Cause Septoria leucanthemi, a fungus that overwinters on plant refuse and on living plants. Spores can be spread by splashing water from rain or irrigation. Once on the leaf, spores germinate and enter through stomata. More than 12 hours of wetting is required for spore germination and leaf infection. Optimum temperature for disease expression is 74°F to 79°F. This species has been reported in both Oregon and Washington.
Alternaria sp. has been identified twice by the OSU Plant Clinic in leaf spots from both Oregon and Washington. This fungus also overwinters on plant refuse and on living plants. Spores are spread by rain or wind.
Symptoms Dark brown circular or semicircular zonate spots with a central sunken pale or whitish area surrounded by somewhat elevated concentric markings. The central pale area may fall out leaving leaves with a shothole appearance. Abundant minute black fruiting bodies (pycnidia) may be seen around the rim of the shothole. Lower leaves tend to show symptoms first. If the disease is severe, leaves may shrivel and die; however, they generally stay on the stem. Alternaria is very similar but will not produce fruiting bodies.
- Remove and destroy diseased leaves and plants at the end of the growing season.
- Keep plant tops as dry as possible by applying water to soil, not sprinkling over plants.
- Rotate planting beds.
Chemical control Focus on cultural control methods first. Spray thoroughly, including leaf undersides.
- 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.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Captan 80 WDG at 1.5 lb/100 gal. Group M4 fungicide. 48-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.
- Emblem at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
- Hi-Yield Captan 50 W Fungicide may be used in home gardens at 2 Tbsp/gal water. H
- 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. Group 12 fungicide. 12-hr reentry.
- Ortho MAX Garden Disease Control at 2 teaspoons/gal water. H
- Ortho Rose & Flower Disease Control 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.
- 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.
- 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.
Reference Punithalingam, E. 1967. Septoria leucanthemi. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 138. Surrey, England: Commonwealth Mycological Institute.