Question: Why do zinnias in the
garden develop leaf blight or mildew, and decline and die?
Answer: There are two important zinnia leaf spots
in Indiana, one bacterial, caused by Xanthomonas zinniae,
and the other fungal, caused by Alternaria zinniae. Bacterial
leaf spots are small, angular, brown spots often surrounded by
a yellow margin. Alternaria leaf spots are larger and reddish brown
with grayish centers. Both diseases result in leaf blighting, and
death of foliage, and eventuallly plant death. Powdery mildew,
covering leaves with grayish white superficial growth also causes
foliage to die. Some of these zinnia diseases are seed-borne, so
purchase seeds from a reliable source. Fungicides may be beneficial
for powdery mildew and Alternaria leaf spot, but will not control
bacterial leaf spot. Avoid wetting foliage while watering. Fall
garden clean-up will help to reduce carry over of inoculum for
next year's plantings.
--Dr. John Hartman, Extension Plant Pathologist, University
of Kentucky, Lexington
Edited for Indiana by Gail Ruhl, Purdue University
Information listed is valid only for the state of Indiana.
The information given herein is supplied with the understanding
that no discrimination is intended and no endorsement by the
Purdue University Cooperative Extension Service is implied. Any
person using products listed assumes full responsibility for
their use in accordance with current direction of the manufacturer.
Purdue University is an equal opportunity/equal access institution.