Roses are subject to a wide array of diseases. Once disease is present, its management can be a complicated story. The best thing to do is to start with roses that are disease-resistant. In the last two decades, NYBG's rose curator replanted most of the Peggy Rockefeller Rose Garden to prioritize that strategy. But naturally that is not the way most of us start with our roses and not all diseases can be avoided with rose selection. Even disease resistant roses are not immune to disease, but they are likely to be less impacted when some common diseases are present.
Keep in mind that many rose diseases are more devastating to a plant that is stressed by challenging growing conditions or without an attentive care routine. Get to know the cultural requirements of the plant so that your roses will have the strength to fight off assaults. The value of caring for the soil in which you plant your roses, cannot be overstated. You can find more information on siting your rose in a place that it will grow well and giving it the best start in our guide to Planting Roses.
Ask your nursery to recommend disease-resistant roses or get in touch with us at Plantinfo@NYBG.org for recommendations of disease-resistant roses with the particular characteristics you prefer.
Rose blackspot is among the most common rose diseases and one of the most serious. Active during the cooler, wet weather in the early and late season, it is caused by the fungal pathogen Diplocarpon rosae, of which there are numerous strains. Spores of blackspot fungi need to stay wet for approximately six hours to germinate and the disease is then spread in water splashing from leaf to leaf. Blackspot disease reveals itself as dark spots with indistinct borders on the leaves of your rose and progresses to yellowed, spotted leaves that fall from the plant. It is early defoliation that limits the plant's ability to generate energy and can lead to reduced vigor and longevity. Lesions may occur on rose canes as well. Blackspot overwinters in leaf litter and on the canes of the plant.
Treating fungal disease once it is present is an uphill battle. Curative fungicides and home remedies may have some effect but need to be applied thoroughly and repeatedly, often with meager results. The best approach begins with plant selection. Look for disease-resistant roses when you buy. Then cultivate your roses with the measures necessary to limit fungal disease. You accomplish that through plant hygiene and, possibly, a program of spraying with preventative fungicides.
The essentials of rose hygiene are designed to keep fungal-supportive moisture from lingering on susceptible rose leaves long enough for spores to germinate.
If your rose is highly susceptible to blackspot and you live in an area of moist, humid summer weather, you might also consider a program of preventative fungicide application beginning early in the season before the disease is present. Products have a period of effectiveness that can be as short as a week and may be washed off by rain, so consistent, repeated application is necessary. Fungicides vary from mild biorational products like baking soda solutions to stronger chemical ones and their effectiveness will depend upon the weather in your area and the vulnerability of your rose type.
Once blackspot occurs, these preventative fungicides are no longer effective and the disease strain may even become fungicide resistant after multiple applications. For a discussion of rose fungicides and their use, refer to Fungicides Made Simple from the American Rose Society linked in the side panel of this guide. You should contact your county's cooperative extension office to find out which products, including organic fungicides, are currently licensed and effective for home use in your area. If you need help locating your county's cooperative extension office, please contact us at the email address plantinfo@NYBG.org and we would be happy to help direct you.
Another of the most common diseases affecting roses, powdery mildew is a fungal disease caused by Sphaerotheca pannosa var. rosae that leaves a superficial, white, dusty coating on leaves, stems and sometimes the flowers. Powdery mildew tends to create a problem for gardeners in middle to late summer, reducing the strength and damaging the appearance of infected plants, but rarely killing them. The fungal spores penetrate the leaves of a rose and deprive it of moisture and nutrients. The spores themselves are unusually rich in moisture so, unlike many fungal problems that need wet conditions to take hold, powdery mildew thrives with warmth (temperatures around 70 to 80º F.) and humidity and is inhibited by rain. The disease is spread when spores are blown from one plant to the next and flourishes when some warm humidity is added to the picture.
Not surprisingly, good maintenance practices and smart gardening are the key to reducing the incidence of powdery mildew as they limit the impact of humidity around the plant. Planting in an area where air circulation in high and sun is strong is important. This fungus is present in soil but a healthy plant is always more resistant to disease problems. The rules of garden hygiene found above under Rose Blackspot will help to limit powdery mildew as well.
The chief impact of this disease is the unattractive appearance it creates but a slow weakening of the rose will result if the plant is affected to the point of losing its ability to photosynthesize. Do your best to limit the incidence of powdery mildew through rose selection and garden hygiene and then allow minor infections to run their course. If your rose has a history of aggressive and threatening occurrences, remove the worst stems and apply the most environmentally friendly product that will be effective to the rest of the plant as soon as you notice the powdery mildew. Available products change constantly, so contact your county's cooperative extension office to find out which products, including biorational fungicides, are currently licensed and effective for home use in your area. If you need help locating your county's cooperative extension office, please contact us at the email address plantinfo@NYBG.org and we would be happy to help direct you.
Rose rosette disease is an increasingly common viral problem in many parts of the United States. This devastating and untreatable disease is caused by rose rosette virus, transmitted by the eriophyid mite Phyllocoptes fructiphilus. The mite becomes a carrier when it feeds on an infected wild or cultivated rose and then is blown by the wind or carried on garden tools, clothes, equipment or a nursery plant to a new location (though they will survive off of a plant for less than a day). The primary host for the disease is wild multiflora roses (Rosa multiflora) which are considered invasive plants in much of the United States. The disease can also be transmitted when material from an infected plant is grafted onto another rose.
Symptoms of new infections typically appear in late summer. Initially they may look like normal red-colored stem growth but gradually become unusual, distorted growth of red stems with stunted leaves that do not turn green as they mature. Rose flowers may be disfigured and discolored, including colorations completely different from the host plant. There are other variable symptoms, including excessive or soft thorns, thickened stems and stunting or distortion of other plant parts. The mites will survive on a rose through the winter and produce multiple generations each year. There is no cure for this disease and plants become weak and susceptible to other problems, including frost damage. The rose typically dies within several years but the transmitibility of rose rosette disease makes addressing the problem quickly advisable.
Rose rosette disease is systemic and it is not possible to remove the virus by cutting out the noticeably affected parts of the plant. Instead you should remove the entire rose plant from your garden after carefully covering the plant in a bag to limit the transfer of mites. Destroy the plant, including the roots and any fallen leaf debris, by burning it or disposing of it fully bagged. Soil does not need to be treated but you should wash your clothes and tools before moving to other cultivated areas of the garden. Do not plant new roses in this area until you are certain that all the rose root material has been removed.
You can limit the risk of rose rosette disease by cultivating roses only in areas where Rosa multiflora is not present within 100 feet, pruning roses back hard in late winter to reduce overwintering mites before spraying with horticultural oil, giving roses space from each other to limit potential spread and avoiding use of lawn blowers around roses. If you are in an area with a high incidence of the disease, horticultural oils can be applied to growing tips weekly in early summer to smother mites before they spread disease. Disease-resistant rose selections, like Knock Out® roses are, sadly, not resistant to this virus.
You can find help identifying this disease on your plants and a map of reported locations on the Rose Rosette Organization website, supported by the American Rose Society, the USDA, agricultural colleges and industry entities.
Caused by the oomycete Peronospora sparsa, downey mildew may be mistaken for rose blackspot in its early stages. Its ability to defoliate a rose in days makes it a serious disease. The spores germinate rapidly (in about four hours) if water is present on the leaves. It needs very high humidity (above 85%) and mild temperatures between 40 and 75º F. to become active. These conditions are less common in the garden than in a grower's greenhouse environment in the spring.
This disease almost always arrives on the plant when you purchase it. It is not caused by a fungus which explains the ineffectiveness of fungicides in treating the disease. The best remedy is to remove it from your garden and dispose of the plant and any fallen debris immediately.