Need to know why my mandevilla plant is turning brown leaves and all of the blooms are falling off?

Question:

Answers:
Mandevillas are beautiful plants.I happen to have 2 growing on my front porch. Without observing your plant in person, I can only make several guesses as to why your mandevilla is losing its leaves and blooms;

1) Mandevillas do not like to sit in standing water and are prone to root rots. I lost mine in this manner earlier this year by having the pot sit in a saucer-filled water. Remember that they prefer to be in well-drained soil that is kept moist and not water-logged.

2) There are two kinds of foliar diseases (that fit your description) found to attack Mandevillas; Corynespora and Cercospora (see pics at this site; http://www.velvetmandevilla.com/diseases...

As stated at their site, these diseases usually occur during the warm and wet months of the summer and can spread rapidly when leaves are left wet overnight. Avoid overhead watering and treat with a fungicide such as "Cleary's 3336" which contains the active ingredient "Thiophanate methyl". (You might be able to find it in some of the local specialty garden shops)

Sanitation is the best way to prevent further infestation...remove all dead leaves and blooms and discard in the trash.

3) Check for scale around the base of the leaves near the petioles and under the leaves for mites. When these insects become large in numbers, they can make leaves turn brown and at times cause leaves/blooms to drop.

IF you see any of these, treat them with Safer's Insecticidal Soap, making sure you get good coverage of the entire plant.

NOTE: Mandevillas prefer a soil of a pH of 5.5 to 6.5 (slightly acidic). They don't require much nitrogen so you can fertilize with a 10-20-10 in the early spring and again in early summer.

Sorry I can't be more specific but hopefully one of these suggestions will help solve your problem. GOOD LUCK!

-Certified Professional Crop Consultant with over 30 years of experience and a Degree in Plant Science