White rot of onion

The fungus Stromatinia cepivora (syn. Sclerotium cepivorum) is the causative agent of white rot, one of the most destructive soil-borne diseases affecting the Allium genus. This pathogen is highly specialized and can persist in soil for over a decade in the form of dormant sclerotia. These structures are triggered to germinate by chemical exudates released specifically by the roots of onion and garlic plants.

The disease affects a wide range of crops including onions, garlic, shallots, leeks, and chives. It is a cosmopolitan pathogen that can be found in most temperate regions where Allium crops are cultivated. Because the sclerotia can be easily transported by contaminated equipment, soil movement, or water runoff, white rot remains a difficult problem to contain in commercial vegetable production.

Symptoms typically begin with leaf yellowing and wilting, which progresses from the tip downwards, eventually causing the death of the foliage. When an infected plant is lifted, a fluffy white mycelial growth is clearly visible on the roots and the base of the bulb. As the infection advances, the bulb tissues soften and decay into a watery mass. Tiny, round black sclerotia, roughly the size of poppy seeds, eventually form on the decaying tissues.

White rot thrives in cool, moist soil conditions, with temperatures between 10°C and 20°C being optimal for fungal growth. Once a field is infested, the pathogen is extremely difficult to eradicate. Yield losses can be total in heavily infested areas, particularly in high-density plantings or fields where the rotation cycle for onions is too short. The disease remains a constant threat, causing both pre-harvest field loss and post-harvest decay.

Effective management requires an integrated approach. Cultural practices such as long crop rotations and ensuring proper field drainage are essential. Growers must strictly avoid introducing infested material into clean soil. In high-pressure environments, the application of specialized fungicides at the time of planting is the primary chemical defense. Additionally, soil solarization and the use of specific microbial bio-pesticides are gaining recognition as viable tools for reducing sclerotia populations in the soil.