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What is it?

The fungus alternaria zinnia causes this common and widespread leaf spot disease of zinnias.

What does it look like?

Leaves develop irregular, circular brown to reddish spots up to 1/2 inch diameter. The centers of these spots may be white or grayish and often the blossoms are also similarly spotted. When severe infection takes place, the leaves, stems and flowers turn dark brown and eventually die. The bases of plant stems may develop sunken, dark lesions. Frequently when the entire plant becomes infected it wilts and dies, often over a short period of time.

How does it manifest?

The fungi flourish in moist conditions, with fungal spores spreading through splashing water or wind. Fungi survive in the soil on infected seeds and on infected debris. It can also survive on stored seeds. This species of fungi also will cause damage to seedlings and they will damp-off.

What can you do about it?

Remove any dying zinnia plants that have become severely infected, and spray infected plants with a captan or chlorothalonil containing fungicide. Pick off and destroy infected flowers and leaves as soon as you notice them. Clean up all other plant debris as you notice it collecting around the plant, and in the fall. Mainly, avoid overhead watering as this spreads spores and causes the fungus to flourish more than any other means of contact.