white rot
Beginner
Scientific Name: Sclerotium cepivorum
Identification
- First above-ground symptoms are a yellowing and dieback of the leaf tips, followed by a collapse of affected leaves
- When bulbs and roots are examined, a white, fluffy mould and soft rot will be observed
- Fungus causes a watery rot and disintegration of infected bulbs
Often Confused with
Onion smut
Period of Activity
Disease development is favoured by cool, moist soil conditions. White rot does well when temperatures range from 10 - 24˚C (50-75˚F). Warm soil temperatures, greater than 25°C (75°F) are less conductive to white rot.
Scouting Notes
Scan the field for patches of stunted plants with yellowing leaves and tip dieback. Destructively sample plants and inspect roots and bulbs for fluffy mycelium or tiny, black sclerotia.
Thresholds
None established.
Advanced
Scientific Name: Sclerotium cepivorum
White rot is a very destructive disease that begins in the field and can carry over into storage. White rot typically develops in patches in the field. Recent research indicates that sclerotial germination stimulants (such as garlic oil, diallyl disulphides, etc.) can significantly reduce white rot.
Identification
- First above-ground symptoms are a yellowing and dieback of the leaf tips, followed by a collapse of the affected leaves
- When the bulbs and roots are examined, a white, fluffy mold and soft rot will be observed
- The fungus causes a watery rot and disintegration of infected bulbs
Often Confused with
Onion smut
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 arising from the sclerotia near the soil surface. Infection by spores is more common and widespread.
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. White rot can be spread by infected seed, 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. There are many naturally occurring competitors in the soil; at least 30 species of fungi and bacteria have been shown to exert some level of control over the white rot population.
Period of Activity
Disease development is favoured by cool, moist soil conditions. White rot does well when temperatures range from 10 - 24˚C (50-75˚F). Warm soil temperatures, greater than 25˚C (75˚F) are less conductive to white rot.
Scouting Notes
Scan the field for patches of stunted plants with yellowing leaves and tip dieback. Destructively sample plants and inspect roots and bulbs for fluffy mycelium or tiny, black sclerotia.
Thresholds
None established.
Management Notes
- Remove culled onions from field area and wash all implements used in infested fields
- Field flooding followed by a crop rotation has been shown to reduce white rot in muck soils
