Alternaria Blight of Zinnia
Click for larger image Alternaria blight on zinnia (Zinnia) - left leaf; bacterial leaf spot on zinnia - right leaf

Only two diseases are commonly troublesome on zinnias (Zinnia elegans), powdery mildew and Alternaria blight. Alternaria blight is the most common and conspicuous.

Symptoms and Diagnosis

Spotting of the foliage caused by the fungus, Alternaria zinniae, is the most conspicuous symptom of zinnia blight. Individual spots are at first circular in outline but rapidly become irregular. The spots may vary from 2 to 10 millimeters or more in diameter, are reddish brown, and have grayish-white centers on the upper leaf surface. Differentiation of margin and center is lacking on the lower surface. As the spots increase in size and number, they coalesce and the affected leaves become brown and dry.

Blossoms may be severely affected. Brown spots, 1 or 2 millimeters in diameter, with grayish-white centers sometimes appear on the petal tissues of the ray flowers. Affected plants soon darken and wither, causing blossoms to become unsightly.

Numerous, small, reddish spots, sometimes with grayish-white centers, may be seen on stem internodes. Such spots usually are superficial. Spots that develop at nodes, however, usually do not remain superficial. Instead, they grow or coalesce into larger lesions that frequently girdle the stem causing the upper portions of the affected stem to die back to the node. Dark brown to black cankers with sunken centers are also common at the base of the stem of diseased plants. Affected plants often wilt completely, even when the basal cankers do not encircle the stem.

The outer tissues of affected roots may become dark gray, rot completely, and slough off, resulting in wilting and death of the plant. Damping-off of seedlings also may occur.

Life Cycle

The fungus survives on seeds and in the soil associated with host debris. Midsummer and early fall are normal times of outbreak.

Integrated Pest Management Strategies

1. Sanitation. Clean up debris; bury or burn if possible. Fungus may remain associated with plant material for 2 years.

2. Rotation. Use a long crop rotation of three years if growing commercially. A two-year rotation schedule is satisfactory for home garden and small isolated areas where disease spread can be limited.

3. Resistant varieties. Use resistant varieties of Zinnia elegans when possible. But be warned, no cultivars to date have been found to be totally resistant to Alternaria blight.

4. Propagation. Seeds may be infected with the fungus. Before planting, treat the seeds by placing them in hot water at 125 degrees F for 30 minutes. Then cool and dry. Older seeds can be injured by this treatment.

5. Watering. Do not use overhead irrigation. Water at the base of plants.

6. Fungicides. Use protectant fungicides. Spray seedlings and young plants with azoxystrobin (Heritage), chlorothalonil (Daconil), copper, ferbam, mancozeb, or maneb.

Organic Strategies

Strategies 1 through 5 are strictly organic approaches. Of the fungicides listed in Strategy 6, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.