Dormant Mandevilla with white spots


We live in Scarborough. I overwintered 3 mandevilla vines in our cool basement and discovered they now have white spots on the stems ( no leaves on plant) which I think might be a mold. The white spots are mostly at intersecting stem and branches but some near base of stem. We neglected to water them over the winter, unfortunately. My husband put them outside in this nice weather. I think we should have climatized them inside but will bring them in now that weather is cooling off. We have an unheated glass porch facing north with western exposure as well but not east ( bushes) . Shall I place them in there or elsewhere and for how long ( after May 24? )? Outside they grow in a container with a wooden slatted trellis facing east. I have included a picture. Please advise and thank you!


Thank you for your question.  We have had previous questions abut overwintering Mandevillas & will include some of the content in this answer.

It sounds like you have let the plant go dormant over the winter.  Ideally this would involve placing it in a dark area which is kept at 10 – 15 C.   Cutting it back to about 30cm above the soil line. Washing the remaining leaves with a damp rag or wet paper towel to remove any pests. Inspecting and treating for insects if needed. Watering it about once per month or when the top 2.5cm of soil begins to dry would have been beneficial. Leaves will drop during dormancy.

If you are now seeing any early spring growth since you have had them outside, this would indicate the plant is breaking dormancy and has survived.. The mandevillas can then can be moved to the sunniest room in your home where the temperature is at least 15 C. Resume normal watering and fertilization. Move it back outdoors when temperatures are consistently above 15C.  At this point the plant can be repotted in a larger pot with fresh potting mix.

It is difficult from the picture you  have submitted to determine exactly what the white spots on the stems might be.  The common pests that inhabit Mandevilla are mealybugs.

Mealybugs are a type of scale insect. As they feed, they cover their bodies with a characteristic white, cottony substance for protection.

These pests damage indoor and outdoor plants by sucking out their sap. They also excrete honeydew over leaves and stems. This sticky exudate is an ideal growth medium for sooty mold which gives the plants a dirty sooty look. It also attracts other insect pests.

Although your plant is dormant, you could try to physically remove as many mealybug scale as possible from your Mandevilla. If the plant is small, you could remove the pests individually, knocking them into a container of soapy water. Or, you could spray the plant with a hard blast of water, collecting and disposing of the bugs that fall off, and then picking off and dispose of any adults that remain on the plant.

You can also apply an insecticidal soap, carefully following the manufacturer’s instructions which may help with population control at the nymph stage of the mealybug.

More information about life cycle and photos can be found here.

April 17, 2023