What's wrong with my Dipladenia?
I live on Illinois and bought three Dipladenia plants at Menards. When I first got them they were very pretty and full. Now, leaves and becoming yellow and falling off and the flowers have white-ish spots on them. There are also far fewer flowers. What am I doing wrong? I know these are tropical plants, but it is the summer and they get plenty of sunlight. Am I overwatering? Is it insects? I'd really like help, as I think these plants are so pretty. I've attached a picture below. Thanks!
Will County Illinois
It's hard to tell without close-ups, but I suspect your Mandevilla ("Dipladenia" is an older name that is no longer in formal use) may be suffering from an insect attack, as well possible over-watering. There appear to be white spots on the underside of leaves, which could be insects or nymphs; as well as some areas that might be congregations of mealybugs.
The yellowing of the leaves may be a sign of over-watering; the good news is a lack of distinctive brown spots that would come with the more common fungal infections associated with Mandevilla. What is unusual is the injury to the flowers, which looks like it might be physical damage caused by chemicals or by effects of overhead watering during intense sun. Fungal and bacterial pathogens rarely invade Mandevilla flowers.
Mandevilla likes somewhat moist, but not wet, soil. It's better to err on the side of dryness -- let the top inch or two of medium dry out before giving them a good drink. Apart from insect pests, Mandevilla tend to stay healthy if not compromised by wet feet (or lack of sun, which can contribute to soggy soil). As with all plants, it's important to practice good sanitation -- clean up dead plant material on and around the plant, pluck infected leaves, remove insects, avoid water standing on leaves -- to maintain a healthy plant.
Mealybugs, whitefly, aphids, scale and mites all like Mandevilla. Many of these can be controlled with a good "spray shower" when you see the first signs of a problem. Look for insects, their eggs or larvae, or the sticky "honeydew", webs or cottony growth in branch crotches that indicate their presence. Use a strong spray from the hose to "blast off" whitefly, mites and aphids. Scale and mealybugs can be a bit harder to dislodge; try an alcohol-soaked cotton swab to wipe them off, and rinse well. If water and swab control isn't enough, rather than resorting to stronger insecticides, try an "insecticidal soap". These are milder chemicals, but they are still pesticides -- read all labels and follow instructions carefully.
Here's a fact sheet on growing Mandevilla from the Clemson University extension:
The University of Minnesota extension has a comprehensive article on "Houseplant Insect Control", which covers the pests that like to make themselves at home on Mandevilla, regardless of whether grown indoors or out:
The article includes up-close photos you can use for identification, as well as control strategies that apply in both environments.
Good luck -- I hope a good shower and some "dry feet" will send your Mandevillas back to good health!