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Powdery mildew growth on tree poinsettia leaves.

Floriculture and Ornamental Nurseries

Powdery Mildew

Pathogens: Erysiphe spp., Leveillula (= Oidiopsis) taurica, Oidium sp., Sphaerotheca spp.

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

Powdery mildew is the name given to diseases resulting from infection by fungi that produce a white, powdery growth on the surfaces of leaves and sometimes other plant parts. Leaves may yellow, then brown and die. Infected tissues may be distorted and misshapen.

COMMENTS ON THE DISEASE

There are many kinds of powdery mildew fungi, and most are highly specialized. For example, the powdery mildew that infects squash plants will infect other plants in the cucurbit family but will not infect roses, and the powdery mildew from roses will not attack zinnias and vice versa, although the fungus that infects zinnias also infects many other members of the composite family. Powdery mildew fungi are obligate parasites; that is, they can grow only on living plant tissue. When the mildew-infected plant part dies, so does the mildew unless cleistothecia (resting stages of the fungus) are formed.

Most powdery mildew fungi grow over the surface of the leaf, sending short food-absorbing projections (haustoria) into the epidermal cells. The fungi produce masses of spores (conidia), which become airborne and spread to other plants. Powdery mildew spores are unique in that they require no external moisture for germination; most other fungi require free water in the form of dew, guttation, rain, or water from overhead irrigation for germination and infection or growth whereas the conidia of powdery mildew (except those that infect grasses) die in water. Spores may be dispersed, however, by splashing water.

The fungus survives in the absence of susceptible host tissues by forming a sexual stage (cleistothecium) resistant to drying and other adverse environmental conditions. With many perennial plants, such as rose, the fungus survives as mycelium in dormant buds or actively on plant tissues. Powdery mildews are particularly severe in semiarid regions, such as most of California, and are less troublesome in high rainfall areas.

Powdery mildews are favored by warm days and cool nights and moderate temperatures (68° to 86°F). At leaf temperatures above 90°F, some mildew spores and colonies (infections) are killed. Shade or low light intensities also favor powdery mildew fungi. Greenhouse conditions are often ideal for development of the disease.

MANAGEMENT

The best control is through the use of resistant cultivars. However little attention has been paid to development of resistant cultivars of flower crops. Because high relative humidity (greater than 95%) favors some powdery mildew fungi, increased air movement around the plants in the greenhouse tends to reduce infection potential in these mildews.

In general, there are two types of fungicidal control: eradication of existing infections and protection of healthy tissues. In practice, some products provide both protection and eradication, especially when good wetting of the plant is achieved. To achieve good wetting, some of these products may require the addition of surfactants.

The fungus has developed resistance to some of these fungicides. Rotate the different fungicides to help slow down the development of fungal strains that are resistant to the fungicides. Plants that have been treated with antitranspirants are less likely to develop powdery mildew infections.

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.
 
PROTECTANTS (Must be applied to healthy tissues before infection takes place)
A. WETTABLE SULFUR# 3 lb /100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Use a wetting agent. Effectiveness of sulfur increases with increasing temperature, but the likelihood of plant injury increases also. Plant damage may result if sulfur is applied at temperatures exceeding 90°F. Some plants, such as melons, are sensitive to sulfur. Sulfur can be applied as a dust or as a spray. Repeated applications are generally necessary to protect new growth and also to renew deposits removed by rain or irrigation.
 
B. MYCLOBUTANIL
  (Hoist) 40WSP 4 oz/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant of rusts or powdery mildew on carnations, crepe myrtle, gerbera, roses, and snapdragons.
 
C. AZOXYSTROBIN
  (Heritage) 1–4 oz/100 gal water 4
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Acts as a protectant but has some eradicant properties. A locally systemic fungicide that is an eradicant and protectant against some powdery mildews.
 
D. FENARIMOL
  (Rubigan) AS 3–10 fl oz/100 gal water 12
  MODE OF ACTION: A DMI (Group 3)1 pyrimidine fungicide.
  COMMENTS: Apply on a 10- to 14-day interval. A systemic fungicide used for prevention or eradication of powdery mildew on roses and field and container-grown ornamentals.
 
E. TRIADIMEFON
  (Strike, Bayleton) Label rates 12
  MODE OF ACTION: A DMI (Group 3)1 triazole fungicide.
  COMMENTS: A long-lasting systemic fungicide that provides for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries.
 
F. THIOPHANATE-METHYL
  (FungoFlo, Cleary's 3336F, etc.) Label rates 12
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Not as effective against powdery mildew as other materials.
 
G. PROPICONAZOLE
  (Banner Maxx) 5 fl oz/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A preventive fungicide.
 
H. POTASSIUM BICARBONATE#
  (Kaligreen) 1–1.5 lb/half-acre 4
  MODE OF ACTION: An inorganic salt.
  COMMENTS: Primarily a protectant but it eradicates some existing infections with thorough coverage. Apply in 75–100 gal water/half-acre at first signs of infection. Thorough coverage is essential for good protection. Labeled for use on roses, field ornamentals, and greenhouse ornamentals; make no more than 8 applications/season.
 
ERADICANTS
A. STYLET OIL#
  (Organic JMS Stylet Oil) 1 oz/gal water 4
  MODE OF ACTION: A contact fungicide with smothering and barrier effects.
  COMMENTS: A good eradicant for mild to moderate powdery mildew infections; oils work best as eradicants but also have some protectant activity. Registered for use on chrysanthemum, diffenbachia, philodendron, poinsettia, and roses. May be phytotoxic, especially on greenhouse roses. Do not apply to plants suffering from heat or moisture stress. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
 
B. NEEM OIL#
  (Triact) 70 Label rates 4
  MODE OF ACTION: Unknown.
  COMMENTS: A broad-spectrum botanical pesticide derived from the neem tree that is effective against various fungal diseases including black spot on roses, powdery mildew, downy mildew, anthracnose, and leaf spot. Registered for landscape and nursery ornamentals; oils work best as eradicants but also have some protectant activity. When using as a protectant, apply on a 14-day schedule; as an eradicant, apply on a 7-day schedule. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
 
C. PIPERALIN
  (Pipron) 84.4 Ec 4–8 fl oz/100 gal water 12
  MODE OF ACTION: An amine (Group 5)1 piperidine fungicide.
  COMMENTS: Requires thorough coverage. For use in greenhouses only. A foliar spray that eradicates powdery mildew on rose, lilac, dahlia, phlox, zinnia, chrysanthemum, and catalpa.
 
D. LIME SULFUR 28%# 1.5–3 pt/100 gal water 48
  MODE OF ACTION: A multi-site contact (Group M2)1 inorganic fungicide.
  COMMENTS: Primarily an eradicant but has some protectant properties. Plant damage may result if applied when temperatures exceed 80°F. Not as effective against powdery mildew as other materials. Not for use in greenhouses.
 
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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