Iris leaf spot
Iris leaf spot is a common disease caused by the fungus, Mycosphaerella macrospora (formerly Didymellina macrospora). Infection is usually confined to the leaves, however stems, flower stalks, and buds may occasionally be infected. The fungus survives winter in plant material infected the previous year. Spores produced in the spring are rain splashed to emerging iris leaves, where they start new infections.
Iris leaf spot initially appears as small brown spots surrounded by water-soaked margins on the upper portion of the leaf. The spots eventually turn gray and develop red-brown borders. Early in the growing season spots enlarge slowly, then, after the iris blooms, the spots enlarge rapidly. Multiple spots may coalesce on the leaf tissue causing premature death of the leaves. If iris leaf spot occurs during several consecutive years food reserves may be depleted causing a reduction in bloom. Plants with severe repeat infections may eventually weaken and die.
Leaf spotting is more common during wet seasons. Moisture on leaves and high humidity present favorable conditions for fungal infection. Plant growth in crowded or shaded conditions often become potted earlier in the season. These leaves tend to be more severely infected than leaves affected later in the growing season.
Cultural practices will reduce the severity of iris leaf spot during dry years. Remove and destroy spotted or dead plant material, especially in the fall, to reduce the number of spores available for infection the following spring. Plant iris in full sun and space at least eight inches apart. Spacing irises in this manner increases air circulation around leaves and provides room for growth. Fungicide application may be necessary during wet years. Fungicides may also be applied after infected portions of the leaves are cut off to prevent further infections. Currently labeled fungicides include chlorothalonil (Daconil 2787), Bayleton, and thiophanate-methyl (Cleary's 3336).
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Chad Behrendt, Crystal Floyd