At this time of year, the refreshing warmth of spring has given way to the constant heat and humidity of summer in the South. And although the air is humid, we can go for weeks without significant rain, making the temptation to use sprinklers hard to resist. And, many of our insect pests are in full swing chewing away on just about everything that doesn’t move. As a result, our yards and gardens are getting that beaten down look, some of the saddest victims being roses. At the Master Gardener of Hamilton County booth at the weekly Chattanooga Market, “why do my roses looks so bad” is one of the most common questions this time of year.
One major reason for sad looking roses is blackspot — the apt name for a fungus (Diplocarpon rosae) that starts as black spots on the leaves and can eventually defoliate a rose plant. Like all fungi, blackspot thrives in warm humid weather, becoming active when the temperature rises to 75 degrees Fahrenheit. It needs about 7 hours of these conditions to germinate and then symptoms will begin to appear on rose foliage within three to ten days. According to research, humidity alone will not allow for the fungus to germinate – the leaf surface must also be wet. From then on spores are produced every three weeks. If left to its own devices, blackspot can infect an entire rose garden and eventually strips the leaves from plants leaving you with bare rose canes. Spores can over winter in the garden so autumn cleanup is critical to break the cycle of the process continuing the next spring and summer, and so on.
The good news is that while blackspot may look like it is killing your rose, it probably won’t if the plant is otherwise healthy. The bad news is that unless you control blackspot, it will come back again and again, year after year to haunt you.
Prevention is the best strategy
As with all diseases, prevention is the best strategy. First, you can look for disease resistant rose varieties, those that have shown less susceptibility to blackspot and other fungal diseases. The Ohio State University Extension Service has a publication on blackspot that includes a listing of rose varieties that have been reported to be resistant. In general, shrub roses like Knockout Roses and and Rugosa roses have shown to be more disease resistant, while many hybrid tea roses tend to be more prone to the disease. The University of Tennessee is conducting research trials on any rose that is claimed to be disease resistant. You can read the results in this brochure.
Starting in spring, you can give your roses a head start by spraying the still dormant plants with fungicidal soap and wettable sulphur. Sulphur is a long-used preventive measure as it creates a film on the plant, a barrier through which the fungal spores cannot germinate. Since both the fungicidal soap and sulphur wash off in rain, you need to repeatedly spray your roses to be effective.
If you water your rose plants, use a soaker hose or other drip irrigation rather than a sprinkler to avoid wetting leaves and stems. As mentioned above, the fungus needs a wet surface to germinate and wetting leaves will give the trigger to a new infection. If you choose to use a sprinkler, use it early in the morning so that the leaves will dry during the day. It’s also important to prune your plants and keep them well spaced to provide for good air circulation as well. Susceptible roses like hybrid tea roses should be spaced 4 feet apart; more resistant varieties may be planted more closely together.
You can also help your roses stay healthy giving them good nutrition throughout the growing season. At this time of year, roses will benefit from a final fertilizer application in mid-July. Apply fertilizer to rose beds at the rate of three pounds per 100 square feet. Spread the fertilizer around evenly and scratch it into the surface; then follow-up by watering the roses and the soil. If you don’t want to use commercial fertilizer, you can create your own using recipes available in this post. Avoid fertilizing roses after mid-August—a late application may cause the roses to put on new growth which will be damaged by frosts.
Finally, you should practice good “housekeeping” in your gardens, cleaning away garden debris to help prevent fungal spores from overwintering.
What can you do if your roses are already infected?
The best thing to do is to prune off the damaged parts of the plant and gather the diseased foliage. The best practice is to burn or bag this garden litter. Do NOT add it to your compost pile – the fungus will come back again unless your composter reaches temperatures of in excess of 120 degrees Fahrenheit.
After you remove the diseased parts from your rose bushes, you will need to apply a preventative formula to minimize further attack. You can use fungicidal soaps and sprays or sulphur several times, especially after rains since these products will wash off. If you prefer a more organic or “homemade” approach, you can make your own antifungal spray out of baking soda: dissolve 1 teaspoon baking soda in a quart of water, add a few drops of liquid soap to the mix to help it cling better to the foliage, spray infected plants thoroughly. Manure tea is another homemade solution that fights blackspot, as well as mildew and rust and provides nutrition to the plant: place one gallon of well-composted manure in a 5-gallon bucket and fill with water. Stir the mixture well and let sit in a warm place for three days. Strain the mixture through a cheesecloth or mesh and use the resulting tea to spray disease affected plants. You can apply the solids left behind around the base of the plant as fertilizer.
For more information and Roses and Rose Diseases
Horst and Clyde, Compendium of Rose Diseases and Pests, Second Edition, a publication of the American Phytopathological Society (2007)