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Leaf Blight - Onions

Botrytis squamosa






Initially appears as small oval or circular white leaf flecks, 1 – 5 mm in diameter, with greenish water-soaked margins. The centre of these lesions become straw coloured and sunken and as the disease continues to colonise the infected leaves they become blighted, shrivel until ultimately the whole leaf dies back. Leaf dieback can occur 5 – 12 days after the appearance of the first spots.


Overwinters as sclerotia in the soil or crop debris. Given suitable conditions these release conidia which invade leaf wounds or damaged tissue. At least 6 hours of leaf wetness (with favourable temperatures) are required for successful infection to occur. Conidia released from established leaf lesions give rise to further spread and secondary outbreaks.

Favourable Factors

High rainfall and high humidity.


Prevalent in most onion growing areas particularly in the west of the UK. Can be a problem in salad onions where the quality of the foliage is important but less damaging to bulb onions or leeks.

Host specific to Allium spp.


  • Apply fungicides at the first signs of infection.

  • Crop rotation

  • Removal of crop debris

  • Decision support systems have been developed in some areas for this disease

Severe leaf blight