How to Diagnose and Treat Rose Black Spot

What Black Spot Looks Like on Your Roses

Rose black spot shows up as roughly circular, dark brown to black spots, up to 1/2 inch across, on the upper surface of the leaves. The edges are often feathery instead of clean, the leaf yellows around the spots, and then it falls. To treat it, clear away infected leaves, keep the foliage dry, prune out infected canes while the plant is dormant, and protect new growth with a labeled fungicide if the disease is severe.

The cause is a fungus, Diplocarpon rosae. The University of California’s IPM program notes that it marks stems as well as leaves, so check the canes too. The Royal Horticultural Society describes the leaf patches as purplish or black and fast to enlarge, and adds that a leaf often drops while much of it is still unmarked. That early leaf fall is the real harm. University of Wisconsin Extension warns that severe black spot leads to yellowing and defoliation.

Why Black Spot Spreads and When It Is Worst

Water drives this disease. The California program states that leaves must stay wet for more than about 7 hours before the fungus can produce spores and infect. By that measure, a rose that dries quickly after rain or watering gives the fungus less of a chance than one that stays damp overnight.

The fungus spends the winter on fallen leaves and in infections on the canes. In spring, spores move to the emerging leaves by wind or splashing rain, according to Wisconsin. Clemson’s Home and Garden Information Center reports that black spot is most severe after long, wet, warm periods in spring. The Royal Horticultural Society tells gardeners to expect leaf markings from spring onward.

Early Detection: What to Check

Look at the upper surface of the leaves, since that is where the spots form and where the fungus produces its spores. The two details to match are the size, which tops out at about 1/2 inch, and the feathery margin. A yellow zone around a dark spot confirms it.

Then look over the canes. The Royal Horticultural Society notes that the fungus winters in dormant infections on young stems and buds as well as on fallen leaves, so stem lesions found now are the ones to prune out later.

A spotted leaf will not recover. The Royal Horticultural Society notes that the markings last for as long as the leaf stays on the plant. What you can change is how many clean leaves get infected next.

Treating an Active Outbreak

Start with the leaves on the ground. Wisconsin’s first instruction is to remove diseased leaf litter promptly. The California program adds that fallen leaves and pruned stems should be disposed of away from the roses.

Next, change how the plant is watered. Use a soaker or drip hose at the base so the leaves stay dry. If you must wet the foliage, do it in the morning instead of the evening or at night, so the leaves fall short of that 7 hour wet period.

Sprays are a third step, and sources disagree on them. Clemson says fungicide sprays are needed to control black spot effectively, even on resistant varieties, and tells gardeners to pick one if the disease is severe enough to warrant control: chlorothalonil, mancozeb, myclobutanil, propiconazole or copper fungicides. Wisconsin names chlorothalonil, copper sulfate and myclobutanil, adds neem oil as an organic product shown to give some control, and gives the timing as every 7 to 14 days from bud break until wet weather subsides. The California program lists neem oil, potassium bicarbonate, sulfur, triforine and chlorothalonil, and describes them as preventive fungicides for places where weather favors severe disease. The Royal Horticultural Society takes the opposite position and recommends that home gardeners do not use fungicides, citing effects on biodiversity and soil health. Whichever product you choose, apply it according to the directions on the label. Our comparison of organic fungicide types goes through the organic options one by one.

Pruning and Sanitation Through the Year

Pruning removes the half of the overwintering fungus that leaf cleanup cannot reach. The California program recommends pruning off infected stems during the dormant season. The Royal Horticultural Society times the job for spring before the leaves appear. Wisconsin supplies the distance: cut diseased branches 6 to 8 inches below any obvious infection. Wisconsin also says to disinfest the pruners between cuts by treating them for at least 30 seconds with 70% alcohol, which it prefers for metal tools, or a 10% bleach solution. Thorny canes make this slow work, and a pair of gloves made for roses helps.

Use the same session to thin crowded growth. The California program recommends pruning canopies to increase air circulation, which helps the leaves dry before they reach the 7 hour wetness threshold.

In autumn, collect the fallen leaves and destroy them. The Royal Horticultural Society offers an alternative for gardens where raking every leaf is not practical, which is to bury the leaves under a layer of mulch.

Preventing Black Spot Next Season

Site and spacing decide how long rose leaves stay wet. The California program advises against planting roses where they get less than 6 to 8 hours of full sun, and against setting them too close together. Wisconsin gives the same advice in other words: a high light position, with the shrubs spaced far apart for airflow and quick drying.

Variety choice matters as much. Wisconsin lists several roses with excellent resistance to black spot:

• The hybrid shrub rose ‘Meipotal’, sold as Carefree Delight
• Rugosa roses such as ‘Blanc Double De Coubert’ and ‘Fru Dagmar Hastrup’
• The climbers ‘William Baffin’ and ‘Henry Kelsey’
• Roses in the Knock Out series

Resistance has limits. The Royal Horticultural Society cautions that popular hybrid teas, floribundas, climbers and patio roses are often susceptible, and that planting the newest cultivars sold as resistant may gain a few years’ respite that usually does not last, because new strains of the fungus arise. Plant a resistant rose and keep up the cleanup and dry-leaf watering anyway.

Black spot is one of two common leaf diseases on roses. A white, powdery coating on the leaves is a different fungus, covered in our guide to powdery mildew on flowers. If the damage on your plants is chewed instead of spotted, the leaf damage identification chart sorts out the common insect culprits.