Rose growers might joke that the only absolute cure for blackspot is artificial roses, while the United States National Arboretum recommends Integrated Pest Management. IPS is a combination of weapons that includes using disease resistant varieties, careful cultural techniques and vigilant monitoring combined with natural control methods and sprays. Given a gardener’s tolerance for some minor damage, IPS minimizes the need for chemical spraying to control blackspot.
Choosing Your Rose
Rosarians know that "resistant" does not mean immune and can be limited to locations or climates, but these roses are a first defense against blackspot. There are lists of blackspot-resistant roses, but many growers consult forums where gardeners list cultivars they know are blackspot free. Many antique, heritage and species roses are resistant, as are Knock Outs and some rugosas. All may show a few small black spots if infected.
Blackspot spores can be present in any soil. They emerge when temperatures warm to 24 degrees C (75 degrees F). If splashed onto lower leaves by rain or careless watering, spores germinate on the wet surfaces. The fungus attacks the plant, leaving telltale irregular black spots with yellow borders.
Blackspot produces spores every three weeks and weakens the rose by defoliating the entire plant. Spots, yellow leaves or bare branches should be treated immediately.
Nurturing Healthy Roses
Garden columnist David Hobson writes, “A cure is elusive, but for the last couple of years, I've had success by being diligent about preventing black spot.” His proactive plan is to spray in late winter with lime sulfur and repeat with a diluted mixture every few weeks and after every rain. Sulfur works by lowering the pH, preventing spores from germinating.
A well-watered and -fed rose with good air circulation has the best chance of resisting blackspot. Roses need sunshine, space between plants and no crowding or shade from neighboring plants.
Keep Clean Beds
Danny Craft writes for the American Rose Society that blackspot can be controlled with a good fungicide every seven to 10 days, but, “Prevention is the key to controlling blackspot.”
To absolutely discourage blackspot, keep rose beds free of all debris, especially in the fall. Burn diseased leaves and cuttings to destroy the spores. Do not compost them; spores can overwinter and reinfect plants in the spring.
Prevention and Treatment
Spores survive on the canes, so annual late winter pruning that removes canes below infected areas is essential.
The National Arboreteum recommends spraying a neem-based fungicide weekly. Reapply after every rain to minimize spores. If blackspot appears, rotate spraying with fungicides containing chlorthalonil, thiophanate methyl or propiconazole to prevent the fungus from becoming resistant to one.