Gray mold or Botrytis rot is one of the serious and common diseases in all strawberry production areas and is a disease of concern in most years. It is caused by the fungus Botrytis cinerea.
The disease is a problem not only in the field, but also during storage, transit, and marketing of strawberry fruit, due to onset of severe rot as the fruits begin to ripen. Other parts infected by the fungus include leaves, crown, petals, flower stalks, and fruit caps.
The disease is most severe during bloom and harvest in seasons with lengthy periods of cloud and rain complemented by cool temperatures and can cause great fruit losses.
Gray mold may be present during all stages of strawberry fruit development and young blossoms are usually very susceptible to infection.
DISEASE CYCLE
The fungus overwinters as minute, black, fungus bodies (sclerotia) or as mycelium in plant debris, such as dead strawberry or raspberry leaves. Nearly all of the overwintering inoculum in strawberry plantings comes from mycelium in dead strawberry leaves within the row or planting. In early spring, the mycelium becomes active and produces large numbers of microscopic spores (conidia) on the surface of old plant (leaf) debris in the row.
Spores are spread by wind throughout the planting where they are deposited on blossoms and fruits. They germinate when a film of moisture is present and infection can occur within a few hours. Temperatures between 20 to 27 degrees centigrade and free moisture on the foliage from rain, dew, fog, or irrigation water are ideal conditions for disease development. The disease can develop at lower temperatures if foliage remains wet for long periods.
Strawberries are susceptible to Botrytis during bloom and again as fruits ripen. Recent research shows that most fruit infection occurs during bloom, but symptoms usually do not develop until close to harvest. During bloom, the fungus colonizes healthy or senescing flower parts, often turning the blossoms brown. These blossom infections establish the fungus within the receptacle of the young fruit as a “latent” or “quiescent” infection. The fungus generally remains latent in developing green fruit until the fruit starts to mature, at which time the fungus becomes active and symptoms appear.
Some fruit infection occurs by direct growth from dropped flower petals or dead leaves.
SYMPTOMS
The most diagnostic symptom is rotted fruit with velvety gray mold growth.
Light brown lesions usually develop on the stem end of the fruit due to flower infections but may also occur on the sides of fruit where soil, standing water, or infected berries or flower petals are in contact.
Fruit infections usually appear as soft, light brown, rapidly enlarging areas on the fruit. If infected fruits remain on the plant, the berry usually dries up, i.e. becomes mummified and becomes covered with a gray, dusty powder, which gives the disease its name “gray mold.”
Fruit infection is most severe in well-protected areas of the plant, where the humidity is high and air movement is poor.
Usually, the disease is not detected until fruits are mature at harvest time. After picking, mature fruits are extremely susceptible to gray mold, especially if bruised. During picking, the handling of infected fruit will spread the fungus to healthy ones. Under favorable conditions for disease development, healthy berries may become a rotted mass within 48 hours after picking.
DISEASE MANAGEMENT
Chemical control
Fungicides are an important disease management tool and the most critical period for applying them to control gray mold is during bloom.
The following fungicides are recommended for use in prevention and eradication of the disease;
- ABSOLUTE 375SC 10ml/20l
- EXEMPO CURVE 250SC 15ml/20l
- EXPLORER 3 SL 10ml/20l
- IPRODE 500SC 10ml/20l
- KATERINA 720SC 40ml/20l
- MEGAPRODE LOCK 525WP 20g/20l
- MILESTONE 250SC 10ml/20l
- PROVIDENCE 400WP 50g/20l
- RANSOM 600WP 15g/20l
- CHARIOT 500SC 20ml/20l
- RIMETA GOLD 300SC 40ml/20l
- TRINITY GOLD 425WP 50g/20l
Non-chemical control methods
- Select a planting site with good soil drainage and air circulation.
- Practise rotations with non-host crops
- Plant resistant/tolerant cultivars
- Plants should be exposed to direct sunlight.
- Plant rows with the direction of the prevailing wind to promote faster drying of foliage and fruit.
- Use disease free planting materials.
- A good layer of mulch between the rows or around the plants aids greatly in controlling fruit rots. The mulch acts as a barrier that reduces fruit contact with the soil.
- Proper spacing of plants and timing of fertilizer applications are also important.
- Good weed control is very important because weeds prevent air movement in the plant canopy and harbour the fungus.
- Pick fruit frequently and early in the day as soon as plants are dry.
- Cull out all diseased berries but do not leave them in the field.
- Handle berries with care to avoid bruising.
- Avoid overhead irrigation.
- Refrigerate fruits promptly at 0-10 degrees centigrade to check gray mold.
Note;
- Whenever spraying, mix the fungicide with INTEGRA 3ml/20l. This is a sticker, spreader, and penetrant which improves the efficacy of the fungicide.
- In order to prevent the fungus from gaining resistance on any of the fungicides, it is advisable to alternate several fungicides throughout the crop season.
- Fungicides should be reapplied after 1-2weeks.
- Timely control of the disease prevents/reduces yield losses
