Grey Spots and Holes on Knockout Rose Bushes
Our knockout rose bushes have greyish spots on the leaves, which seem to eat a hole in the leaf right where the spot is. Our bushes are covered with them. We have looked carefully and it is not an aphid or any kind plant pest. Can you tell me what this is and how to get rid of it?
Certified GKH Gardening Expert
At first, I thought you might have powdery mildew. However, the powdery mildew typically does not make holes in the foliage. Some worms and caterpillars, on the other hand, come out at night to do feast on plants, so it still could be a pest of some sort that you just are not able to see due to the timing of the attacks. I would recommend spraying the bushes down with some Ortho Bug B Gon or similar insecticide.
Sometimes water droplets get on the leaves from sprinkling systems or even a mild rain. Then the sun comes out hot and intense and the droplets act like little magnifying glasses on the foliage, causing burn spots that then turn into holes where the leaf has been burned. This can happen with the application of some water soluble fertilizers and other sprays that are applied at the wrong time of day.