My Sunflowers Are Growing With Brown Spots on Their Leaves

Alternaria Leaf Spot

• Alternaria leaf spot or blight caused by the fungus Alternaria zinniae affects the leaves of sunflowers and other plants by forming small, oval, reddish-brown spots on the upper surface of leaves accentuated by a white or gray center. The brown spots grow larger as the white center eventually turns dry and falls away, leaving behind a hole. The foliage begins to take on a ragged appearance, especially when further damage is inflicted by buffeting wind and rain.

Damage

• These same brown spots form at the stems of a sunflower and may girdle, or strangle, the stem, causing the upper foliage and stems of the plant to wither, brown and die as water and nutrients are blocked from reaching that area. Alternaria leaf spot or blight also produces cankers near the soil line and seeps into the roots causing rot and decay. The same brown spots appear on the petals of the sunflower. Seedlings are likely to collapse and die.

Sunflower Rust

• The fungus Puccinia helianthi causes sunflower rust, a condition that produces brown spotting on the lower leaves of a sunflower and then moves on to affect upper leaves as well. The condition normally develops when sunflowers are at or past their blooming stage. The circular brown spots measure approximately 1/16 of an inch in diameter with smaller spots eventually merging into larger collections. Sometimes the brown spots may be surrounded by a yellow border.

Rust Management

• As the weather cools, the spots become an even darker brown or black. The fungus overwinters on sunflower litter and spreads easily to new sunflowers in the spring, especially when warm and wet weather exists. Sunflower rust should be managed by ensuring that new sunflowers are not planted the following year in areas where rust previously occurred. Sunflowers that are planted early and short season hybrids typically experience fewer cases of rust. Promoting good air circulation to keep the leaves dry is especially helpful in sunflower rust management.