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| Snapdragon -- Downy Mildew |
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| Cause:
The fungus-like organism Peronospora
antirrhini causes a systemic infection of young snapdragons. The disease is favored by cool, wet conditions and/or high humidity. Spores are produced between 44 and 72°F (7 and 22°C), though 50°F (10°C) is optimum. Conditions that keep leaves wet for 6 to 10 hours favors this disease. Oospores are produced in abundance in infected tissue. Seed transmission may occur but has not been proved.
The cultivars Potomac Rose, Potomac Apple, and Potomac Dark Orange are less susceptible than Rocket White, Potomac Ivory, and Potomac Royal.
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| Symptoms:
Affected plants are stunted and pale yellowish green. Leaves curl in and droop down. Dark-color cultivars develop purple spots; light-color cultivars develop yellow spots. Leaves' undersides may develop a white to purplish fungal growth (conidia and conidiophores). The growing point usually dies, and the plant often breaks off at the base. |
 The upper leaf surfaces show diffuse yellow discoloration and distortion. |
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Cultural
control: - Reduce greenhouse humidity by a combination of heating, venting, and moving air. Space plants for good air circulation.
- Scout plants carefully for symptoms of the disease especially the undersides of leaves and any stock plants or plants held over from the previous year.
- Remove and destroy infected plants and plant debris.
- Plant resistant cultivars.
- Maintain adequate fertility.
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Chemical
control: Use before symptoms develop. Rotate fungicides that have a different mode of action for resistance management.
- Alude at 1.25 to 2.5 quarts/100 gal water applied as a foliar spray at 14- to 21-day intervals. 4-hr reentry.
- Compass O 50 WDG at 1 to 2 oz/100 gal water plus a spreader sticker. Do not use organosilicate additives. Rotate to a different fungicide with a different mode of action after each application. 12-hr reentry.
- Cygnus 50 WG at 3.2 to 6.4 oz/100 gal water plus a non-organosilicone spreader-sticker. Best used before symptoms develop. 12-hr reentry.
- Endorse at 1.1 to 2.2 lb/100 gal water. 4-hr reentry.
- Exotherm Termil at 1 can/1000 sq ft of greenhouse. It
ignites to form a vapor that condenses back on the plants. See label for details. 24-hr reentry with no ventilation or
12-hr reentry with ventilation.
- Fenstop at 7 fl oz/100 gal water. For greenhouse use only. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. 4-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. May injure certain cultivars with repeated use of this product at high rates. 4-hr reentry.
- Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. 12-hr reentry.
- Kocide 2000 T/N/O at 0.75 to 2 lb/A (or 1 to 2 Tbl/1,000 sq ft) when new growth is present. 24-hr reentry.
- Mancozeb-based products can be used as mixing partners and provide some protection. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Pentathlon DF at 1 to 2 lb/A or per 100 gal water.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Phyton 27 at 1.3 to 2.5 fl oz/10 gal water. 24-hr reentry.
- Stature SC at 6.12 to 12.25 fl oz/100 gal water. Do not make more than 2 sequential applications. 12-hr reentry.
- Subdue MAXX at 0.13 to 0.25 fl oz/100 gal water as a seeding drench. Zero hr reentry.
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References:
Byrne, J. M., Hausbeck, M. K. and Sconyers, L. E. 2005. Influence of environment on atmospheric concentrations of Peronospora antirrhini sporangia in field-grown snapdragon. Plant Disease 89:1060-1066.
Francis, S.M. 1981. Peronospora antirrhini. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 685. Surrey, England: Commonwealth Mycological Institute.
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| Content edited by:
Jay W. Pscheidt on
January 1, 2009 |
