Downy Mildew: Cool-Weather Obligate Parasites of Ornamentals

Downy mildews are highly specialized obligate parasites of vascular plants—distinct from powdery mildews. Though historically grouped with fungi, they belong to the Oomycota (with Phytophthora and Pythium), requiring different chemistries than true fungi.

Symptoms

• Colonies often first on leaf undersides, sometimes bluish-tinged
• Can grow systemically throughout plant
• Upper leaf: small yellow spots
• Underside: white to bluish-white fluffy growth, darkening to gray as tissue dies
• Infected leaves/branches may distort and die
• Can be confused with Botrytis or powdery mildew macroscopically; microscopically distinct

Life History & Epidemiology

• Favored by cool temps (58–72°F / 15–23°C) and >85% RH at leaf surface
• Some overwinter locally in debris, soil, weeds; others blow in annually from southern regions
• Sexual reproduction: oogonia + antheridia; asexual: sporangia
• Infection to new spores: 4–10 days
• Sporangia germinate directly (germ tube) or indirectly (zoospores in Sclerospora, Plasmopara); Peronospora, Pseudoperonospora, Bremia rarely form zoospores

Management

• Remove debris: bury, burn, or sealed container
• Space plants for air circulation; rapid leaf drying after irrigation/rain
• Avoid overhead irrigation in cool weather
• Critical: Scout highly susceptible crops (roses, pansy, impatiens) during cool, damp spring weather for first infections
• Contact Extension for fungicide recommendations (chemistries similar to Pythium/Phytophthora control)

Key References

1. Agrios, G.N. 1997. Plant Pathology, 4th ed. Academic Press.
2. Farr et al. 1989. Fungi on Plants and Plant Products in the U.S. APS Press.
3. Yarwood, C.E. 1947. Snapdragon downy mildew. Hilgardia 17:241-250.