Dipladenia stalk turning brown and squishy

[Lel5120]

Hello,
I bought this dipladenia plant yesterday from my local supermarket, and only noticed afterwards that it had brown patches on some leaves and squishy stalks (I had to cut off all the flower buds as they seemed to be wilting on the plant!) The next day I noticed more of its branches were beginning to turn brown. I don't know what's wrong, does anyone have an idea?
I live in france and it's quite warm here at the moment, I keep it in a well lit room out of direct sunlight and haven't watered it yet, as it's still quite wet from the shop watering it.

Thanks in advance!

 

[Marck]

Greetings, welcome to the Forums.

Immediately unpot the plant and inspect the soil and pot for drainage issues. Also remove all dead and rotting leaves and stems. If the container does not have drainage holes, repot into a different container or drill holes into the pot. Place this plant in full sun or bright light. Full sun is ideal, but increase the light level in stages to avoid suns scorch due to a sudden change. Let the soil partly dry before the next irrigation. Fertilize monthly with a complete, liquid fertilizer.

Dipladenia (Mandevilla species and hybrids), in the Dogbane Family (Apocynaceae), are evergreen vines and shrubs popularly gorwn for there large, colorful flowers. Overall the genus is native to Arizona, New Mexico, Texas, Mexico, Cuba, Central America, and South America, but many of the more commonly grown varieties are hybrids of South American species.