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Scientific Name: Botrytis squamosa

In Ontario, botrytis leaf blight is one of the top two most common fungal diseases seen in onion crops and may cause a dramatic reduction in onion bulb growth and yield.  .     

Identification

• Small (1 to 5 mm) grayish-white oval shaped lesions
• Spots often surrounded by slivery-white ‘halo’
• Active lesions are grayish-white colour
• Older lesions tend to be brownish-white and desiccated (dehydrated or dried out)
• When numerous lesions are present the leaf tips begin to dieback eventually affecting the entire leaf

Often Confused with
Iris yellow spot virus
Herbicide burn
Pelting rain and/or hail

Biology
Botrytis leaf blight overwinters as sclerotia in the soil, on onion debris and in cull piles.  The sclerotia in onion fields will produce conidia (spores) which infect sprouting bulbs and other onion plants.  Sclerotia on leaf debris produce conidia and also ascospores (sexual spores) which will infect leaves of other onion plants.  The ascospores reproduce sexually resulting in possible new strains of the pathogen and potential tolerance to fungicides.  Conidia are produced repeatedly, up to four times, by the germination of the sclerotia.    

Period of Activity
Development of the fungal disease usually occurs after mid-June, typically when temperatures and leaf wetness are ideal for infection.  Warm (16-28 °C), wet weather is most favorable for disease development; however, warm nights and high humidity also provide ideal development conditions.    

Scouting Notes
To scout for botrytis leaf blight, examine the three oldest green leaves on 50 to 100 plants and then determine the number lesions per leaf.

Thresholds

	THRESHOLD
COOKING/SPANISH ONIONS	1 ACTIVE LESION/LEAF
GREEN ONIONS	0.5 ACTIVE LESION/LEAF

Management Notes

• Practice a 2-3 year crop rotation and remove cull/volunteer onions from the field
• Plant spacings that permit better air movement and irrigation schedules that do not extend leaf wetness periods may be helpful.
• To reduce the incidence and severity of botrytis leaf blight, remove cull piles and cull onions from field areas, rogue out volunteer onions and rotate crops.

There are two ways to determine whether or not a preventative fungicide should be applied:

• BOTCAST considers the relationship of weather variables (e.g. leaf wetness, temperature and relative humidity) in determining the probability of sporulation and infection.  Each day, disease severity values are determined using weather variables and added to the previous day's value (CSDI) until a threshold is reached, triggering a fungicide application. The threshold commonly used to trigger a spray application is 20 CSDI.
• Field Monitoring.  Spray application decisions should be based on field monitoring. If conditions surpass the threshold, then a fungicide treatment is recommended.