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Beginner

Scientific Name
Sclerotinia spp.

Identification

• Appears as water-soaked spots anywhere on the plant, usually on leaves nearest the ground, or on the head
  
• Affected tissue often turns grey, giving rise in wet weather to fluffy white mould
  
• Eventually mould develops black dots in the field or storage

Often Confused With
Bacterial soft rot
Rhizoctonia

Period of Activity
Spore production occurs only when a strict set of environmental conditions have been met: soil moisture in the top 2 to 3 cm (1 in.) must remain near saturation for 10 days and must be at least 10ºC (50°F).

Scouting Notes
Prevention of white rot requires good control of chewing insects and diseased areas.

Thresholds
None established.

Advanced

Scientific Name
Sclerotinia spp.

White rot of crucifers is a disease of temperate climates. The fungus that causes this disease has a wide host range and will attack more than 350 species of plants. In addition to crucifers, the most susceptible vegetables are celery, carrot, bean, artichoke and lettuce.
                          
In Ontario, this disease can cause losses in the field, in storage or in transit. While losses, on average reach levels of 1% in crucifers, occasionally a single location will experience losses of 40- 50%.

Identification
This disease may begin near the ground on older leaves, or on mature heads. It is especially noticeable on cabbage heads. Initial symptoms include dark, water-soaked areas on leaves near the soil. These areas enlarge, the leaves wilt and other plant parts become infected. Cabbage heads in the field will retain their shape but will be filled with a soft, watery rot. 

Infected heads become covered with a white cottony fungal growth containing numerous small, oval, hard black sclerotia. Theses sclerotia have the appearance of mouse droppings and are the means by which the fungus survives unfavourable conditions.

Often Confused With
Bacterial soft rot
Rhizoctonia

Biology
Sclerotinia is a weak pathogen and must have an energy source in order to penetrate plant tissue. Damaged tissue, from insect feeding or disease, can act as sufficient sources of food energy for this fungus. Infection can take place directly by fungal mycelium (threads), in the soil or by spores produced on apothecia, small disc-shaped bodies resembling golf tees, arising from the sclerotia near the soil surface. Infection by spores is more common and wide spread.

Spores from apothecia are readily blown on the wind. They are encased in a sticky substance and are able to stick to plant surfaces when they land.  Sclerotinia can also be spread by infected seed and mechanical transmission on machinery, animals and man.

The fungus survives as sclerotia in the soil, or on diseased plant tissue. It will also survive on weed hosts and has been found in abundance near dandelion and wild clover. Temperature and pH have little affect on the ability of this fungus to survive. It will survive for 3 to 8 years in most soils of temperate climates. There are, however, many competitors that occur naturally in the soil. At least 30 species of fungi and bacteria have been shown to exert some level of control over the Sclerotinia population.

Period of Activity
Spore production occurs only when a strict set of environmental conditions have been met: soil moisture in the top 2 to 3 cm (1 in.) must remain near saturation for 10 days and must be at least 10ºC (50°F).

Scouting Notes
Prevention of white rot requires good control of chewing insects and diseased areas.

Thresholds
None established.

Management Notes

• Plant in well drained soil.
  
• Rotate with non-susceptible crops such as beets, onions, spinach, corn, cereals or grasses. Because the sclerotia persist in soil and the choice of resistant crops is limited, short-term crop rotation is not an effective control measure. At least three years of non-host crops (cereals, corn, grasses, onions) are required to reduce the probability of problems due to the fungus.
  
• Space plants so that air circulation can prevent water-logging of the soil.
  
• Deep ploughing of diseased residue will ensure that most sclerotia are below the level at which development of apothecia takes place.
  
• Use only seed free of small sclerotia - only spiral cleaners can remove such sclerotia.