Fungi on Knockout Roses

Significance

• Knockout roses promise to be virtually disease-free, displaying more disease resistance than conventional roses. While you'll notice far fewer pest and disease problems with your knockout rose than other rose plants, these special roses can indeed get fungus. They'll be harmed less than other roses and can receive less fungicide treatment. Fungus on roses can be caused by wet weather, other infested plans or growing roses in shade.

Black Spot

• The major symptoms of this fungal disease is black spots on the leaves of rose plants. The spots are surrounded by a yellow ring. New leaves are most vulnerable. Knockout roses can generally withstand black spot, although the plants will be unsightly. This disease occurs more often in warm, wet summer weather. Rake up and remove infected leaves to prevent the disease from spreading to other, more vulnerable, plants.

Powdery Mildew

• Powdery mildew, a fungal disease that can affect many plants, is more common in moist fall and spring weather. Symptoms include white powdery discoloration of plant leaves and curling leaves. With severe infections, all leaves may fall. Roses planted in the sun and pruned to allow air circulation between plant tissue display greater resistance to this disease.

Downy Mildew

• If leaves develop yellow, brown or purple splotches, downy mildew is the likely culprit. The top of the plant generally gets sick first, and leaves drop off the rose. Rain and watering from overhead, which gets leaves wet, engenders this fungal disease. Knockout roses are just as susceptible as other rose cultivars to downy mildew. Roses planted in the sun with good air circulation are less likely to contract downy mildew, as are plants irrigated at the root level only.