Cause This fungal disease, caused by Alternaria alternata, has not been formally reported from the Pacific Northwest but has been found on a few samples sent to the OSU Plant Clinic. The disease is favored by prolonged cool, moist conditions and low nitrogen fertility. Older leaves are particularly susceptible if they are senescing. The fungus survives on dead leaves on the soil surface and spores are spread by air currents.
Symptoms Dark brown, irregularly shaped spots appear on leaves. Spots range in size from barely visible to 0.33 inch in diameter. Larger spots may show several dark concentric rings within the spot. Spots may be surrounded by a diffuse yellow halo. Spotting is mostly on the lower leaves, although new growth may be affected. Numerous spots close together on leaves may coalesce. Severely infected leaves shrivel, turn black, and fall off. Only leaves are affected.
Cultural control
- Space plants for good air circulation.
- Remove and destroy infected leaves and fallen plant debris.
- Maintain optimum plant fertility.
- Water at base of plants and/or with drip irrigation. Do not wet foliage.
Chemical control Use just before conditions favor disease development.
- Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Group 2 fungicide. 12-hr reentry.
- Compass 50 WDG at 2 to 4 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- CuPRO 5000 at 1.5 to 2 lb/A. Group M1 fungicide. 48-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Disarm 480 SC at 1 to 4 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Fame SC at 1 to 4 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
- Heritage at 4 to 8 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-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.
- Medallion WDG at 1 to 2 oz/100 gal water. Use with oils or adjuvants may damage plant. Some geranium cultivars may become stunted or chlorotic when higher rates are used. Group 12 fungicide. 12-hr reentry.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Nu-Cop 50 DF at 1 lb/100 gal water. 48-hr reentry. O
- Orkestra at 4 to 6 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Pageant at 4 to 8 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Postiva at 10 to 28 fl oz/100 gal water. Group 3 + 7 fungicide. 12-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trinity TR at 1 can/3,000 sq ft of greenhouse. Group 3 fungicide. 12-hr reentry without ventilation or 4-hr with ventilation.
Reference Strider, D.L. 1985. Diseases of Floral Crops, vol. 2. New York: Praeger Publishers.