Roses (Rosa spp.) have lavish blooms, romantic fragrance and lush, dark green foliage that adds beauty and interest to any home landscape. Unfortunately, these garden divas are highly susceptible to fungal diseases such as black spot (Diplocarpon rosae). As the name implies, this dreaded disease causes small black circles to appear on the foliage. Infected leaves often yellow and defoliate early, causing plants to look straggly and bare. Black spot disease can also reduce the number, size and quality of blooms. Combining fungicidal treatments with nonchemical control measures can can help you manage black spot disease and keep your roses healthy and happily blooming.
Captan-based fungicidal products are commonly used by homeowners to control rose black spot disease. Carefully read and follow the instructions on the manufacturer's label since directions do vary. One captan product recommends mixing 2 tablespoons of concentrate for every 1 gallon of water. Use a small garden sprayer to thoroughly cover the foliage. For optimal protection, start spraying your rose plants when the spring foliage starts emerging. Repeat treatment every seven to 14 days up until the first frost.
Spray fungicides only on calm days when no rain is expected for at least 24 hours after treatment. Retreat roses if your area receives more than 1/4 inch of rain or overhead irrigation. Captan can cause skin or eye irritation on contact. Reduce the risk of exposure by wearing protective eyewear, a face mask, long sleeves, pants and shoes with socks when spraying your plants. Captan is toxic to fish, so don't spray it near bodies of water.
The Baking Soda Solution
Back in the 1980s, Dr. R. Kenneth Horst at Cornell University came up with a baking soda and horticultural oil recipe that helps manage black spot disease. The baking soda kills off fungal spores, while the oil helps the solution spread out to better cover rose foliage. This mixture is moderately effective in controlling the disease and very effective in preventing it.
The baking soda solution requires mixing 4 teaspoons of baking soda and 2 tablespoons of lightweight horticultural oil into 1 gallon of water, according to Good Earth R.O.S.E. Care. Getting the proportions right is important, says the American Rose Society, to avoid damage to the roses. Combine the ingredients thoroughly and use a handheld spray bottle or a small garden sprayer to apply the solution. Completely moisten all rose foliage, including beneath the leaves, but avoid spraying the solution on open flowers. Repeat treatment once a week for as long as your plants show symptoms.
Nonchemical Control Measures
Horticulturists at the Ohio State University Extension report that fungicides will not be effective unless you also follow the correct cultural and sanitation techniques. Removing and discarding infected leaves as soon as you spot them helps keep the disease from spreading to healthy foliage. Throw affected plant tissue into a covered trash can. Avoid composting the diseased debris or you risk spreading it to your other rose plants.
Not only do black spot fungal spores spread via splashing water, but they also require seven hours of moisture to germinate and infect rose foliage. Minimize surface wetness by not watering plants using overhead methods. Keep the leaves dry by using a soaker hose to water just the soil at the base of the plant. Seattle's Green Gardening Program recommends removing all leaves within 12 inches of the soil line so water won't splash the spores onto the lower foliage.
Rose lovers that dread dealing with black spot disease should avoid susceptible cultivars, including most copper or yellow rose varieties, and plant disease-resistant cultivars instead. Modern and old shrub roses typically require less vigilance, note experts at the Penn State Extension. Carefree Beauty (Rosa "Bucbi"), and Bonica cultivars (Rosa "Meidomonac") are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9 and Knock Out (Rosa "Radrazz") is hardy in USDA zones 5 through 9, with all having a good resistance to the disease.