downy mildew
Beginner
Scientific Name: Pernospora destructor
Identification
- Early symptoms include a purple-grey, velvety growth
- Usually starts out patchy, then spreads quickly through field
- Diseased leaves turn pale-green, yellow and then collapse
Often Confused with
Purple Blotch
Period of Activity
The disease often starts in patches in the field and is favoured by cool (<22˚C), humid weather. It is most common in cool, wet seasons.
Scouting Notes
Regular field scouting is the best way to assess disease levels. Scout for downy mildew at the same time you scout for Botrytis leaf blight.
Thresholds
A forecasting model called DOWNCAST has been developed to help predict downy mildew outbreaks. If the forecasting model is not available, it is recommended that a protective fungicide program be initiated during cool and humid weather conditions.
Advanced
Scientific Name: Pernospora destructor
Downy mildew is a water mould that does well under cool to moderate temperatures, with prolonged leaf wetness periods. Early identification is often difficult because symptoms often don’t appear until 5 to 7 days later. Infected plants are often invaded by secondary pathogens such as soft rot bacteria.
Identification
- Early symptoms include a purple-grey, velvety growth
- Usually starts out patchy, then spreads quickly through field
- Diseased leaves turn pale-green, yellow and then collapse
Often Confused with
Purple Blotch
Biology
Downy mildew is often considered a sporadic disease; however, under the right conditions, this disease can explode and be very destructive to onions. Infection occurs when onion foliage remains wet for 2–6 hours at 3˚C–14˚C (37˚F–57˚F). The incubation period lasts for 10–16 days. A relative humidity of 95% or greater promotes further infections. Research has shown that downy mildew spores germinate at night and spread during the day by wind and/or rain.
Period of Activity
The disease often starts in patches in the field and is favoured by cool (<22˚C), humid weather. It is most common in cool, wet seasons. In Ontario, disease outbreaks are common in July and August. It's important to note that once an onion plant is infected it can remain asymptomatic for up to 16 days as the fungus grows within the leaf.
Scouting Notes
Regular field scouting is the best way to assess disease levels. Scout for downy mildew at the same time you scout for Botrytis leaf blight.
Thresholds
A forecasting model called DOWNCAST has been developed to help predict downy mildew outbreaks. If the forecasting model is not available, it is recommended that a protective fungicide program be initiated during cool and humid weather conditions.
Management Notes
- Managing downy mildew involves the use of cultural practices integrated with fungicide applications where registered.
- Crop rotation for 3 years will reduce the overwintering oospore populations in soil.
- Controlling alternative weed hosts in and around the field is also important to eliminate overwintering inoculum.
