Black Spot of Rose

Introduction

Black spot is one of the most common and important diseases of roses throughout the world. It is caused by the fungus Diplocarpon rosae. Black spot will cause a general weakening of the plant so that progressively fewer and fewer blooms are formed if the disease is left unchecked. Plants so weakened are increasingly subject to winter injury.

Symptoms

As the name implies, infected leaves show black spots especially on the upper leaf surface. The spots can be up to 1/2" in diameter and typically have fringed borders. Yellowing of the leaf begins surrounding the spots and the entire leaf may yellow and eventually drop off. Close inspection of the spots will reveal the presence of tiny black spore producing bodies. The fungus may also infect the canes where lesions appear purple at first and later black.

Environmental Conditions

As is true with most fungi, this fungus requires free water for infection to occur. The spores must be wet for at least 7 hours before they can germinate. A temperature of 65^oF is best for spore germination and the disease develops most rapidly at about 75^oF. Temperatures of 85^oF and above inhibit the spread of the disease.

Survival and Dispersal

The spore producing structures (acervuli) form within two weeks of the initial infection. These structures release spores which are blown or splashed or otherwise carried to new tissues initiating new infections. The fungus survives the winter in fallen leaves and at infection sites on the canes. Spores will not survive in the soil and individual spores do not survive longer than one month.

Management

1) Rake and discard all fallen leaves because they are the main source of spores in the spring. 2) Also prune and discard any obviously infected canes. 3) Avoid wetting the foliage especially during dark cloudy days. 3) Grow plants in an open sunny location to promote rapid drying of the foliage. 4) Do not plant in dense plantings and avoid windbreaks to allow good air circulation. 5) Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings. 6) Removing infected leaves as they arise early in the season may help to retard the rate of disease spread. 7) Several fungicides are registered for control of black spot (see tables).  Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.

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