Downy mildew builds up fast during cool, humid mornings, and by the time the fuzzy grey-violet growth is visible on the leaf surface, the infection has usually already spread through the field. Caused by the fungus Peronospora destructor, it shortens leaf life right when the bulb needs it most, and the weakened foliage it leaves behind opens the door to purple blotch and Stemphylium blight as well. Spraying the right fungicide at the first sign of trouble is what keeps the canopy standing through bulb development.
Signs to Watch For
- Pale green to yellow patches appearing on the leaves
- A fuzzy, greyish-violet fungal growth on the leaf surface during cool, humid mornings
- Affected leaves turning yellow and collapsing at the point of infection
- Leaves dying back and the crop looking thin and patchy from a distance
- Increased susceptibility to purple blotch and Stemphylium blight moving into the same weakened tissue
Causes
- Cool nights followed by humid, overcast mornings with heavy dew
- Prolonged leaf wetness from fog, drizzle, or overhead irrigation
- Dense planting that restricts airflow and keeps the canopy damp for longer
- Infected bulbs, sets, or crop debris left over from a previous onion season
- Poor field drainage that keeps humidity high around the base of the crop
1. Azoxystrobin 18.2% + Difenoconazole 11.4% w/w SC
Combines a strobilurin with a triazole, giving both protective and curative action against the fungus, with the systemic triazole component moving through the plant to reach new growth.
Product Recommendation
- Syngenta Amistar Top Fungicide
- Katyayani Azodharma Fungicide
- JU Agri Azole Fungicide
- EBS Azolex Fungicide
- Dhanuka Godiwa Super Fungicide
2. Metalaxyl M 3.3% + Chlorothalonil 33.1% SC
Combines a systemic component that moves through the plant to stop the fungus from spreading internally with a protective contact fungicide that shields untouched leaf surface from new infection.
Product Recommendation
3. Azoxystrobin 8.3% + Mancozeb 66.7% WG
Pairs a systemic strobilurin with a protective contact fungicide, giving broad, preventive cover against downy mildew spread across the crop.
Product Recommendation
Cultural Practices to Keep Disease Pressure Down
- Use disease-free, certified seed and treat seedlings before transplanting
- Avoid overhead irrigation late in the day so leaves don't stay wet overnight
- Space plants to allow good airflow through the canopy and faster drying after rain or dew
- Improve field drainage to keep humidity down around the base of the crop
- Rotate onion out of a field for at least one season if the disease has appeared previously
- Remove and destroy infected leaves and crop debris rather than leaving them in the field
Disclaimer: Always confirm current label registration, approved dosage, water volume, and pre-harvest interval (PHI) for your state before using any product, since labels are periodically revised by regulatory authorities.
This guide is a general advisory. Do your own research and verification before using any product. Always confirm suitability for your crop stage, region, and field conditions before making any purchase, spray, or crop-management decision.
