Fungus on Zinnias

Zinnia plants (Zinnia spp.) are colorful, summer- and autumn-blooming annuals that belong to the daisy family (Asteraceae) and grow well across the United States. Common zinnias (Zinnia elegans) range between 1 and 4 feet in height and make lovely additions to borders and flower beds. Creeping zinnias (Zinnia angustifolia) make attractive ground covers since they reach just 9 to 18 inches in height. Two troublesome plant diseases occasionally cause fungus to appear on zinnias.


Blights are potentially serious fungal diseases that can quickly turn entire plants unsightly. The most common zinnia ailment is zinnia blight (Alternaria zinniae), which initially causes small, red-brown spots to appear on the upper surfaces of leaves. The spots have noticeable, gray-white fungal spores growing in the centers. The blighted areas enlarge and merge, causing affected leaves to dry out and turn brown. Similar spots may appear on the flowers, making the blossoms wither or darken. Zinnia stems frequently develop dark brown cankers that can girdle the entire stem and cause dieback or wilted foliage.


Zinnias sometimes suffer from powdery mildew (Golovinomyces cichoracearum), a fungal disease that causes a white or grayish-white powdery growth to form on leaves, flowers and stems. The powdery patches sometimes grow until they cover entire leaves or stems. Severe infections cause yellowing or browning foliage, dead leaves and deformed flowers. Although powdery mildew is rarely fatal, it can cause premature defoliation and make your plant look unattractive.

Leaf Spot

Leaf spot often occurs on common zinnias, although creeping zinnias show great resistance to this plant disease. The fungal pathogens (Cercospora zinniae) cause affected leaves to develop small, red-brown to deep purple spots with white to pale gray fungal spores in the centers. The fungal spores often fall out of the leaves, leaving little gaping holes behind. Sometimes several leaf spots merge, which kills the entire leaf. Severe infections cause the foliage to dry out and turn brown.

Chemical Treatments

You can often effectively treat mild to moderate fungal diseases with horticultural or plant-based oils, such as jojoba or neem oil. The University of Wisconsin Cooperative Extension recommends spraying fungus-infected plants with a solution containing 3 tablespoons of lightweight horticultural oil and 1 1/2 tablespoons baking soda for each gallon of water. Start treatment as soon as you spot any fungal disease symptoms and repeat applications every seven to 10 days. Fungicidal oils work only on contact, so be sure to completely cover the foliage when you spray. Carefully read and follow label instructions and safety precautions.

Cultural Controls

Fungal pathogens often overwinter in plant debris, so rake up and destroy fallen leaves and decaying plant material when the growing season ends. Plant your zinnias in rich, well-drained soil that receives full sun. Not only do these annuals thrive in fully sunny locations, but fungal diseases also prefer to infect plants growing in shady, moist conditions. Water your zinnias about once a week. Using a soaker hose, water just around the base of the plant until the top 8 to 12 inches of soil receives moisture. Water early to mid-morning so the foliage has a chance to quickly dry in the sun. Space multiple zinnias so each plant receives plenty of air circulation. Ample airflow helps keep fungal pathogens off of your plants.

Photo Credits

  • Medioimages/Photodisc/Photodisc/Getty Images
bibliography-icon icon for annotation tool Cite this Article