Rose black spot

What it is

Black spot is a fungal disease specific to roses. It rarely kills a bush, but repeated early leaf loss year after year weakens the plant, so you get fewer flowers and spindly growth. Gardens with clean air tend to see more of it, as the fungus is sensitive to air pollution, and wet western summers suit it well.

Symptoms

The tell-tale sign is round, purple-black spots on the upper surface of the leaves, often with a feathery or fringed edge rather than a crisp one. Leaves around the spots turn yellow and fall, sometimes leaving a plant half bare by August. Small dark, blistered patches can also appear on young stems. Spots on rose leaves with a clean edge and no yellowing may be something else, such as a minor leaf spot or spray damage.

Cause

Spores are spread by splashing rain and watering, and need a few hours of wet leaf to infect, so humid, showery weather from late spring onwards brings the first outbreaks. The fungus overwinters on fallen leaves and in lesions on stems, which is why the same bed tends to be hit each year from early in the season.

Control

Pick off spotted leaves as they appear and rake up every fallen leaf, binning or burning them rather than adding them to the compost. In late winter, prune out stems with dark lesions and open up the centre of the bush. Water at the base in the morning, never over the foliage. A well-fed, mulched rose recovers quickly and will often reflush with clean leaves. Fungicides are a last resort only; follow the label carefully if you use one.

Prevention

Resistance varies hugely between roses, so read catalogue descriptions and favour varieties sold as disease resistant; many modern shrub and rambling roses are far tougher than older hybrid teas. Plant in an open, sunny spot with space around each bush, mulch in spring to bury spores on the soil, and avoid replanting a highly susceptible variety where a badly infected one grew.

Plants often affected

• Rose

Could it be something else?

These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.

• Box blight
• Downy mildew
• Drought stress
• Honey fungus

Months to keep an eye out: May, June, July, August, September and October.

Sources

Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.

• Wikipedia — Diplocarpon rosae
• University of Minnesota Extension — Black spot of roses