Stemphylium blight moves fast once warm, humid weather sets in, and by the time whole leaves are collapsing, the infection has usually already spread across the field. Caused by the fungus Stemphylium vesicarium, it cuts short the leaf life the bulb depends on to size up properly, so catching the early signs and spraying the right fungicide in time is what protects both yield and bulb quality.
Signs to Watch For
- Small, pale yellow to orange flecks appearing near the leaf tip
- Flecks developing into long, spindle-shaped brown-to-black streaks running down the leaf
- No distinct purple centre in the lesions, unlike purple blotch
- Leaves dying back from the tip and eventually collapsing or toppling over
- Seed stalks showing similar lesions and breaking over in a badly affected crop
Causes
- Warm days combined with high humidity or prolonged leaf wetness from dew, rain, or overhead irrigation
- Dense plant spacing that restricts airflow and keeps the canopy damp for longer
- Weakened or stressed foliage, including leaves already damaged by purple blotch or thrips feeding
- Infected crop debris left in the field from a previous onion season, which carries the fungus over
- Excess nitrogen application, which produces soft, lush growth more prone to infection
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-spectrum cover against Stemphylium blight along with the other leaf spots that often move in on the same weakened tissue.
Product Recommendation
4. Tebuconazole 38.39% w/w SC
A systemic triazole that's 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
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 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.
