Onion crops are especially vulnerable during humid, cloudy weather, when leaf diseases can spread fast and cut short the leaf life the bulb needs to size up properly. Catching the early signs and spraying the right fungicide in time is what protects both yield and bulb quality.
1. Purple Blotch
How to Spot It
Caused by the fungus Alternaria porri, purple blotch starts as small, water-soaked spots on leaves and seed stalks that quickly turn into sunken lesions with a purplish-brown center and a yellow halo around them. As the lesions enlarge, concentric rings appear inside them, and badly affected leaves eventually collapse and dry from the tip downward.
Solutions to Control It
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
Azoxystrobin 8.3% + Mancozeb 66.7% WG - Pairs the same systemic strobilurin with a protective contact fungicide, giving broad-spectrum cover against purple blotch along with the other leaf spots that often show up alongside it.
Product Recommendation
Difenoconazole 25% EC - A systemic triazole fungicide that moves into the leaf tissue and disrupts fungal cell membrane formation, giving both protective and curative control against purple blotch.
Product Recommendation
2. Stemphylium Leaf Blight
How to Spot It
Caused by Stemphylium vesicarium, this disease is often mistaken for purple blotch but tends to start as small, pale yellow to orange flecks near the leaf tip that grow into long, spindle-shaped brown-to-black streaks running down the leaf, without the distinct purple centre seen in purple blotch. Severely affected leaves die back from the tip and eventually topple over.
Solutions to Control It
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
Azoxystrobin 8.3% + Mancozeb 66.7% WG - Pairs the same systemic strobilurin with a protective contact fungicide, giving broad-spectrum cover that also holds off Stemphylium moving in on weakened leaf tissue.
Product Recommendation
Tebuconazole 38.39% w/w SC - A systemic triazole that is absorbed rapidly into the leaf and interferes with the fungus's growth from within, giving strong knockdown of established Stemphylium infection along with protection for new growth.
Product Recommendation
3. Downy Mildew
How to Spot It
Caused by Peronospora destructor, downy mildew shows up as pale green to yellow patches on the leaves, with a fuzzy, greyish-violet fungal growth developing on the surface during cool, humid mornings. Affected leaves gradually turn yellow, collapse at the point of infection, and the crop becomes noticeably more prone to purple blotch and Stemphylium moving in on the same weakened tissue.
Solutions to Control It
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
Azoxystrobin 8.3% + Mancozeb 66.7% WG - Pairs the same systemic strobilurin with a protective contact fungicide, giving broad, preventive cover against downy mildew spread across the crop.
Product Recommendation
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
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
- Rotate onion out of a field for at least one season if any of these diseases 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.
