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Iris, Bulbous and Rhizomatous (Iris spp.) - Leaf Spot
Cause Bipolaris iridis (asexual: Drechslera iridis), a fungus that overwinters on diseased debris and infected bulbs. New spores that form in spots can spread through the air to healthy foliage. Wet weather is suspected to favor disease development. The disease is common in western Washington.
Symptoms Small, dark-brown spots first appear on leaves, stems, and sometimes flowers. They enlarge, fuse, and cause the leaf's collapse. Ink spot can be confused easily with Cladosporium leaf spot (fire). Spores formed in the centers of the ink spots are brownish black compared to the greenish spores of Cladosporium. Bulbs of "Dutch-type" iris are not affected often, but on Iris reticulata irregular, ink-black stains appear on husks. Fleshy scales may exhibit small yellow dots or irregular sunken black craters with distinct raised margins.
Cultural control
- Remove and destroy affected leaves in fall.
- Maintain proper plant spacing to provide good air circulation.
- Rotate plantings.
- Divide plants frequently enough to avoid serious congestion.
Chemical control Begin applications before disease appears and repeat every 10 to 14 days while conditions favor disease development. Many materials registered for leaf spot may also work on ink spot.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water plus a resin-based surfactant. Group 2 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 2 fl oz/gal water. Group 3 fungicide. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
Reference Ellis, M.B. and Waller, J.M. 1974. Drechslera iridis. CMI Descriptions of Pathogenic Fungi and Bactera. No. 434.