I have a half dozen Knockout roses that have been destroyed. I have seen no beetles (this being my first thought) and the leaves do not seem to look quite as if they had been eaten; some sections do look so but just as many areas have dried out patches throughout the leaf. Within a short time the entire plant was defoliated.
I do not know if a photo would help in identifying the issue but I have several on my PC I could send if this is possible?
I have another half dozen that are look extremely healthy; these are separated from the dead ones by a 2-foot concrete patio, but next year I fear they will fall victim to this malady if it is not treated correctly.
The Gardener’s Answer
Hi William in New Jersey: Knockout roses are more disease-resistant than some of the old-fashioned roses, but certainly not pest-free. As with all plant material, they are going to be more susceptible to insect and disease issues if they are not growing in ideal conditions. Roses demand a minimum of six hours of direct sun and nutrient-rich soil. That being said, it is difficult to give you a specific answer not being able to see your roses but your first thought is certainly a possibility. At this time of year, the beetles are not going to be on your roses but the damage they could have done earlier in the season is still evident on the foliage. After they are done eating, the foliage looks clear, kind of lacy, and when it rains it pushes the remaining foliage through, leaving you with holes. Rose slug is another possibility. For a positive diagnosis you can take a sample to your County Cooperative Extension Service or to a reputable garden center. For now, be sure to remove all fallen foliage and other plant debris since this provides a great place for insects to over-winter. Good sanitation practices are important in terms of a healthy garden.