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UC Pest Management Guidelines


Grayish downy mildew fungal growth on the underside of a spinach leaf.

Floriculture and Ornamental Nurseries

Downy Mildew

Pathogens: Peronospora spp., Plasmopara spp., and Bremia sp.

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

The name downy mildew is somewhat descriptive of the appearance of the white, lavender, or purple sporulation that occurs usually on the undersides of leaves. Downy mildew leaf lesions are often angular in shape and delimited by veins. Pale yellow or necrotic areas often are visible from the upper side of the leaf. Extensive disease can result in the death of large portions of the leaves.

In some plants when young shoots are infected the fungus may become systemic and the resultant growth is stunted, malformed, and discolored.

COMMENTS ON THE DISEASE

In contrast with powdery mildews, the downy mildews are more host specific and require very wet conditions to flourish. Water is required for infection and humidity above 90% is needed for sporulation. Growth of the pathogens is favored by cool temperatures.

Downy mildew spores are usually short-lived although they may survive several days under cool, moist conditions. They are airborne and when they land on a susceptible plant and there is free water, germination and infection occurs, generally in 8 to 12 hours. Some downy mildews also produce a sexual spore that can survive dry conditions, which enables the pathogen to survive in the absence of the host. Downy mildews are favored by moist and cool conditions (40° to 60°F).

Common name Amount to Use R.E.I.+
(trade name)   (hours)

  Calculate impact of pesticide on air quality
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
 
A. MANCOZEB
  (Dithane) 75 1–1.5 lb/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: A protectant fungicide. Thorough coverage is important for control.
 
B. MEFENOXAM
  (Subdue Maxx) 1-2 oz/100 gal water 48
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
  COMMENTS: Use allowed in California under a 24(c) registration.
 
C. COPPER 50%# 1–2 lb/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: A protectant fungicide. Growth of some plants may be reduced by this material; follow label directions carefully to reduce the risk of phytotoxicity. Not all copper compounds are approved for use in organic production; be sure to check individual products.
 
D. FOSETYL-AL
  (Aliette) 2.5 lb/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: For control of downy mildew on roses. Spray to wet using no more than 400 gal/acre.
 
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
# Acceptable for use on organically grown ornamentals.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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