White rot: Black sclerotia around the neck of the bulb
Pathogen: Sclerotium cepivorum
Symptoms
Innumerable small black sclerotia form in the fungal growth and on the affected parts. Fluffy mycelium may be on remaining roots and bulb. White fluffy or cottony growth of mycelium with abundant black sclerotia resembles mustard grain developing on the infected bulbs.
Disease development
White rot fungus can stay in the soil on plant debris or as sclerotia. The sclerotia can remain dormant in the soil for many years germinating as soon as onions or a related crop are cultivated on the land again. White rot can spread quickly from root to root. Cultivation machinery, planting material, crates or footwear can also spread the disease. White rot doesn’t cause a lot of damage in the first year of infection, but in the following years considerable numbers of plants may fail because the number of sclerotia can increase rapidly.
Disease management
Avoiding infection is essential. Follow good hygiene protocols on the farm. Avoid soil from infected plots being carried by machines to other plots. Correctly registering infected plots can prevent it from spreading to uninfected plots. Inspect the propagating material (for example onion sets) for the presence of white rot. If the disease is detected for the first time in a certain plot, remove and destroy as many infected plants as possible to prevent the rapid spread of the infection.
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