stemPHYLIUM LEAF BLIGHT
Beginner
Scientific Name: Stemphylium vesicarium
Identification
- Symptoms often start as small yellow to tan, water-soaked lesions that develop into elongated spots which turn dark olive brown to black when spores develop
- Often found on the leaves damaged by diseases, insects, herbicides or environmental stresses
- Development of this disease is favoured by warm (15 – 26°C) humid conditions and long periods of leaf wetness (6 hours or more)
- Onions leaves begin to die-back from the tip resulting in early lodging which can reduce bulb size and yield
- This disease can start to appear as early as the end of June and may show up first on damaged, diseased or dying leaf tissue
Often Confused with
Purple Blotch
Herbicide Injury (oxyfluorfen, flumioxazin)
Tipburn (a physiological disorder that can be common around the time the onions bulb)
Period of Activity
Ideal infection occurs when warm weather, 15-26°C (59-79°F), coincides with prolonged dews or leaf wetness (6 hours or more). Weaker/stressed plants are at a highest risk for infection.
Scouting Notes
Regular field scouting is recommended. Look specifically at older leaves as they are more commonly affected than younger tissues. Plants with herbicide damage are more likely to develop stemphylium leaf blight symptoms.
Thresholds
None established.
Advanced
Scientific Name: Stemphylium vesicarium
Identification
- Symptoms often start as small yellow to tan, water-soaked lesions that develop into elongated spots which turn dark olive brown to black when spores develop
- Often found on the leaves damaged by diseases, insects, herbicides or environmental stresses
- Development of this disease is favoured by warm (15 – 26°C) humid conditions and long periods of leaf wetness (6 hours or more)
- Onions leaves begin to die-back from the tip resulting in early lodging which can reduce bulb size and yield
- This disease can start to appear as early as the end of June and may show up first on damaged, diseased or dying leaf tissue
Often Confused with
Purple Blotch
Herbicide Injury (oxyfluorfen, flumioxazin)
Tipburn (a physiological disorder that can be common around the time the onions bulb)
Biology
Stemphylium vesicarium has a wide host range which also includes asparagus, garlic and pear. This pathogen overwinters on infected crop debris as pseudothecia, which can release ascospores in the spring. On onions, conidia germination can occur between 4 - 35°C although ideal temperatures for appressoria development is 15 - 24°C. Ideal infection conditions are temperatures of 15-26°C (59-79°F) with a leaf wetness period of 6 hours or more. After initial symptoms are observed, sporulation can occur within 6 days at 18 - 24°C. Conidia are spread mainly by wind but can also be spread by splashing from rainfall or irrigation.
Period of Activity
Ideal infection occurs when warm weather, 15-26°C (59-79°F), coincides with prolonged dews or leaf wetness (6 hours or more). Weaker/stressed plants are at a highest risk for infection.
Scouting Notes
Regular field scouting is recommended. Look specifically at older leaves as they are more commonly affected than younger tissues. Plants with herbicide damage are more likely to develop stemphylium leaf blight symptoms.
Thresholds
None established.
Management Notes
- Minimize herbicide injury which may provide entry points for the fungus
- Mitigate the effects of environmental stresses by ensuring the crop has adequate moisture and nutrients throughout the growing season
- Apply registered fungicides preventatively to improve efficacy
- Monitor and control onion thrips levels using only surfactants recommended on the label
- Reduce inoculum by destroying infected crops, remove or relocate cull piles
- Rotate to non-host crops (e.g. carrot, celery, lettuce)
