Control Plant Diseases
in yard and garden

Mimeo BP 1-7
Revised

DISEASES OF IRIS

The iris was named for the Greek
goddess of the rainbow and was venerated
in ancient Egypt. It is believed by botanists
that the "lilies of the field" in Biblical lands
were really irises, and one of our names for
the older garden iris was "Fleur-de-lis."
There are several destructive diseases
of the iris that may spoil anticipated blooms
or destroy the plant completely.

Leafspot
The most common and most widely dis-
tributed disease of iris is leafspot. It is
generally responsible for the ragged appear-
ance of iris in mid-summer, and is sometimes
serious enough to cause premature death of
the foliage. Repeated attacks for several
consecutive seasons will weaken the iris plant,
may be responsible for the plant’s failure to
produce satisfactory blooms, and may oc-
casionally kill the plant.
Leafspot on iris becomes evident in
late May. It takes the form of small, gray-
ish-brown spots with watersoaked margins.
These initial infections will increase in size
and number until they involve a large portion
of the leaf surface. After the plants bloom,
the leaf spots enlarge more rapidly, run to-
gether and destroy many leaves prematurely.
The older spots become gray in the center
surrounded by a reddish-brown border.
Iris leafspots are caused by several
fungi (Phyllosticta iridis, Alternaria iridicola and Didymellina macrospora) which live as
parasites in the infected spots on the foliage.
Microscopic spores are produced from the
center of each spot during damp weather.
These may be seen with a strong hand lens as
black, hair-like growths. They are splashed
to the surface of healthy leaves by rains or
watering and develop into many new leafspot
infections. Iris leafspot fungi live through
the winter months in dead, diseased leaves,
which serve as a source of infection the fol-
lowing spring.
Sanitation is a major step towards pre-
venting leafspot. Late in the fall or before
growth starts in the spring, rake up and
burn all plant debris around the iris. During
the early summer, destroy all foliage showing
extensive leafspot development.
In years of frequent spring and early
summer rains, 4 to 6 sprays at 7 to 10 day
intervals will be needed to control leafspot.
Spraying should start when the new growth is
4 to 6 inches high and should be discontinued
just before the first blossoms open.
Any of the following fungicides will pre-
vent leafspot:
	Phaltan--2 tablespoonsful per gallon of
water.
	Captan (Orthocide)--2 1/2 tablespoonsful
per gallon of water.
	Zineb (Parzate or Dithane Z-78)--2
tablespoonsful per gallon of water.
	Maneb (Manzate or Dithane M-22)--l 1/2
tablespoonsful per gallon of water.
Adding 1/2 teaspoonful of liquid house-
hold soap to each gallon of spray will help im-
prove leafspot control.

Cooperative Extension Service PURDUE UNIVERSITY Lafayette, Indiana
Department of Botany and Plant Pathology, Life Science Building