• Topics: Plant Pathology | Mertely, James C | Peres, Natalia A | Strawberry Diseases | Botrytis cinerea

Botrytis Fruit Rot or Gray Mold of Strawberry¹

Botrytis fruit rot, also known as gray mold, is caused by the fungus Botrytis cinerea and is one of the most important diseases of strawberry worldwide. The disease affects fruit in the field, resulting in severe pre-harvest losses. It also affects fruit after harvest, since infections that begin in the field continue to develop during storage and transit at refrigeration temperatures.

Symptoms

Strawberry flowers are highly susceptible to B. cinerea, and may be blighted directly (Figure 1). However, symptoms usually are observed later on green and ripening fruit. Lesions typically develop on the stem end of the fruit and are often associated with infected stamens or dead petals adhering to the fruit or trapped beneath the calyx (Figure 2). Lesions begin as small, firm, light brown spots that enlarge quickly (Figure 3). During periods of rainy weather, heavy dews, or high relative humidity, lesions become covered with masses of tan to gray spores (Figure 4). Large numbers of spores are released as visible gray puffs when infected fruit are disturbed. Botrytis may consume and mummify the entire fruit (Figure 5).

Figure 1. 

Flower blighted by Botrytis cinerea.

Credit: UF GCREC
[Click thumbnail to enlarge.]

Figure 2. 

Botrytis lesion from colonized petal (arrow).

Credit: UF GCREC
[Click thumbnail to enlarge.]

Figure 3. 

Botrytis lesion without spores.

Credit: UF GCREC
[Click thumbnail to enlarge.]

Figure 4. 

Botrytis lesion with spores.

Credit: UF GCREC
[Click thumbnail to enlarge.]

Figure 5. 

Botrytis-mummified fruit.

Credit: UF GCREC
[Click thumbnail to enlarge.]

Disease Development and Spread

B. cinerea is a common colonizer of strawberry foliage in the nursery, and is also present on dying vegetation around strawberry fields. After transplanting, spores produced on old dying leaves rapidly colonize new emerging leaves without causing visible symptoms. These spores (conidia) are dispersed by air, water, and harvesters to infect flowers during the main bloom period in January and February. Cool to mild temperatures and prolonged leaf wetness promote spore production, germination, and infection of stamens, petals, and other floral parts. Flower infections often progress slowly, with lesions becoming visible on green and ripening fruit 2 to 4 weeks after infection. Direct infection of fruit by spores is not considered important in the field or after harvest. However, the pathogen also spreads from diseased fruit to healthy fruit by direct contact (Figure 6). As the epidemic progresses, diseased fruit, mummified fruit, and decayed flowers and pedicles become important new sources of inoculum. Botrytis fruit rot is especially damaging in annual production systems characterized by prolonged flowering and fruiting cycles. In Florida, the second crop of fruit that ripens in February and March are more seriously affected than the first crop of fruit that ripen in December and January.

Figure 6. 

Fruit-to-fruit spread of Botrytis cinerea.

Credit: UF GCREC
[Click thumbnail to enlarge.]

Control

Botrytis fruit rot can be controlled by both chemical and cultural measures. Cultural practices include the use of resistant cultivars and the physical removal of infected plant parts (plant sanitation). Although there are no commercial cultivars highly resistant to this disease, 'Camarosa', 'Carmine', and the newly released 'FL Radiance' and 'FL Elyana' are less susceptible to Botrytis fruit rot than 'Strawberry Festival', 'Treasure' and 'Sweet Charlie'. The Californian cultivar 'Camino Real' has been proven highly susceptible under Florida conditions. Cultivars with large clasping calyces are generally more susceptible because moisture trapped between the calyx and the receptacle promotes spread of the pathogen from stamens and petals to the developing fruit. Removal of senescing and dying leaves after establishment helps to eliminate a potential source of inoculum. However, studies have shown that leaf pruning modestly reduces disease incidence, but does not increase marketable yield, and is not practical due to the high cost of labor. Yields may even be reduced when pruning includes the removal of partially green leaves. However, the removal of diseased and culled fruit from the plant canopy during normal harvest operations is considered vital to successful management of Botrytis fruit rot.

In commercial fields in Central Florida, fungicide applications are usually necessary to suppress sporulation and protect flowers from infection. A good disease management program is based on regular applications of a broad-spectrum protective fungicide such as captan or thiram. Applications at low rates should begin after overhead irrigation for plant establishment has ended, and continue throughout the season. Strawberries bloom from November to March in Florida, but peak blooms occur in November and January/February. Disease incidence is usually low in the first bloom and the regular protectant applications are sufficient to prevent significant early-season losses. During the second peak bloom, fungicides with good activity against Botrytis fruit rot can be substituted for protective applications. Captevate®, Elevate®, Pristine®, Scala®, and Switch® are among the most effective fungicides for control of Botrytis fruit rot (Table 1). The first application should be made at 10% bloom (usually late January). Susceptible cultivars may require up to four applications at weekly intervals to protect flowers throughout the bloom period. Applications are especially critical during periods of mild temperatures and prolonged wetness caused by rains, fog, or heavy dews. Once this critical period has ended, normal applications of captan or thiram can be resumed, usually at high label rates. Applications of protectant fungicides are usually sufficient to control Botrytis fruit rot in March, when the disease is naturally suppressed by hot weather.

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. Nick T. Place, Dean.

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