RE: TB: Companion Plants and Leaf Spot
Jeff is quite right in pointing out that there are two different sorts of leaf
infections. The fungal kind (*Didymellina macrospora*) start out as a
wet-looking spot after rain or overhead irrigation (if you do that sort of
thing). They soon turn yellowish tan to brown, and have a ring look to them.
A lot of fungal leafspot can run together and look pretty bad. Injuries from
sharp, small hail are especially bad about providing entry points for the
The other sort, the bacterial, (*Xanthomonas tardicrescens*) appears also
after water--rain or irrigation by sprinkler--but usually starts near or at
the tips of the leaves and spreads downward in irregular jagged splotches.
The foliage looks awful!
TWOI (*The World of Irises*) has a chapter on diseases of irises written by
John Weiler and starts on page 334. On page 335 there is a photograph in
black and white of four leaf conditions--the first bacterial leaf blight, the
second fungal leaf spot, the third virus mottling, and the last, also virus,
is of mosaic type lesions (photo by Barnett).
The first two photos are those Jeff and I describe--bacterial leaf blight and
fungal leaf spot. Both these can be reduced in severity or incidence by a
clean mulch of bark chips, crushed rock or some such ground cover that does
not harbor the bacteria or fungal spores, or a non-competing, low growing
ground cover (if such a thing is possible).
The *worst* outbreaks of the bacterial and fungal problems will occur if there
is a lack of sanitation (removal of dead or diseased foliage) or lack of
cultivation and hoeing. Weeds that impede air movement in and around the iris
foliage--or planting too densely--prevents the leaves from drying quickly,
thus helping the infection get a toehold. Damp foliage that stays damp for a
long time invites these diseases.
Spring burning or an oil-emulsion dormant spray as is used in fruit orchards
(March in Idaho, earlier in the South and mid-winter, I think, in So.
California) does wonders in preventing these diseases--they kill the fungus
spores and dormant forms of the bacteria.
Few of us have access to the orchard spray treatment--but the lucky ones who
have a neighbor with orchards might ask the grower to give a blast to the iris
patch. Won't hurt--and does a whale of a lot of good.
Neil Mogensen z 7 western NC mountains
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