Strawberry Disorders

Botrytis Fruit Rot & Powdery Mildew

Botrytis Fruit Rot

• Symptoms
• Development & Spread
• Control

Powdery Mildew

• Symptoms
• Development & Spread
• Control

Botrytis Fruit Rot

Botrytis fruit rot--also known as gray mold--is caused by the fungus Botrytis cinerea. This fungus is one of the most important diseases of strawberry worldwide. B. cinerea is not limited to strawberries, but infects a wide range of fruit, vegetable, and weed species.

In Florida, this fungus affects fruit before and after harvest. In the field, an epidemic results in severe pre-harvest losses. Infections continue to develop during storage and transit at refrigeration temperatures.

Symptoms

Strawberry flowers are highly susceptible to B. cinerea, and may be blighted directly. However, symptoms usually are observed later on green and ripening fruit.

Lesions

Lesions typically develop on the stem end of the fruit. These are often associated with infected stamens or dead petals stuck to the fruit or trapped beneath the calyx. Lesions begin as small, firm, light brown spots and quickly enlarge.

Spores

During periods of rainy weather, heavy dews, or high relative humidity, lesions become covered with a white fungal mycelium and gray to brown spores. Large numbers of spores are released as visible gray puffs when infected fruit are disturbed. Botrytis may consume and mummify the entire fruit.

Development & Spread

Generally, B. cinerea colonizes strawberry foliage in the nursery. The fungus can also be found on dying vegetation around strawberry fields. After runner transplants are planted, spores produced on old dying leaves rapidly colonize new emerging leaves without causing visible symptoms. During the main bloom period in January and February, these spores (conidia) are dispersed by air, water, and harvesters to infect flowers.

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. After infection, lesions generally take two to four weeks to become visible on green and ripening fruit.

The pathogen spreads from diseased fruit to healthy fruit by direct contact. However, direct infection of fruit by spores is not considered important in the field or after harvest. 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 ripen in February and March are more seriously affected than the first crop of fruit that ripen in December and January.

Control

Botrytis fruit rot can be controlled by both chemical and cultural measures.

Cultural Practices

These include the use of resistant cultivars and the physical removal of infected plant parts. Although commercial cultivars are not highly resistant to this disease, 'Camarosa', 'Carmine', and 'Treasure' are less susceptible to Botrytis fruit rot than 'Strawberry Festival' and 'Sweet Charlie' under Florida conditions.

Cultivars with large clasping calyces are generally more vulnerable. The calyx and the receptacle can trap moisture, enabling the pathogen to spread from stamens and petals to the developing fruit. Removing any aging and dying leaves after establishment helps to eliminate a potential source of inoculum.

UF studies have shown that, while leaf pruning can slightly reduce disease incidence, it does not increase marketable yield. As such, pruning 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.

Fungicides

Good disease management requires regular applications of a broad-spectrum protective fungicide, such as captan or thiram. After overhead irrigation for plant establishment has ended, begin applications at low rates and continue them throughout the season.

In Florida, strawberries bloom from November to March, but peak blooms occur in November and between January and February. Botrytis fruit rot is not as common during the first bloom, so regular protectant applications will prevent significant early-season losses. During the second peak bloom, fungicides with good activity against Botrytis fruit rot should be used instead.

The first application should be made at ten percent bloom, usually in late January. Susceptible cultivars may require up to four applications at weekly intervals to protect flowers throughout the bloom period.

It is critical to use protectant applications during periods of mild temperatures and prolonged wetness caused by rains, fog, or heavy dews. Once these weather conditions have passed, resume normal applications of captan or thiram, 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.

Powdery Mildew

Sphaerotheca macularis is an obligate parasite that infects leaves, flowers and fruit. S. macularis only infects living tissue of wild or cultivated strawberries.

Symptoms

Early infections are characterized by small white patches of fungus growing on the lower leaf surface. These patches generally have a powdery appearance, caused by dense mycelial growth and numerous chains of conidia (spores). Under favorable conditions, the patches expand and coalesce until the entire lower surface of the leaf is covered.

In some cultivars, relatively little mycelium is produced, so that white patches are difficult to see. Instead, irregular yellow or reddish brown spots develop on colonized areas on the lower leaf surface, and eventually break through to the upper surface. The edges of heavily infected leaves curl upward.

At times, dark round structures (cleistothecia) are produced in the mycelia on the undersides of leaves. 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. Both types of infection may reduce fruit quality and marketable yields.

Development & Spread

In temperate areas, the pathogen may survive by producing cleistothecia. In nurseries, living, green leaves become infected easily, meaning that infected transplants are normally the primary source of inoculum for fruiting fields in Florida. When conditions are favorable, conidia produced on infected plants are dispersed by wind.

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. It usually subsides in January and early February, but may reappear in late February and March.

Control

As with Botrytis fruit rot, there are several methods of controlling the fungus.

Cultural Practices

The best method for controlling powdery mildew is to use transplants free of the disease. However, even disease-free fields can become infected by conidia that blow in from neighboring fields.

Cultivars differ widely in their resistance to powdery mildew. Unfortunately, the most popular cultivars--"Strawberry Festival" and "Camarosa"--are very susceptible to the disease. Fields with susceptible cultivars should be surveyed regularly for powdery mildew, especially early in the season.

Fungicides

Controlling foliar infections helps to prevent fruit infections. 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 can effectively control powdery mildew if the pathogen population has not become resistant to them. Strobilurin fungicides can be used to suppress powdery mildew, but they are more effective in controlling anthracnose fruit rot.

For resistance management, these fungicides should not be applied more than four or five times per season. Reserving strobilurin fungicides for anthracnose control may be a better use of the recommended limited number of sprays. Growers should evaluate the time of season, strawberry variety, and weather conditions when deciding whether to use strobilurin spray for powdery mildew control or for anthracnose control.