Blackspot? Anthracnose? Rust?
by Paulette Mouchet
Originally published in "The Rose Garden" newsletter, May 2003. Revised February 2007.
Nationwide, blackspot causes a lot more grief to rosarians than mildew or rust because blackspot frequently kills plants. "The worst roses for me are hybrid teas," writes Rhoda Villanueva, who lives in New York state, a disease hotspot. "I've lost so many that I don't even bother with them anymore, no matter how tempting they are. Climbing Peace is the sole survivor, but it is a wreck after its June flowering. I have mildew, but it isn't as much of a killer for me as blackspot."
Blackspot is a fungal disease caused by Diplocarpon rosae. It starts out by producing small black spots on the upper leaf surfaces. It's important to note that the edges of the spots are feathery. Leaves lower down on the bush are affected first. As the disease progresses, the feathery spots enlarge and often become ringed with yellow, then whole leaves turn yellow and drop from the plant. Where blackspot is severe, plants die due to defoliation.
The blackspot fungus becomes active during warm, wet weather and requires free water on the leaves for 7 hours at 75 degrees to germinate. Symptoms will appear in approximately 3 days. The dry Southwest has few problems with blackspot while coastal areas that have very high humidity and warm temperatures often have disease outbreaks. As with mildew, prevention is your first line of defense.
- Plant resistant rose varieties
- Plant them far enough apart to allow lots of air circulation
- Plant in full sun
- When pruning, remove all diseased leaves and canes and discard to the trash
- After pruning, strip all the leaves from all your roses and discard to the trash
- After pruning, use a dormant season horticultural oil spray. If you have a severe infection year after year, you might want to use spray with a copper or lime sulfur compound during the dormant season.
If blackspot appears in your garden during the growing season, don't panic! Remove diseased leaves to the trash and wash your hands and tools. Be sure to pick up and remove any diseased leaves that have fallen to the ground. Although blackspot does not exist on the ground, fungus on the leaves can spread to other plants. The Cornell Baking Soda Formula is effective for blackspot.
This fungal disease is often confused with blackspot because it also produces black spots on leaves. However, anthracnose spots have defined edges whereas blackspot edges are feathery. In the early stages of the anthracnose disease, the circular spots are red and quite small: 1/16- to- 1/8-inch in diameter. As the disease progresses, the spots enlarge and the centers turn light brown with a dark red ring around them. Eventually, the center drops out leaving a hole. In severe cases, the entire leaf will turn yellow and fall off.
Anthracnose develops during cool, moist conditions, which are common in Southern California during the spring. As soon as the weather warms up, anthracnose disappears. If you have anthracnose in your garden, follow the Prevention Guidelines above for blackspot.
Rust on underside of leaves
One of the most common and easily identifiable rose diseases. It's caused by one of nine species of the phragmidium fungus which, in the spring, forms powdery, light orange to yellow spots on the undersides of the leaves. In the summer, the spots turn bright orange and in the fall, they turn black. Spore germination requires cool summer temperatures and continuous moisture for at least 2 hours.
Good sanitation is key to controlling rust and other diseases in your garden. Follow the Prevention Guidelines listed above for blackspot.
Backspot photo Copyright Regents of the University of California.
Anthracnose and rust photos by Jack Shoultz.