How to Manage Pests

UC Pest Management Guidelines

COMMENTS ON THE DISEASE
There are many kinds of powdery mildew fungi, and some 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
(trade name)		Amount to Use
	PROTECTANTS (Must be applied to healthy tissues beoe infection takes place)
A.	WETTABLE SULFUR	3 lb /100 gal water
	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
	(Systhane) 40WSP	4 oz/100 gal water
C.	AZOXYSTROBIN
	(Heritage)	1-4 oz/100 gal water
	COMMENTS: Acts as a protectant but has some eradicant properties.
D.	FENARIMOL
	(Rubigan) AS	3-10 fl oz/100 gal water
	COMMENTS: Apply on a 10- to 14-day interval.
E.	TRIADIMEFON
	(Strike, Bayleton) 25WDG	2-4 oz/100 gal water
	COMMENTS: Provides general control of some powdery mildew fungi.
F.	THIOPHANATE-METHYL
	(FungoFlo, Cleary's 3336F, etc.)	Label rates
	COMMENTS: Not as effective against powdery mildew as other materials.
G.	PROPICONAZOLE
	(Banner Maxx)	5 fl oz/100 gal water
H.	POTASSIUM BICARBONATE
	(Kaligreen)	1-1.5 lb/half-acre
	COMMENTS: Potassium bicarbonate is 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. Labeled for use on roses, field ornamentals, and greenhouse ornamentals; make no more than 8 applications/season.
	ERADICANTS
A.	STYLET OIL
	(JMS Stylet Oil)	1 oz/gal water
	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) 90EC	Label rates
	COMMENTS: 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
	COMMENTS: Requires thorough coverage. For use in greenhouses only.
D.	LIME SULFUR 29%	1.5-3 pt/100 gal water
	COMMENTS: 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.

PUBLICATION

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

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