Q.Dieffenbachia
My Dieffenbachia has white spots on the underside of leaves, which then yellow and drop. I have tried insecticidal soap and soapy water, with no success.
Spider Mites are a common pest on a Dieffenbachia, though this could also be Mealybug.
Neem Oil is a good treatment.
Here are some links with more information.
https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm