• Topics: Plant Pathology  |  Gulf Coast REC  | Strawberry Diseases  | Mertely, James C  |  Peres, Natalia A 

Powdery Mildew of Strawberries¹

Powdery mildew, caused by Sphaerotheca macularis, occurs in most areas of the world where strawberries are grown.

Pathogen and Symptoms

Sphaerotheca 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 strawberry 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. 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 60° to 80° 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.

Control

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 Treasure, 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 have some limited curative action. These include Rally^®, whose active ingredient is myclobutanil, and which was formerly named Nova^®. Sonoma^® is a competing brand that also contains myclobutanil. Fungicides in the same chemical class as Rally^® and Sonoma^® include Procure^®, Bumper^®, and Orbit^®. These products are treated as a group since they belong to the same fungicide class and have similar properties. All share a common, single mode of action and, for this reason, should be rotated with other fungicides with different properties to avoid the development of resistance. Quintec^® is a recently introduced and effective fungicide with a different mode of action than other powdery mildew products. Other rotational options include the benzimidazole fungicide Topsin M^® and the strobilurin fungicides Abound^®, Cabrio^®, and Pristine^®, but caution should be taken to not exceed four applications of these products per season. In addition, powdery mildew was recently added to the label of Switch^®. Controlling foliar infections helps to prevent fruit infections.

Figure 1. 

Mycelia of Sphaerothecs macularis on strawberry leaf surface.

Figure 2. 

Lower leaf surface of strawberry covered with powdery mildew.

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.

Figure 6. 

Sphaerotheca macularis on seeds.

Tables

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. Millie Ferrer, Interim Dean.

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