Brown Spots on Newly Planted Sunflower Leaves

Insects

• Sunflowers are popular food for a wide variety of insects. As the sunflowers grow, caterpillars and grasshoppers eat their foliage, but they seldom cause damage of concern. More problematic are banded sunflower moths, which cause brown spotting to the leaves. Watch for evidence of the moths and, when detected, spray the plant with insecticide to destroy the moths and their larvae. Once larvae get into the head and seeds, they are difficult to eradicate. Banded sunflower moths are best avoided by planting in June or July.

Diseases

• In cool, wet soils, fungi can become attached to seeds or seedlings. Seeds may be treated with fungicide prior to planting. A variety of disease may cause surface spots or yellow patches but have no impact on yield. The most threatening disease for sunflowers is a white mold known as sclerotinia.

Rust

• Rust usually appears on sunflowers when the seed head has formed completely. Conditions for infection are at their prime when the sunflowers have grown to their maximum height and the plants have a dense canopy. Warm weather, particularly at night, and water drops on the leaves also help the rust take hold. Small, brown dots, also known as uredia, appear on lower leaves, and these may merge together forming a larger shaped pustule, sometimes surrounded by a yellow border. Rust can be best managed with fungicides. Experts from the National Sunflower Society also recommend not planting sunflowers in the same place two years running.

Alternaria

• High yield loss may occur from leaf defoliation caused by alternaria, which is most prevalent in hot, humid environments. The disease defoliates the leaves by creating dark spots. The stem is also infected, and stem breakage may occur. Fungicide is the only effective way to control alternaria.

Phoma

• Phoma, also known as black stem, is a common sunflower disease. It results from soil-borne fungus. Infection usually starts on the leaves and moves to the stem, causing dark lesions. These become most noticeable once the petals have dropped. There is no fungicide treatment that controls this disease. Crop rotation is the best form of management in the long term.