry out when they are in containers, even with the bottom saucers removed I've still had problems, last night when I knew we had a big storm coming I moved some of my plants inside my screened in porch or under the eave's to try to help them out, my poor plants are having a rough go of it this summer. I hope someone comes along with some good advice for you, I wasnt much help..
Christine

My Mandevillas do well in the hot humid weather (and heavy rainfall), and also in the dry weather. They're in the ground in raised soil. I think the issue will be drainage, and that of course is more likely to show up when there's heavy rain.

Hi
having a similar problem with mine since the rainy season started . The answer of course is a fast draining mix
What mix are you using?? Suppose you can't put it in the ground in your area or a way to keep rain off it??
I'm using a mix of fir bark fines along with a bit of top soil
works great during dry season rainy not so much lol.
There must be a compromise but haven't found it as yet. but this has been an exceptionally rainy season this year gary

Thank you all for the insight:) I planted them in a potting soil & compost mixture with chipped mulch (a little mixed in & a layer on top.) One plant is doing ok, having a little new growth and blooms. The other is pitiful:( it looks even worse now than it did in the pic (top left.)

They just need a bit of time to recover.

What kind of soil are they in? It looks peaty to me. I don't care how much rain you've had. If the soil is peaty, it can easily turn 'hydrophobic' in sun and heat so that maybe your plants are not getting ENOUGH H2O! Have had plenty of experience with this due to relying on prepared mixes (i.e., cr/p), in the past!