Hi, Some of leaves on knockout roses have brown/yellow spots and etc. Please see attached pic. What is wrong with my roses and what can I do? Thank you.
This looks like pesticide burn from spraying roses foliage on cloudy days or late in the day. The product stays on the leaves in liquid form too long and burns the foliage. Usually a spray is mixed just a bit to heavy or above the on the label instructions and thus burns the foliage. It can also be caused by water from lawn sprinklers getting on the roses foliage during the hotter parts of the day and the water acting like a magnifying glass directing the suns rays onto the foliage, thus burning it. It does not look like insect damage.