White spots, holes on Knock Out Roses
We had several Knock Out Roses planted in April. They have done well until now. Recently, I have noticed white spots and holes on the leaves. Is this a fungus or an insect? I have not seen anything moving and have looked under the leaves. How do I correct this problem?
This looks like damage from rose slug sawfly feeding. These insects are typically very active in May-June but they can continue to be active through the fall. Check the surfaces of the leaves (upper and lower) for very tiny green caterpillars. This damage may be from earlier in the season, so you may not see insects present. Starting in May next year, check your plants regularly to see if there are sawfly larvae present on the foliage. You can use horticultural oil or spray with spinosad to manage these insects. You will find additional information here: https://extension.umd.edu/hgic/topics/rose-slugs-shrubs
Other insects, such as Japanese beetles, can also cause chewing damage on rose foliage in June-July. https://extension.umd.edu/hgic/topics/japanese-beetles