Rose Black Spot: Symptoms, Prevention, and Control

Black spot, caused by the fungus Diplocarpon rosae, is one of the most common and damaging diseases of roses. It attacks young expanding leaves and canes, leading to defoliation and weakened plants.

Disease Cycle

Fungal spores overwinter on infected fallen leaves and canes. In spring, rain or sprinkler irrigation splashes spores onto new lower leaves. Spores require several hours of leaf wetness to germinate and infect. Under warm temperatures, symptoms appear in as few as 3 days, and new spores form within 10 days, allowing rapid spread.

Symptoms

Leaves

Dark spots up to 1/2 inch in diameter with "feathery" edges on some cultivars; smaller spots with distinct edges on others. Spots often have a yellow halo. Infected leaves turn yellow and drop prematurely.

Canes

Small purplish spots form on the current year's growth.

Management

Cultural Practices

• Avoid dense plantings to promote air circulation.
• Maintain good weed control.
• Avoid wetting foliage during irrigation; use drip irrigation or water early so leaves dry quickly.
• Rake and destroy fallen leaves.
• Prune out canes showing black spot symptoms.

Resistant Cultivars

Plant resistant varieties when feasible. Highly resistant include: 'David Thompson', 'Bebe Lune', 'Coronado', 'Ernest H. Morse', 'Fortyniner', 'Grand Opera', 'Lucy Cromphorn', 'Sphinx', 'Tiara', 'Carefree Beauty', 'Simplicity'. The American Rose Society also recommends: 'Bride's Dream', 'Canadian White Star', 'Cary Grant', 'Dainty Bess', 'Duet', 'Electron', 'Elina', 'Elizabeth Taylor', 'Helmut Schmidt', 'Keepsake', 'Lady', 'Lady Rose', 'Lady X', 'Las Vegas', 'Mikado', 'Nantucket', 'Olympiad', 'Otto Miller', 'Pascale', 'Polarstern', 'Precious Platinum', 'Princess of Monaco', 'Pristine', 'Sheer Bliss', 'Sunbright', 'Tansinnroh', 'Uncle Joe'.

Fungicide Applications

Apply a fungicide with a spreader-sticker to protect young tissues. Begin when daily temperatures reach the high 50s°F, applying every 2 weeks. As leaves emerge, switch to weekly applications. Stop spraying when daytime temperatures average in the 80s°F or during dry weather. Resume in autumn when temperatures moderate and rainfall or dew occurs regularly.