How to protect your roses from black spot disease

Although there is no cure for black spot, it can be effectively managed — and future growth protected — with a bit of vigilance.

What to do if you see those telltale black spots

Caused by the fungus Diplocarpon rosae, black spot disease causes telltale black spots on foliage. Left unchecked, the often-fatal infection will diminish blooms and severely weaken plants, which increases their susceptibility to other diseases, pest infestations and winter injury.

If your plant is affected, prune away all infected leaves and branches and rake all fallen plant parts from the soil. Then, place the debris in a sealed bag and dispose of it in the trash; leaving it on the soil will lead to reinfection. Avoid doing this when the weather is humid, as spores spread rapidly when exposed to moisture.

There are many fungicides indicated for the prevention and treatment of black spot.

The most effective contain the chemical mancozeb. After cleaning up around the plants, apply the fungicide according to the package directions (probably three applications spaced 10 days apart). The fungicide should stop disease progression, and keeping the ground clear of affected leaves will prevent spores from making themselves at home in the soil, where they would otherwise reinfect the plants.

Alternatively, Neem oil and copper fungicide are among the most environmentally friendly treatments for black spot. Apply after cleaning up around plants, as above, and take care to follow package directions carefully.

Next spring, when leaf buds begin to open, apply a preventive fungicide, such as Neem oil, copper or one containing the active ingredient potassium bicarbonate.

Spraying a solution of 1 tablespoon each of baking soda and ultrafine horticultural oil diluted in a gallon of water can also be a preventive against black spot and other fungal and mildew diseases. Don’t be tempted to use more baking soda than recommended, however. Its salt content can injure plants if overused.

Planting onions or tomatoes near roses will also help discourage black spot fungal infections.

Following best practices can go a long way, too: Space plants sufficiently to allow light and air to circulate between plants and into inner branches. Water deeply only once a week (less during rainy periods) using a soaker hose, which directs water straight to roots and keeps foliage dry. Overhead sprinklers and other methods that wet leaves encourage the growth of fungal diseases such as powdery mildew, in addition to black spot.

Selecting resistant varieties, especially in humid conditions, is the best way to avoid the disease. Shrub roses such as those in the Knockout and Oso Easy families, are among the most immune, but there are some cultivars of other species that offer varying degrees of resistance, too. When shopping for plants, you’ll find resistance information on labels and catalog descriptions.

Jessica Damiano writes regular gardening columns for The Associated Press. She publishes the Weekly Dirt Newsletter. Sign up here for weekly gardening tips and advice.

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