Sunflowers (Helianthus annuus) are head turners -- their height, large flowers and bright colors will immediately grab your attention. As with all flowers, even young sunflowers are vulnerable to disease. If you notice brown spots on a new annual sunflower, there are usually measures you can take to treat disease and prevent future problems.
Rust, a fungal disease, can result in brown spots on new sunflower leaves. Spots appear as pustules on leaf undersides and sometimes have a yellow margin. Rust usually afflicts sunflowers that are planted close together; it thrives in hot, wet environments, especially when temperatures remain high at night. Rust is often transmitted through infected soil, so do not grow sunflowers in soil where rust has previously appeared. Rust can be prevented by rotating sunflowers with another crop at least every four years. Neem oil is a good botanical oil to fight and prevent rust, although if leaf spot is already present, the process can be time consuming. Apply at a rate of 2 tablespoons per 1 gallon of water. For prevention, apply every seven to 14 days. If leaf spot is present, apply every seven days until it is gone.
Alternaria Leaf Spot
Alternaria leaf spot is caused by fungus and presents as circular spots with brownish or grayish middles and yellow edges. They may be accompanied by infected stems with brown streaks or black lesions. Rotating sunflowers will prevent alternaria leaf spot from spreading from year to year. A fungicide containing Neem oil can also be used for leaf spot. Mix 2 tablespoons into 1 gallon of water and spray all surfaces thoroughly. For prevention, apply every one to two weeks or every seven days if leaf spot has already afflicted sunflowers.
Bacterial Leaf Spot
Bacterial leaf spot first appears as circular lesions that eat through the plant and are encompassed by a yellow edge. In extreme cases, as lesions grow into each other the entire leaf turns brown and leaves fall in excess. Unfortunately, there is no treatment for bacterial leaf spot, although crop rotation may help to prevent its recurrence.
Verticillium wilt or verticillium leaf mottle usually appears just after sunflowers flower. First signs are yellow or brown spots near the veins of lower leaves. Eventually, the yellow spots grow and begin to eat through the leaf so that it appears green, yellow and mottled. As time goes on, yellow spots become brown and spread throughout the leaves. Eventually, dry leaves die and fall. The only effective method that may help prevent verticillium wilt is to rotate crops.