WE USE A SULFUR BASED FOLIAR SPRAY IN OUR IPM REGIMEN
If you notice a white or yellow residue in the grooves or spots on the leaves of your new plants, it's most likely dried foliar spray, not powdery mildew (PM).
Sulfur is commonly used across many agricultural industries worldwide because it's one of the most cost-effective and efficient treatments for a variety of issues. This is why the presence of sulfur residue on your plants is perfectly normal.
In fact, sulfur is so effective that it eliminates PM on contact. If you're dealing with powdery mildew, a sulfur treatment will take care of it.
Important Note: To avoid a potential phototoxic reaction, wait 7-10 days after a sulfur application before using any oil-based IPM treatments.