Blackspot on roses Diplocarpon rosae
IDENTIFICATION – The name says it all. This fungal disease causes black spots with fringed margins and a yellow halo on leaves. Infected leaves drop early, often leaving the plant almost bare. Blackspot doesn’t kill roses, but it weakens them, making them susceptible to insects, cold and other diseases. Warm, humid summers make blackspot worse.
CONTROL – The best control is prevention: Plant blackspot-resistant varieties. Ask at your local garden center which ones are best. Make sure roses have good air circulation and full sun, and water with a soaker hose. If you use overhead watering, water in the morning so the leaves dry quickly. Rake out fallen leaves around the plant to get rid of as many spores as possible. You can’t get rid of the spots already on leaves, but you can prevent more from forming. Spray your roses with a mix of 1 tablespoon baking soda and 1 tablespoon horticultural oil in 1 gallon of water once a week if you see signs of blackspot. Shake the sprayer to keep the mixture stirred and spray until all the foliage is coated. Try this on a few leaves first, to make sure it doesn’t burn them. If it does, use less baking soda. Neem oil is also effective. Spray on cloudy days or early morning, not in full sun, to avoid burning the leaves.