Cause Several fungi including Septoria tageticola, Alternaria sp., and Cercospora sp. Although there are no published reports of finding these fungi in the Pacific Northwest, the OSU Plant Clinic has found Alternaria leaf spot. Cultivars of the African marigold (T. erecta) are susceptible to S. tageticola, but cultivars of the French marigold (T. patula) are resistant. Generally favored by conditions that keep foliage wet for extended periods of time.
Symptoms S. tageticola produces oval to irregularly shaped spots that are smoky gray to black. Small, black, fruiting bodies (pycnidia) develop in the necrotic area. Infections usually start on lower leaves and progress to higher leaves.
Alternaria tagetica can result in a blight of leaves, stems, and flowers. Dark-brown necrotic spots form initially and may be surrounded by a chlorotic halo. Leaf spots can expand and coalesce leading to wilting and drying of the leaves with eventual plant death. Flower petals can become totally darken and shriveled.
Cultural control
- Keep leaves dry.
- Plant resistant cultivars.
- Remove and destroy infected leaves.
Chemical control Best used before symptoms develop and in conjunction with cultural controls.
- 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.
- Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- 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.
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Insignia at SC 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Medallion WDG at 1 to 2 oz/100 gal water is registered for several leaf-spotting organisms. Using with oils or adjuvants may damage plant. 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. Group M1 fungicide. 48-hr reentry general or 24-hr reentry for greenhouse.
- Orkestra at 4 to 10 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Ortho Rose & Flower Disease Control at 2 fl oz/gal water. Group 3 fungicide. H
- Pageant at 8 to 12 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.
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 5 to 8 fl oz/100 gal water. 12-hr reentry.
- Infuse Systemic Disease Control at 2 Tbsp/gal water. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Spirato GHN at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Ziram 76 DF at 1.5 to 2 lb/100 gal water. Group M3 fungicide. 48-hr reentry.
References Cotty, P.J., Misaghi, I.J., and Hine, R.B. 1983. Production of zinniol by Alternaria tagetica and its phytotoxic effect on Tagetes erecta. Phytopathology 73:1326-1328.
Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants, 5th ed. New York: John Wiley & Sons, Inc.