1 Answer

Iris leaf spot (blotch or fire) is the most common and widespread
disease of both rhizomatous and bulbous species of iris. It is
caused by the fungus Mycosphaerella (Didymellina) macrospora
(imperfect state Cladosporium iridis = Heterosporium gracile).
Other hosts of the leaf spot fungus include daylily (Hemerocallis),
freesia, gladiolus, and narcissus.
Leaf spot devitalizes plants and makes them unsightly. Repeated
severe attacks reduce blooming and may kill plants after several
years since premature death of the leaves gradually weakens the
rhizomes or bulbs. Heavily infected plants are also more prone
to cold damage, other diseases, and insect pests.
The disease is favored by prolonged periods of mild and very
damp weather. Leaf spot is most severe in low-lying areas where
air movement is poor and where diseased iris debris has been
allowed to accumulate.
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