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What happened to my knock out roses?

What happened to my knock out roses? The caller stated, “They used to be so beautiful.”

Knock out roses are popular because there are very few pest or disease issues. They are fairly forgiving in their site conditions given well-drained soil and plenty of sunlight. I have a few at my house that barely receive any sun but still flower albeit not prolifically. Knock outs are available in multiple colors and do not require much routine maintenance.

In regard to pruning, you might get away with pruning for size every other year but you should prune every year. Pruning late in the winter when the buds begin to swell is the best practice. I like to prune them hard to cause a good growth response, routinely knocking them back to around 18 inches.

Looking at the most common diseases, the first is Black Spot. This shows on the leaves as small, circular black spots but then multiple black spots appear eventually causing the leaf to turn yellow (chlorotic). Once the leaves turn chlorotic, the senesce and the bush begins to defoliate. Black spot is most severe after long wet, warm periods in the spring. Ultimately, plants will become stunted and produce less and less flowers. To keep this disease from knocking out your roses, sanitation is your first line of defense. Clean up old leaves in the fall along with any mulch where infected leaves have fallen. Remove infected canes and destroy them. Remove infected leaves during the season as soon as you see them. Use fungicides or consider planting resistant varieties if the infection is severe.

Powdery mildew is another fungal disease that I often see on many plants including roses. This infection looks like the leaves have been covered with baby powder. Typically, this is an issue during warm, dry periods, usually toward the end of summer when we have high humidity at night and low humidity during the day. Powdery mildew results in leaf distortion, leaf drop, and poor-quality flowering to flower buds that just don’t ever open. Again, sanitation is your greatest defense. Remove and destroy infected leaves as soon as possible. Fungicides are available to help. Make certain to kill two birds with one stone by using a fungicide that controls black spot as well.

These common diseases were not the issue here, unfortunately. This was the dreaded rose rosette virus. Just like in humans, there is no cure for viruses. Symptoms of this disease include witch’s broom arrangement of the leaves, red leaves, leaf distortion, succulent stems, and an excessive number of thorns. A witch’s broom arrangement is where the leaves emerge nearly on top of one another making it look like the end of an old straw broom. This can also be a symptom of glyphosate (active ingredient in Round-up and many other herbicides) damage on roses. The red leaves will resemble young leaves just out of the bud but they will not turn green. The leaf distortion can actually mimic synthetic auxin damage from herbicides like 2, 4-D. The leaves will be long and skinny, also called strapping. Succulent stems will be thickened in size, excessively fleshy, and pliable. There is no mistaking the excessive number of thorns, however. This is the tell-tale sign that the rose rosette is an issue.

The rose rosette virus is spread by tiny mites that attack the leaves and petiole of the plant. They are too small to see with the naked eye and feed on so little of the plant that they are impossible to defend against. Wind currents carry them from place to place and plant to plant. As a carrier of the disease, once they feed on the rose, it is infected. There is no cure but to destroy the bush. While there are disease-resistant roses, it is not recommended to go back with them in the same area. There are only two that have resistance according to NC State University, Rosa carolina and Rosa setigera. It is time to look at another flowering bush to fill that vacant area.

If you are having trouble with growing in your home landscape, call the Extension office at (252)946-0111 or email me at gene_fox@ncsu.edu. I will have a commercial ornamentals and turf class coming up in February/March. If you need pesticide or landscape contractor credits, please keep an eye out for details on this class. I am also planning a series of classes beginning in February that will go into soils, fruits, vegetables, ornamentals, and lawns. Keep an eye on our website or the Beaufort County Master Gardener Facebook page for more details or register at this link: https://go.ncsu.edu/2024grow. Until then, Happy Gardening!