Group Title: Plant Pathology Documents
Title: Powdery mildew of strawberries. Plant Pathology 208
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Permanent Link: http://ufdc.ufl.edu/UF00066813/00001
 Material Information
Title: Powdery mildew of strawberries. Plant Pathology 208
Series Title: Plant Pathology Documents
Physical Description: Book
Creator: Peres, N. A.
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Plant Pathology Department -- Institute of Food and Agricultural Sciences -- Gulf Coast Research and Education Center, Dover, FL
Publisher: Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: March 2005
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066813
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

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Powdery Mildew of Strawberries

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Powdery Mildew of Strawberries1

N.A. Peres and J.C. Mertely-

Powdery mildew, caused by SphL,/i. i, //?L tI macularis, occurs in most areas of the world where
strawberries are grown.

Pathogens and Symptoms

S. macularis infects leaves, flowers and fruit. Early foliar infections are characterized by small white
patches of fungus growing on the lower leaf surface. On susceptible cultivars, dense mycelial growth
and numerous chains of conidia (spores) give these patches a powdery appearance (Figure 1 ).
Under favorable conditions, the patches expand and coalesce until the entire lower surface of the leaf
is covered ( Figure 2 ). In some cultivars, relatively little mycelium is produced, making it difficult to
see the white patches. Instead, irregular yellow or reddish brown spots develop on colonized areas on
the lower leaf surface, and eventually break through to the upper surface ( Figure 3 ). The edges of
heavily infected leaves curl upward (Figure 4 ). At times, dark round structures (cleistothecia) are
produced in the mycelia on the undersides of leaves ( Figure 5 ). Cleistothecia are initially white but
turn black as they mature. The fungus also infects flowers, which may produce aborted or malformed
fruit. In addition, S. macularis colonizes older fruit producing a fuzzy mycelial growth on the seeds
(Figure 6 ). Both types of infection may reduce fruit quality and marketable yields.

Disease Development and Spread

Sphaerotheca macularis is an obligate parasite that only infects living tissue of wild or cultivated
strawberry. In temperate areas, the pathogen may survive by producing cleistothecia. The fungus
readily infects living, green leaves in the nursery. Thus, infected transplants are normally the primary
source of inoculum for fruiting fields in Florida. When conditions are favorable, conidia produced on
infected plants are wind dispersed. Development and spread of powdery mildew is favored by
moderate to high humidity and temperatures between 600 to 800 F. Rain, dew and overhead irrigation
inhibit the fungus. Because dry conditions and high humidity are common in greenhouses and plastic
tunnels, powdery mildew is typically more severe in protected culture. In open fields in central
Florida, the disease is typically most severe in November and December, usually subsides in January
and early February, but may reappear in late February and March.

Powdery Mildew of Strawberries

Use of transplants free of powdery mildew is a good method for controlling the disease but even
disease free fields can become infected by conidia blown in from neighboring fields. Cultivars differ
widely in their resistance to powdery mildew. Unfortunately, the most popular cultivars, Strawberry
Festival and Camarosa, are quite susceptible to the disease. Fields with susceptible cultivars should
be surveyed regularly for powdery mildew, especially early in the season. Fungicides should be
applied at the first sign of disease to control powdery mildew on susceptible cultivars. This is
especially important when using protectant fungicides such as elemental sulfur. Systemic fungicides
like the benzimidazoles (Topsin M) and the sterol inhibitors (Nova and Procure) can effectively
control powdery mildew if the pathogen population has not become resistant to them. Strobilurin
fungicides such as Abound, Cabrio, and Pristine are also effective in suppressing the disease,
but caution should be taken to not to exceed four to five applications per season. Controlling the
foliar infections helps to prevent fruit infections.


Figure 1. Mycelia of .SliL ,, %hll, macularis on strawberry leaf surface.


Figure 2. Lower leaf surface of strawberry covered with powdery mildew.

Powdery Mildew of Strawberries


Figure 3. Reddish-brown spot reaction caused the Sphaerotheca macularis in some cultivars.


Figure 4. Curling leaves on severely infected plants.


Figure 5. Micrograph of cleistothecia on leaf surface.

Powdery Mildew of Strawberries


1. This is document PP-208 a publication of the Plant Pathology Department, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of
Florida, Gainesville, FL. Publication date: March 2005. Please visit the EDIS Website at

2. N.A. Peres, assistant professor, Plant Pathology Department, and J.C. Mertely,
coordinator programs/services, Gulf Coast Research and Education Center--Dover, FL.
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information.
UF/IFAS does not guarantee or warranty the products named, and references to them in this
publication does not signify our approval to the exclusion of other products of suitable composition.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution
authorized to provide research, educational information and other services only to individuals and
institutions that function with non-discrimination with respect to race, creed, color, religion, age,
disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For
more information on obtaining other extension publications, contact your county Cooperative
Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida
A. & M. University Cooperative Extension Program, and Boards of County Commissioners
Cooperating. Larry Arrington, Dean.

Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural
Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all
conventions, but permits free reproduction by all agents and offices of the Cooperative Extension
Service and the people of the State of Florida. Permission is granted to others to use these materials
in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the
publication, its source, and date of publication.

* PHI = Post harvest interval; REI = Restricted entry interval.

Recommendations given in this fact sheet are based on experimentation and statements from the
manufacturer. Consult the label of all products for specific use requirements and restrictions.

Powdery Mildew of Strawberries


Figure 6. Sphaerotheca macularis on seeds.


Table 1. Products labeled in Florida for control of powdery mildew of strawberry.

Trade Name AHI Comments
Active ingredient Type
or REI

Do not add silicone
.sufactants or mix
Abound azoxystrobin strobilurin 4 with E
with EC

p, Do not mix with
Armicarb, .
Armicarb, potassium bicarbonate salt 4 highly acidic
No more than two
Cabrio EG pyraclostrobin strobilurin 24 sequential
S. Do not apply more
Nova 40W myclobutanil sterol inhibitor 24 than 30 oz/A/season.
than 30 oz/A/season.

S. boscalid + carboxamide + I No more than 115 oz
Pristine 24
pyraclostrobin strobilurin product/A/season.
No more than 32 oz/
Procure 50WS triflumizole sterol inhibitor 24 A/season.

Topsin M No more than 4 lbs
7oWSB thiophanate methyl benzimidazole 24 N rod t/eason.
70WSB F product/A/season.

Suppresses mites,
Wettable sulfurs including predatory
(numerous trade sulfur elemental 24 populations in
names) biological control

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