Mandevilleas need to dry out between soakings, so they prefer a fast draining sandy soil. Also, I don't know how hot Odisha gets, but they might also need afternoon or filtered shade to thrive. They are native to a part of South America that has dryish summers, with cooler nights than than most of the true tropics--or my climate, for that matter.

My first thought was over watering. That's the #1 reason plants rot. Look at how these plants grow in nature, then try to duplicate that environment. Many plants just won't grow here in NY no matter what I do. How do I stop a Polar Vortex?

Some kinds of plant seem to be almost immune to root rot, while others seem to get it with the slightest excuse. With my experience with Mandevilleas, I wouldn't be the least surprised at it getting it during an Indian monsoon. The high temps, high humidity, and extreme rainfall would make many of the Phytpthora fungi very happy--and whichever plant they attacked, very unhappy!

Sometimes it's the long extended periods of wetness combined with cool temperatures that causes fungi to go rampant. That's when I go looking for mushrooms and get lucky.

Some fungi, such as botrytis, like cool wet conditions, while others, such as some Phytopthora species, prefer warm to hot, wet conditions.

Many thanx 4 d answer.

You're welcome, Chintamani!