Question
I have a fairly large diffenbachia that is now covered with some white powdery substance and the plant seems to be losing big leaves one by one.  I was using a plant light, now I moved the plant to a more natural sunlight area and not much has changed. I water once a week with room tempature water. Please advise.

Answer
Hi Kathy,

The 'white powdery substance' is either spider mites are mealybugs. The former look more like tiny white dust particles and they make very tiny webs if you look closely. Mealybugs look like small pieces of white cotton. Treat the spider mites with a solution of soapy water, using liquid dish soap. For mealybugs, use a solution of 5 parts soapy water and 1 part rubbing alcohol.

The key to successful eradication of indoor plant pests is thoroughness of your spray coverage. All leaf and stem surfaces must be thoroughly drenched until they are dripping wet to make sure you get all of the critters. Yes, this is messy, but it is essential to really get rid of them.

Most plant pests thrive on plants that are under stress. The loss of leaves is not caused by the bugs but by something else. You didn't provide a lot of detailed information, but it seems that your Dieffenbachia has not received enough light. A plant light alone is not enough. Your Dieffenbachia must be very close to a moderately sunny window that is left uncovered throughout the day. However, it will take several months for you to see positive results and you will continue to lose older leaves in the interim.

Watering by a calendar schedule is not a good idea. Because your plant has not received adequate light, its growth rate and water usage have been slowed so weekly watering may be too much. Allow the top quarter of teh soil to dry out before watering.

I have written an article on indoor plant pests that I will email for free to anyone who sends a request to me at my address below.

Please let me know if any of this is unclear or if you have any additional questions.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC