What is that white “fuzz” on my Mexican Petunia?
The first thing you should check for is mealy bugs. They are known to infest Ruellia and can be difficult to control.
If you do find mealy bugs on your Ruellia it is best to trim the plants back hard, bag the clippings and seal the bags before disposing them.
If the temperatures are between 45°-85°F you can spray them will All Seasons Oil after new growth appears. Consult labels on other products to be sure they are labeled for mealy bugs.
What if I can’t find mealy bugs?
If you are sure that there are no mealy bugs it is probable that your Ruellia are infested with a tiny mite called an Eriophyid mite, or Gall mite. The fuzzy growth you see is called “Erineum”. It is the plant’s reaction to the feeding of the mite. The fuzzy tissue provides shelter for the mite to continue feeding, protecting it from pesticides and weather.
The treatment for this mite is similar to the treatment for mealy bugs. Cut the plants back hard and bag the clippings, sealing the bags before disposing of them. You do not want to carry the clippings across your landscape, as these mites can infest other plants and are the vectors for disease including Rose Rosette disease.
Once you have disposed of the clippings, spray the new growth that emerges with All Seasons Spray Oil. Remember that it needs to be between 45° and 85°F with low humidity to work well and not damage the plant. Check other pesticide labels to confirm they can be used for mite control before using.