Sunflower (Helianthus annuus) is a native to the United States and widely grown by commercial gardeners because of their edible seeds and oil. Many casual gardeners also grow sunflowers because the flowers have a distinct, attractive look. If its foliage begins turning black, the plant becomes both unattractive and unhealthy. Black leaves may indicate disease that could kill the plant.
Sclerotinia wilt is a common sunflower problem that typically occurs in summer. The disease lives in soil for up to 4 years. Sclerotinia symptoms appear when the sunflower starts to flower. The wilt makes the flower heads and leaves wilt. Foliage may turn black before it falls from the plant. When left untreated, sclerotinia kills sunflowers. The entire flower may turn black in late stages of the disease before it dies to the ground.
Alternaria leaf and stem spot appears as small, circular spots on the leaves. The spots may be brown or black. As the disease progresses, spots appear on the sunflower blossoms. A fungus causes alternaria leaf spot and it spreads to other garden plants if untreated.
Fungus causes sunflower rust. The rust fungus survives through the winter and attacks plants in the spring. The wind spreads spores that may affect many other plants if rust is not treated right away. In early stages, rust appears as small, rust-coloured spots on the leaves, stems and flower blossoms. When temperatures turn cooler, the spots begin turning black.
Using clean garden tools prevents fungal diseases on sunflowers. Fungi may live on pruning shears, tillers and other tools used on plants and soils. Fungus grows best in moist conditions. Water sunflowers in the early morning, so sunlight will dry the plants during the day. Rotate sunflower plantings every season to further reduce the possibility of fungal disease.