Black Dots and Spots on Clover

Leaf and Stem Spot

• Cercospora is the fungus responsible for a condition known as stem and leaf spot, also called summer black stem disease. This fungal disease most commonly affects clover plants that grow in warm regions of the United States, Europe and Africa. Although the fungal spores may be present throughout the year, the symptoms normally appear after the initial mowing or cutting of the clover plants.

Symptoms

• The appearance of small spots is usually the first noticeable symptom of Cercospora. The spots are slightly circular and can range in size from 2 to 6 cm. They can start out as brown spots on both the upper and lower surfaces of the clover leaves. As the disease progresses, the lesions turn darker and spread to the stems of the affected legumes. Some spots may appear purplish-black. The spots take on a silver tone when dew covers the leaves. This change in color occurs as the fungus produces spores.

Damage

• Cercospora can wipe out entire stands of clover. The disease causes the leaves to turn brown and shrivel, often resulting in leaf drop. The fungus may spread to the blossoms and infect the developing seeds, resulting in contamination and poor germination.

Prevention

• A fungicide that contains cupric hydroxide can help reduce the risk of Cercospora in clover plants. Planting clover in areas with good drainage helps reduce the growth of fungal spores. Removing damaged plants and correcting drainage problems helps minimize the chance of fungal growth. Avoid overwatering or watering late in the evening when cool nighttime temperatures can trap the moisture against the clover foliage. Don't use diseased clover clippings in compost bins or as mulch for other plants.