leaf spots
- To: Multiple recipients of list <i*@rt66.com>
- Subject: leaf spots
- From: B* S* <B*@hsc.edu>
- Date: Sat, 28 Mar 1998 08:00:38 -0700 (MST)
I'm posting an excerpt from THE GARDENER'S IRIS BOOK on leaf spots.
Leaf Spots
Again, there are two possible culprits, a bacterium and a fungus. Both
produce rather similar symptoms, but with practice can be told apart. I
suspect that the prevalence of leaf spotting was behind the old custom of
cutting down the foliage of bearded irises after the blooming season. This
practice may actually have been helpful, at least with bacterial leaf spot.
Bacterial leaf spot
It begins at the margins or tips of the leaves, producing soft, watery
areas that rapidly spread and run together, working downward through the
leaf. Eventually the whole leaf may die. This form of leaf spot is
relatively uncommon, found mostly in northeastern North America, and occurs
during long spells of wet weather.
The only cure is to cut off any infected leaves well below the spotted
region, using tools that are dipped in a bleach solution between each cut
(the bacteria can be spread by tools). Good sanitation helps; keep dead
leaves pulled off, and don't allow irises to become so crowded that air
can't easily circulate amongst them. The spread of the disease slows or
stops when the weather dries off.
Fungal leaf spot
As many as eight different leaf spot fungi attack different sorts of
irises, but the symptoms are all the same. Small spots appear anywhere on
a leaf (but more usually toward the tips) and grow to about a quarter of an
inch in diameter, with a yellowish center and a distinct brown border. 2-32
The spots do not get soft and do not run together as in bacterial leaf
spot, but they can become so abundant that the whole leaf is destroyed.
Heavily infected plants are seriously weakened and will fail to thrive.
This disease, like bacterial leaf spot, happens most often during wet
weather, and may appear in the eastern part of North America and in Europe
in early spring.
Treatment and Prevention
In areas with dry summers, fungal leaf spot comes on with the autumn rains.
For the sake of appearances, badly spotted leaves can be removed. Even
more so than with other diseases, fungal leaf spot can be prevented by
avoiding crowding (the disease spreads from plant to plant) and by cleaning
up dead foliage thoroughly in the fall and burning it. Systemic fungicides
work quickly to limit the spread of the disease, but should be used only in
those rare cases of very heavy infestations. Some authorities say that if
you can keep this disease out for two years, it will not recur unless
brought in on new plants.
Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@hsc.edu>
