purple blotch
Beginner
Scientific Name: Alternaria porri
Identification
- Often found on the leaves damaged by other diseases, insects or environmental stresses
- Small 1-3 cm (0.5-1 in.) brown spots with purplish centres are characteristic of this disease
- Spots form into oval lesions that have a purplish tint with concentric rings
- Infected leaves are often weakened and easily blown over
Often Confused with
Downy mildew
Period of Activity
Infection occurs when warm weather, 18-30°C (64-86°F), coincides with prolonged dews or leaf wetness. Weaker/stressed plants are at 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. However, when plants are infested by onion thrips, younger leaves are more susceptble and, the severity of the disease is much higher when compared to infected plants not infested by onion thrips.
Thresholds
None established.
Advanced
Scientific Name: Alternaria porri
Identification
- Often found on leaves with pre-existing damage caused by other diseases, insects or environmental stresses
- Small 1-3 cm (0.5-1 in.) brown spots with purplish centres are characteristic of this disease
- Spots form into oval lesions that have a purplish tint with concentric rings
- Infected leaves are often weakened and easily blown over
Often Confused with
Downy mildew
Biology
This disease requires three conditions for infection: 1) the disease to be present in the field; 2) water to be present for spore germination; and 3) temperatures between 13 to 34°C, with an optimum temperature of 25°C for growth.
Purple blotch overwinters in crop debris either in the field or in storage sheds. The spores develop in high humidity, rain or persistent dew with an optimal range of 25-29°C (77-85°F). The activity of the spores is reduced at any temperature below 12.7°C (55°F). Fungal spores can spread to onion foliage nd bulbs in the field or storage shed by wind, splashing water splashing, implements, insects or workers.
Period of Activity
Infection occurs when warm weather, 18-30°C (64-86°F), coincides with prolonged dews or leaf wetness. Weaker plants or those affected by other diseases are at high risk.
Scouting Notes
Regular field scouting is important. Look specifically at older leaves of the plant as they will more likely be targeted by the disease. However, when plants are infested by onion thrips, younger leaves are more susceptible and, the severity of the disease is much higher when compared to infected plants not infested by onion thrips.
Thresholds
None established.
Management Notes
Some simple ways to control purple blotch in the field include:
- Reduce inoculum by destroying infected crops, remove or relocate cull piles
- Rotate to non-host crops (e.g. carrot, celery, lettuce)
- Monitor and control onion thrips levels
- Harvest onions during dry weather and ensure proper curing to prevent introduction into storage
