What Causes Roses to Get Black Leaves?

Blackspot Fungus

• Rose bushes may develop blackspot on their leaves if temperatures are warm and the plants are receiving too much moisture. The fungus can take up to seven hours to germinate and grow in temperatures of at least 75 degrees Fahrenheit. Spots appear in about three to 10 days. The fungus will continue to grow every few weeks if the plant is not treated. Spores can survive cold winter months and reappear in the spring.

Managing the Fungus

• Plant management and use of disease-resistant cultivars are good ways to control blackspot on rose bushes. Some older types, like tea roses, for example, are susceptible to blackspot, but newer hybrids are stronger and more resistant to the fungus. Location is important; plant rose bushes with ample space between them in areas of sunlight that can dry rainwater and overnight dew. Remove dead and moldy leaves and infected canes from the plant and its stems.

Fungicides

• Fungicide chemicals can control the spread of blackspot on rose bushes and other flowering plants. Benomyl, chlorothalonil, mancozeb or a variety of other products like fungicidal soap can help; visit your local garden store for assistance on choosing the right mixture for your flowerbeds. The fungicides must be applied several times throughout the spring, summer and fall to protect against spore growth in damp and humid weather.

Sulfur Products

• Sulfur is an effective product in controlling the blackspot fungus because it keeps spores from forming and growing. Copper sulfur dust can be applied weekly with the appearance of new leaf growth as soon as the first blackspot appears. As with other types of fungicides, sulfur must be applied frequently, especially after rainstorms.

Disease Resistant Roses

• Disease-resistant rosebush hybrids and cultivars are in development, with new plants available yearly. The All-America Rose Selections (AARS) organization tests new breeds for a couple of years before introducing the plants to the public. Knock Out, Kardinal, Moondance, Passionate Kisses, Dick Clark and Walking on Sunshine are only a few of the disease-resistant, colorful varieties of roses on the market today. The AARS conducts tests in gardens which have varied climate zones, like Minnesota, California, Kentucky, South Carolina, Massachusetts, Texas, Washington, Illinois and Oregon.