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I have a love/hate relationship with roses. If it isn’t the deer eating them from the top down, it’s the voles eating them from the roots up … and then there are the diseases. Here are the top 10 rose diseases, including pictures to identify what’s wrong—as well as advice on treatment.
Roses seem to be afflicted with more than their share of maladies. In terms or prevention, you can avoid a lot of this angst by selecting a rose that is less susceptible to disease.
Rose cultivars such as hybrid teas, floribundas, and grandifloras will need a proper pest control program with spraying.
This very common and serious disease, caused by a fungus (Diplocarpon rosae), is very easy to spot. The initial signs are small black spots on the lower leaves; these spots are then surrounded by a yellow area; finally, the leaves fall off. Eventually, black spot will spread to higher leaves and defoliate the plant. Leaves may grow back but, if it happens often enough, black spot will weaken the plant.
Black spot occurs most in humid weather in the spring but then continues into summertime; remove and destroy the infected leaves as they appear. Avoid the spread by keeping leaves dry; always water roses at ground level and never from overhead. During the winter, prune back all infected canes (which will have dark lesions) within 1 to 2 inches of the bud union. Spores can overwinter in fallen leaves so make sure to clean them up. Go to your garden center to get a fungicide spray for black spot.
2. Powdery Mildew
Here’s another widespread rose problem that is aptly named. Powdery mildew (caused by the fungus Sphaerotheca pannosa var. rosae) makes leaves, shoots, and buds look like they have been dusted with talcum powder. The leaves may look dry and curl up. The flower buds are also affected and some blossoms may look distorted.
Powdery mildew occurs when the temperatures are cool or mild, yet it’s very humid through the night; it’s more severe in shady areas. Just as with black spot disease, remove infected leaves and canes; spray with the same fungicide you’d use for black spot; remove and destroy infected leaves in the fall.
Roses are not the only ones to suffer from this condition. Phlox, squash plants, lilacs, and bee balm are just a few of the many plants that are also are susceptible. For more info, see the Almanac’s page on powdery mildew.
3. Stem Canker
There are several types of cankers which all ultimately cause rose canes to turn black or discolored and die. Stem canker enters plants through wounds caused by winter injury, improper pruning, or flower cutting, and also strikes plants weakened by black spot or powdery mildew. There are several types of stem canker:
Brown canker starts as small reddish-purple raised bumps on the stems.
Brand canker looks similar to brown canker but bumps develop a brown center.
Stem canker forms yellow-red patches on the bark.
The bumps or patches enlarge until they surround the cane, and they usually spread to other canes. If you see the signs, immediately cut stems off below the affected area to avoid spread. Prune properly, cutting well below the diseased area. Cut 1/4 inch above an outward-facing bud node at a 45-degree angle. Disinfect cutting tools afterwards in a 10% bleach water. There is no fungicide to treat stem canker, only vigilance.
4. Rose Rust
This disease is caused by 9 species of fungi! All cause orange rusty-looking spots on leaves and canes. Eventually, it can defoliate the plant. Rose rust overwinters in canes, leaf debris, on trellises or fences, or in any nearby protected place.
To avoid rose rust, ensure your roses have good air circulation. Don’t plant too near each other and other plants (roses should be spaced a few feet apart), and make sure you prune annually so that air and sunlight can reach the center. As always, never water from overhead, only at the soil surface. Immediately remove and destroy infected leaves. There are fungicides to spray to prevent rose rust; ask your local garden store.
5. Botrytis Blight
The grey mold fungus Botrytis cinerea attacks the blossoms of rose plants, especially hybrid tea roses. It cover petals with a brownish fuzz; it also affects the flower buds.
Botrytis Blight is fostered by hot humid summers and will readily infect lush new growth fostered by too much high nitrogen fertilizer.
The best to avoid blight is to prune off and dispose of your fading blossoms, avoid getting the leaves wet from watering from above, and prune in the early spring to provide good air circulation. There are also fungicide sprays for blight.
6. Downy Mildew
Downy Mildew is caused by a fungus-like pathogen, Peronospora sparsa (which is technically a water mold). It thrives in cool, damp weather, especially in greenhouses and nurseries but also in your landscape.
Resembling black spot, downy mildew starts with yellow spots or lesions on the top part of the leaves (whereas black spot begins near the bottom of the plant). Downy mildew spots also appear along the veins of the leaves. Over time, they turn black or dark-colored, and leaves fall off the plant.
You’ll find the gray or white spores (the reproductive structures of downy mildew) on the underside of the leaves. Downy mildew spreads from water splashing from one infected plant to another. Cane fruits such as raspberries and blackberries are also susceptible to downy mildew so one preventative action is to keep roses far from brambles.
As with most disease, it’s vital to remove all infected leaves and plants; dispose of them by burning or black trash bags. Space rose plants properly for air circulation, avoid watering from ahead, and thin canes to reduce moisture. Speak to your local garden center about approved fungicides.
Roses can also be afflicted by viruses …
7. Rose Rosette Disease
Rose rosette disease is very serious and untreatable. Also called witches broom, rose rosette virus (RRV) is caused by a virus carried by a tiny mite—the rose leaf curl mite (Phyllocoptes fructiphilus) which feeds on cell sap.
RRV causes clusters of deformed leaves, red shoots, and excessive thorns, moving through the entire plant. The plants die within two years; the disease also spreads to other rose plants.
Rose rosette is common in wild roses so make sure you aren’t planting your cultivated roses near them. Any infected rose should be removed and disposed of immediately, including its roots which may still harbor the infected mites. To attempt to stop the spread of disease, spray any roses with a bifenthrin spray every two weeks from spring through fall.
8. Rose Mosaic
Unfortunately, rose mosaic is another virus that, once caught, has no cure or treatment. Remove and dispose of plants immediately; put in black trash bags or burn. Rose mosaic can be caused by several different viruses forming discolored or mottled leaves and blossoms. It can stunt growth and inhibit flowering. It is also spread by insects.
And, finally, there is bacterial disease …
9. Crown Gall
Caused when bacteria enters a wound, usually at the base of the plant, crown gall is easy to spot since it develops a large bumpy growth on the stem called galls or swellings. The galls will also appear near the crown or roots of the plant.
Crown gall is caused by a soil-inhabiting bacterium, Agrobacterium tumefaciens; these galls block the flow of water and nutrients from traveling up the roots and stem! This is another lost cause with no solutions, requiring removal of the plant, roots and all.
The only preventative measure is to avoid wounding the plant during planting and pruning, and also to select disease-resistant roses.
10. Rose Slug
Rose slugs are not actually slugs but sawfly larvae. They can chew holes in foliage or scrape off layers from the top and bottom of each leaf leaving whitish windows.
The damage is unsightly but usually is not enough to kill a healthy plant however a heavy infestation can weaken the plant enough to reduce future blossoming.
Since the larvae are not caterpillars, Bt is not going to kill them. They can be controlled by handpicking, knocking them off with a forceful spray of water, or spraying them with insecticidal soap. Fortunately they have many natural predators including birds, wasps, and beetles. Begin scouting your plants in mid-spring, checking tops and undersides of the leaves for eggs and the small, 3/4 inch long, yellowish-green larvae.
Plus, Rose Insect Pests
While this article focuses on rose diseases, your rose problems could also be caused by insect pests.
If you see holes in the leaves, it’s most probably the famed Japanese Beetle. These metallic-colored eat flowers and flower buds but will also attack foliage. Handpicking and regular applications of Sevin provides the best control. Traps do not work.
Young plants are often attacked by aphids which are sucking insects and cause distortion of shoots. A blast from the hose can knock off aphids. Use insecticidal soaps sprays and natural predators.
If the leaves are turning bronze or grey, this is sign of mites. They are also sucking insects which pierce the underside of rose leaves and suck sap. A blast from the hose directed to the underside of the leaves every couple days can manage mites or use insecticidal soap.
Deformed flowers with flecked petals are from thrips, tiny brown insects. Insecticidal soap can help but it’s difficult to get rid of thrips.
You can best prevent rose problems by recognizing what conditions they need to thrive and also by practicing good sanitation.
Purchase disease-resistant cultivars and certified disease-free plants.
Space plants several feet apart so they get good air circulation! Do not plant rose bushes near tree roots. The soil should be fertile, moist, and well-draining.
Provide full sun, with a minimum of six hours every day. Morning sun is important for roses so they dry off from morning dew.
Water thoroughly so soil is kept moist, but keep the leaves dry and water only at the base of the plant. Drip irrigation or soaker hoses are preferable to overhead sprinklers. During dry weather, water roses about every 7 to 10 days. Water slowly but deeply at soil level.
Don’t over-fertilize. Only fertilize shrubs, landscape, and species roses once in the spring with an organic fertilizers such as compost or an all-purpose 10-10-10 fertilizer. Hybrid teas should be fertilized three times a year: in early spring after pruning, during the first bloom, and in mid-July. Do not fertilize after late July.
Rake up and remove all dropped leaves, hips, and broken or pruned branches. Don’t add them to your compost pile.
When using garden tools with roses to plant or prune, always clean them afterwards to prevent disease. Use a 9:1 water to bleach solution.
Mulch around rose plants to keep the soil moist but also remember to remove mulch in the spring and add fresh mulch to avoid overwintering disease.
Except for some shrub and groundcover roses, most rose plants need deadheading. When the blossoms are spent, clip them off with pruning shears right above the uppermost three-leaflet leaf. Do not deadhead after late summer.
Don’t give up on roses! Through trial and error I finally have a few that grow beautifully and blossom for me every year.