Transparent Spots and Holes on Knockout Roses: Slugs vs. Mildew

Diagnosing Leaf Damage in Chicago

Knockout rose leaves show small transparent areas (underside missing) and irregular holes. Some holes have adjacent transparent tissue—do transparent spots become holes?

Rose Slug (Sawfly Larvae) Damage

• Feed on leaf undersides
• Create "windowpane" effect—top layer dies, turns brown, falls out
• Holes are small, uniform size
• Rarely cause leaf drop unless infestation is severe

Downy Mildew Damage

• Burns top leaf surface
• Large, irregular pale gray blotches with no definite outline
• Kills large leaf areas → leaf drops

Key Difference

Rose slugs = small uniform holes from underside feeding. Downy mildew = large irregular blotches on top surface causing leaf death.