| 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. | 
