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GARDEN BAD GUYS - ANTHRACNOSE
by Nanette Londeree, Consulting Rosarian
Are you seeing spots? Are your nice, new rose leaves dotted with color, or holes in the leaves? Are your rose leaves turning bright yellow and dropping to the ground? With cool to mild spring temperatures and rain, the conditions are ripe for a number of diseases that plague roses and cause these types of symptoms. The most common of the spot producing diseases is blackspot. Another disease that is very similar in appearance to blackspot is anthracnose. This is a fungal disease is caused by Sphaceloma rosarum. Also known as Purple Spotting, Spot Anthracnose or Shot-Hole fungus, it seems to attack wild roses, climbers and ramblers most often, though hybrid tea and bush roses also get the disease.
The first indication of infection is the presence of circular spots, 1/16” to 1/8” in diameter that are red or sometimes brown to purple, mostly on the leaves and occasionally on the stems. Blooms aren’t affected. The spots may be individual, or they might run together. As the disease progresses, the spots may enlarge with the centers of the spots turning gray, tan or white with a dark red margin. The spot itself drops out of the leaf, leaving a circular hole, thus creating a “shot hole” effect, while most of the leaf remains green. In severe cases, the entire leaf will turn yellow and drop.
The symptoms of blackspot are similar. Blackspot lesions vary in size and are often surrounded by a yellow halo, and the yellowing spreads across the leaf.
Like many fungal diseases, the reproductive portion, the spores, are spread by air currents and splashing water (like rain) to newly expanding leaves and stems. In our environment it is usually seen in when we have cool moist spring conditions. The fungus over winters in old lesions on leaves and canes.
This disease is certainly unattractive; it can stress and weaken the plant, and in severe cases, may result in defoliation of the entire plant. Like most diseases, it is normally easier to prevent it rather than eradicate it once it’s established. Prevention begins with good garden sanitation. Remove old leaves from around the base of plants and prune out infected canes before spring buds form as these also carry the spores for this fungus infection. Avoid overhead watering later in the day; if you wet the leaves, do it such that they have time to dry before the evening. Anthracnose usually disappears once we get into our dry season.
If you spray your roses, the materials you use for black spot and powdery mildew are usually effective for anthracnose as well. Products such as Banner Maxx, Clearys 3336F, Compass, Immunox, or Rose Defense are all good choices for control. It’s important to begin your spraying regime early in the season before the disease gets established. A dormant spray with lime sulfur in late winter can be an additional preventive step for this disease.
Photo of anthracnose courtesy of List of Rose Problems and Diseases by Baldo Villegas
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Last Modified: 08/06/2013