Why Do My Roses Have Holes in the Leaves?
The long-lasting color and sturdy structure make roses a focal point in any garden. Unfortunately, roses aren’t just attractive to us, they are also attractive to all kinds of insects and diseases that love to punch holes in those dark green leaves. If you are seeing holes in your rose leaves, chances are it's due to roseslug sawfly. Sawflies may look like miniature caterpillars, but they are technically the larvae of certain flies or wasp-like insects. The roseslug sawfly has several generations throughout the year which keeps the holes forming for most of the growing season. But while their damage is quite conspicuous and alarming, the sawfly is fairly easy to control. This is because of their soft bodies which make many foliar insecticides, such as the organic insecticidal soap, highly effective at controlling them. For heirloom rose species, some people will elect to use systemic insecticides. These are applied to the soil and control the insects when the damaging pests ingest treated leaves. Systemic insecticides tend to cost more and can provide year-long protection.
There is another insect that damages rose leaves. The damage caused by the Japanese beetle is usually much worse than just a couple of holes. The Japanese beetle can completely defoliate a rose during the few weeks that they are flying around as adults. Controlling the Japanese beetle with foliar sprays is much more difficult. Not because there aren't sprays that will work, but because these sprays are also very hard on beneficial and pollinating insects. Some say it is better to use a systemic insecticide before the Japanese beetles visit your roses. There is a non-neonicotinoid product that is safe for bees called Acelepryn. Your plant health care professional will be able to apply this for you. The product is sold as a concentrate only and is best controlled by a trained professional.
Finally, black spot disease of roses can leave dark circles on the leaves. This fungal disease will not cause holes. But the infected leaf will turn yellow and drop off. Some species of roses are more susceptible to this disease than others. The knockout roses seem to be the most resistant. If you have heirloom-grown species on your property you are probably having to do preventative fungicides to control this. Fungal diseases like black spot can be suppressed by good pruning and sanitation efforts. Roses should be pruned into an open vase-shape. This allows light and air to make it all the way through the plant and reduces the favorable areas for spores to develop. Keep mulch away from the base of the plant as this also harbors spores. Blacks spot spores spread from the soil and splash up the plant through irrigation or rainwater. Removing the leaves on the bottom 2 inches of the plant may help stop this upward movement. Avoid overhead irrigation if at all possible.
Roses are quite beautiful and rewarding shrubs to have in your landscape. While it seems that they are attacked on all fronts from various pests and diseases, these issues can be mitigated by a patient home gardener and/or help from a professional.