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.

• Maintain plants far enough apart from each other, and other plants, that good air circulation is available to quickly dry leaves.
• Plant only in full-sun conditions where moisture will dry quickly.
• Remove fallen leaf litter (where fungal spores may be present).
• Water roses from the ground level (rather than splashing water on leaves) and in the morning only (when it has all day to burn off).
• Make sure that you don't fertilize more than recommended or the rose will actually become less robust and more likely to be affected by disease. Profuse, leafy growth in the early and late rose season, just as conditions favor this pathogen, are the most likely to succumb to blackspot.

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. 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.

For more discussion of rose disease, pests and problems, refer to our guide Rose Problems.

Courtesy of NYBG Plant Information Service