Leaf Spot
Photo
1: Courtesy of David F. Graper, Pest Alert,
Volume 10, Issue 2 May 20, 2001,
South
Dakota Cooperative Extension Service
Photo
2: Courtesy of David F. Graper, Pest Alert,
June 30, 1999,
South
Dakota Cooperative Extension Service
Leaf spot is a malady that affects irises in all
parts of the country. While there are two types, fungal and bacterial, the
appearance of both is quite similar. However, in the lesser-common bacterial
leaf spot, the spots eventually become soft and run together. Read on to
discover the differences in cause and treatment for each type.
Fungal Leaf Spot: Fungal leaf spot is quite common. As many as eight
different leaf-spot fungi attack different sorts of irises, but the symptoms
are all the same. Small yellow spots appear anywhere on a leaf (but more
usually toward the tips) and grow to about ¼ inch in diameter. Spots
later turn to a yellowish-brown to gray color with a distinct reddish-brown
border. Dark green spores may also be found in spots. The spots can become so
abundant that the leaf dies. Although the fungus does not attack the
rhizomes, heavily infected plants are seriously weakened and will fail to
thrive, eventually leading to the death of the plant.
Prevention: Both bacterial and fungal leaf spot occurs most often
with wet conditions such as rain, high humidity, or overhead irrigation. Fortunately, systemic fungicides work
quickly to limit the spread of the disease. The key to prevention is
eliminating overcrowding in the garden, as the disease spreads from plant to
plant. Also, care should be taken to thoroughly remove all dead foliage in
the fall. The spores that cause fungal leaf spot can overwinter on dead
leaves or other debris, initiating a new round of infection in the spring.
Cut off and destroy any leaf or part of a leaf that is affected.
Treatment of fungal leaf spot: When conditions for leaf
spot are favorable, a regular biweekly fungicide spray program is the best
course of action. Start spraying when the leaves are 6 to 8 inches tall.
Spray less often if the weather is dry and more often in rainy weather.
Usually 4 to 6 sprays of a fungicide containing chlorothalonil (Daconil),
myclobutanil (Eagle), mancozeb (Fore, Dithane, or Penncozeb), or
trifloxystrobin (Compass) beginning when the leaves are 4 to 6 inches high
and repeated at 7 to 10 day intervals, will control the disease. For maximum
effectivenss, a spreader-sticker or 1/4 teaspoon of liquid bleach should be
added to each gallon of spray. Using two of these fungicides alternately is
best, since resistance to a given fungicide can occur with repeated use. As
with all chemicals, read the directions carefully and follow them. Do not
apply near vegetables or plants that will be eaten. Wear rubber gloves and
long pants during the application.
Bacterial Leaf Spot: While the cause is different, bacterial leaf spot
closely resembles fungal leaf spot in appearance. This form of leaf spot
typically begins at the tips of the iris leaves and produces soft, watery
areas that spread rapidly and run together. These spots work downward,
covering the leaf and eventually killing it. This form of leaf spot is
relatively uncommon and occurs during long spells of wet weather.
Treatment of bacterial leaf spot: The only cure for bacterial leaf spot is to cut off any
infected leaves as far below the spots as practical. Use tools that are
dipped in a bleach solution between each cut for sterilization and always put
cut leaves into a secure container to prevent spread of the disease.
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